430.12. Motor Terminal Housings. The sizes of these enclosures are standardized as provided in the associated tables, which had their sizes significantly increased by about 40 percent in the 1996 NEC. In part (E) the Code rule requires some provision for connecting an equipment grounding conductor at the terminal box where the branch circuit supplies a motor. The grounding connection may be either a ”wire-to-wire” connection or a “fixed terminal” connection, and the ground terminal provision may be either inside the junction box—for connection of an equipment grounding conductor run with the circuit wires within the supply raceway—or outside the junction box—for connection of an “equipment bonding jumper” on the outside of a length of flexible metallic conduit or liquidtight flex, either of which is so commonly used. As required by 348.60 and 350.60, a bonding jumper is required for even short lengths of flex (up to 6 ft [1.8 m]) when the wires within the flex are protected at their origin by fuses or CBs rated over 20 A; and liquidtight flex over metric designator 35 (trade size 1¼ in.) and overcurrent protection over 60 A (see 250.118 for details) must have a bonding jumper for the typical length (up to 6 ft [1.8 m]) used with motor connections. See also 430.245 and Fig. 430-70.
This rule permits either an inside or an outside connection of the bonding jumper to correlate with 250.102(E), which permits the bonding jumper (up to 6 ft [1.8 m] long) to be run either inside or outside the flex.
The exception to this rule eliminates the need for providing “a separate means for motor grounding” at the junction box where a motor is part of “factory-wired equipment” in which the grounding of the motor is already provided by some other conductive connection that is an element of the overall assembly.
430.13. Bushing. Refer also to 300.4(G).
430.16. Exposure to Dust Accumulations. The conditions described in this section could make the location a Class II, Division 2 location: the types of motors required are specified in Art. 502.
430.17. Highest Rated or Smallest Rated Motor. Note that the current rating, not the horsepower rating, determines the “highest rated” motor where Code rules refer to such. See 430.62.
430.22. Single Motor. The basic rule of part (A) says that the conductors supplying a single-speed motor used for continuous duty must have a current-carrying capacity of not less than 125 percent of the motor full-load current rating, so that under full-load conditions the motor must not load the conductors to more than 80 percent of their ampacity. Be aware that this 125 percent factor has absolutely nothing to do with the 125 percent factor involved in calculating conductor sizing for overcurrent device terminations when those wires are subject to continuous loads. The fact that the two numbers are identical is a coincidence. The motor circuit is based on likely overload protective device settings, and addresses the fact that the 125 percent condition, or something close to it, could continue for a substantial amount of time, but probably not 3 h. Do not make the mistake of looking at a continuous duty motor, adding 25 percent as required for this section, and then adding another 25 percent of that (total factor = 156 percent) for the motor circuit conductor size. That amounts to double derating and the NEC does not require it to be done.
For a multispeed motor, part (B) provides that the selection of branch-circuit conductors on the supply side of the controller must be based on the highest full-load current rating shown on the motor nameplate.
For a wye-start, delta-run motor with all six leads (twelve for dual-voltage motors) brought out, the starting voltage is 1/
of the phase-to-phase voltage, and the starting current is (1/)2 or one-third of the full-load current. Because all the leads are brought out, during the normal running condition each phase connection has two wires connected to it, one to each of the two adjacent positions of the delta connection. Each of those two wires carries a current that does not add arithmetically in the final connection, because they connect to different phase windings in the motor. The current in each wire, when added together using three-phase procedures (i.e., multiplied by ) will equal the full-load current taken from the line at the disconnect. This means that each of those wires carries 1/ of the full-load current, or 58 percent of FLC. Part (C) requires that these same wires, which connect the controller and the motor, be “based” on this number for this reason. This is only the first step. If this is a customary continuous duty cycle application, then multiply this number by 125 percent to get the actual wire size on the load side of the controller, or 72 percent of FLC. If the duty cycle is other than continuous, use the multiplier from Table 430.22(E) to get the final factor.
Part (D) requires the comparable conductors that connect a part-winding connected motor to its controller be based on 50 percent of full-load current because each phase winding operates in parallel with its sister.
Figure 430-4 shows the sizing of branch-circuit conductors to four different motors fed from a panel. (Sizing is also shown for branch-circuit protection and running overload protection, as discussed in 430.34 and 430.52. Refer to Table 430.250 for motor full-load currents and Table 430.52 for maximum ratings of fuses.) Figure 430-4 is based on the following:
1. Full-load current for each motor is taken from Table 430.250.
2. Running overload protection is sized on the basis that nameplate values of motor full-load currents are the same as values from Table 430.250. If nameplate and table values are not the same, OL (overload) protection is sized according to nameplate.
3. Conductor sizes shown are for copper. Use the amp values given and Table 310.16 to select correct size of aluminum conductors.
It is important to note that this rule establishes minimum conductor ratings based on temperature rise only and does not take into account voltage drop or power loss in the conductors. Such considerations frequently require increasing the size of branch-circuit conductors.
Part (E) includes requirements for sizing individual branch-circuit wires serving motors used for short-time, intermittent, periodic, or other varying duty. In such cases, frequency of starting and duration of operating cycles impose varying heat loads on conductors. Conductor sizing, therefore, varies with the application. But it should be noted that the note at the bottom of Table 430.22(E) says any motor is considered to be for continuous duty unless the nature of the apparatus that it drives is such that the motor cannot operate continuously with load under any condition of use.
Fig. 430-4. Circuit conductors are sized at 1.25 times motor current. (Sec. 430.22.)
When a motor is used for one of the classes of service listed in Table 430.22(E), the necessary ampacity of the branch-circuit conductors depends on the class of service and on the rating of the motor. A motor having a 5-min rating is designed to deliver its rated horsepower during periods of approximately 5 min each, with cooling intervals between the operating periods. The branch-circuit conductors have the advantage of the same cooling intervals and hence can safely be smaller than for a motor of the same horsepower but having a 60-min rating.
In the case of elevator motors, the many considerations involved in determining the smallest permissible size of the branch-circuit conductors make this a complex problem, and it is always the safest plan to be guided by the recommendations of the manufacturer of the equipment. This applies also to feeders supplying two or more elevator motors and to circuits supplying noncontinuous-duty motors used for driving some other machines. The duty cycle will essentially be intermittent because buildings are not of infinite height.
430.23. Wound-Rotor Secondary. The full-load secondary current of a wound-rotor or slip-ring motor must be obtained from the motor nameplate or from the manufacturer. The starting, or starting and speed-regulating, portion of the controller for a wound-rotor motor usually consists of two parts—a dial-type or drum controller and a resistor bank. These two parts must, in many cases, be assembled and connected by the installer, as in Fig. 430-5.
Fig. 430-5. Wound-rotor motor may be used with rotary drum switch for speed control. (Sec. 430.23.)
The conductors from the slip rings on the motor to the controller are in circuit continuously while the motor is running and hence, for a continuous-duty motor, must be large enough to carry the secondary current of the motor continuously.
If the controller is used for starting only and is not used for regulating the speed of the motor, the conductors between the dial or drum and the resistors are in use only during the starting period and are cut out of the circuit as soon as the motor has come up to full speed. These conductors may therefore be of a smaller size than would be needed for continuous duty.
If the controller is to be used for speed regulation of the motor, some part of the resistance may be left in circuit continuously and the conductors between the dial or drum and the resistors must be large enough to carry the continuous load without overheating. In Table 430.23(C) the term continuous duty applies to this condition.
Conductors connecting the secondary of a wound-rotor induction motor to the controller must have a carrying capacity at least equal to 125 percent of the motor’s full-load secondary current if the motor is used for continuous duty. If the motor is used for less than continuous duty, the conductors must have capacity not less than the percentage of full-load secondary nameplate current given in Table 430.22(E). Conductors from the controller of a wound-rotor induction motor to its starting resistors must have an ampacity in accordance with Table 430.23(C), as shown in Fig. 430-6 for a magnetic starter used for reduced inrush on starting but not for speed control.
430.24. Several Motors or a Motor(s) and Other Load(s). Conductors supplying two or more motors (such as feeder conductors to a motor control center, to a panel supplying a number of motors, or to a gutter with several branch circuits tapped off) must have a current rating not less than 125 percent of the full-load current rating of the largest motor supplied, plus the sum of the full-load current ratings of the other motors supplied, plus capacity for the “other” load(s).
Fig. 430-6. Rules cover conductor sizing for wound-rotor motors without speed control. (Sec. 430.23.)
Figure 430-7 shows an example of sizing feeder conductors for a load of four motors, selecting the conductors on the basis of ampacities given in Table 310.16 and using conductors with a 60 or 75°C insulating rating—or using 90°C-rated conductors at the ampacities of 75°C. UL rules generally prohibit use of 90°C conductors at the 90°C ampacities shown in Code Table 310.16. [Refer to 110.14(C)(1)(a)(4).]
For the overcurrent protection of feeder conductors of the minimum size permitted by this section, the highest permissible rating or setting of the protective device is specified in 430.62. Where a feeder protective device of higher rating or setting is used because two or more motors must be started simultaneously, the size of the feeder conductors shall be increased correspondingly. Note that, as covered in Exception No. 1, if one or more motors is not classified as continuous duty, there is a two-step process involved in determining the highest motor rating. First determine the adjusted current requirements for all such motors by using 430.22(E). Then compare those results with the continuous duty motors taken at the customary 125 percent. The largest result is the number that goes forward as the largest motor for the feeder calculations.
These requirements and those of 430.62 for the overcurrent protection of power feeders are based on the principle that a power feeder should be of such size that it will have an ampacity equal to that required for the starting current of the largest motor supplied by the feeder, plus the full-load running currents of all other motors supplied by the feeder. Except under the unusual condition where two or more motors may be started simultaneously, the heaviest load that a power feeder will ever be required to carry is the load under the condition where the largest motor is started at a time when all the other motors supplied by the feeder are running and delivering their full-rated horsepower.
Fig. 430-7. Feeder conductors are sized for the total motor load. (Sec. 430.24.)
Where other loads are also supplied, conductor sizing is determined as follows:
1. The current-carrying capacity of feeder conductors supplying a single motor plus other loads must include capacity at least equal to 125 percent of the full-load current of the motor.
2. The current-carrying capacity of feeder conductors supplying a motor load and a lighting and/or appliance load must be sufficient to handle the lighting and/or appliance load as determined from the procedure for calculating size of lighting feeders, plus the motor load as determined from the previous paragraphs.
The Code permits inspectors to authorize use of demand factors for motor feeders—based on reduced heating of conductors supplying motors operating intermittently or on duty-cycle or motors not operating together. Where necessary this should be checked to make sure that the authority enforcing the Code deems the conditions and operating characteristics suitable for reduced-capacity feeders, as noted in 430.26.
For computing the minimum allowable conductor size for a combination lighting and power feeder, the required ampacity for the lighting load is to be determined according to the rules for feeders carrying lighting (or lighting and appliance) loads only. Where the motor load consists of one motor only, the required ampacity for this load is the capacity for the motor branch circuit, or 125 percent of the full-load motor current, as specified in 430.22. Where the motor load consists of two or more motors, or a motor(s) and other loads, the required ampacity for the motor load is the capacity computed according to 430.24.
Figure 430-8 shows a typical installation for which calculation of required feeder ampacity is as follows:
Fig. 430-8. Other load must be properly combined with motor load. (Sec. 430.24.)
Step 1. Total Load
430.24 says that conductors supplying a lighting load and a motor load must have capacity for both loads, as follows:
Motor load = 65 A + 40 A + 14 A + 14 A + (0.25 × 65 A) = 149 A per phase
Lighting load = 120 A per phase × 1.25 = 150 A
Total load = 149 + 150 = 299 A per phase leg
Step 2. Conductors
Table 310.16 shows that a load of 299 A can be served by the following copper conductors:
500-kcmil TW
350-kcmil THW, RHH, XHHW, or THHN
Table 310.16 shows that this same load can be served by the following aluminum or copper-clad aluminum conductors:
700-kcmil TW
500-kcmil THW, RHH, XHHW, or THHN
Note that the 90°C rating for the THHN was not considered because of termination limitations. If there were adverse environmental conditions such as high temperature ambient conditions or mutual conductor heating, any required derating would have started at the 90°C rating.
430.26. Feeder Demand Factor. A demand factor of less than 100 percent may be applied in the case of some industrial plants where the nature of the work is such that there is never a time when all the motors are operating at one time. But the inspector must be satisfied with (and grant special permission for, including the provision of written notice) any application of a demand factor.
Sizing of motor feeders (and mains supplying combination power and lighting loads) may be done on the basis of maximum demand current, calculated as follows:
Running current = (1.25 × If) + (DF× It)
where If = full-load current of largest motor
DF = demand factor as permitted by 430.26
It = sum of full-load currents of all motors except largest
But modern design dictates use of the maximum-demand starting current in sizing conductors for improved voltage stability on the feeder. This current is calculated as follows:
Starting current = Is + (DF × It)
where Is = average starting current of largest motor. (Use the percent of motor full-load current given for fuses in Table 430.52.)
430.28. Feeder Taps. This Code rule is an adaptation of 240.21(B)(1) and (B)(2), covering use of 10- and 25-ft (3.0- and 7.5-m) feeder taps with no overcurrent protection at the point where the smaller conductors connect to the higher-ampacity feeder conductors. The adaptation establishes that the tap conductors must have an ampacity as required by 430.22, 430.24, or 430.25.
In applying condition (1), the conductor may have an ampacity less than one-tenth that of the feeder conductors but must be limited to not more than 10 ft (3.05 m) in length and be enclosed within a controller or raceway, and rated not less than 10 percent of the rating of the overcurrent device protecting the feeder.
If conductors equal in size to the conductors of a feeder are connected to the feeder, as in condition (3), no fuses or other overcurrent protection are needed at the point where the tap is made, since the tap conductors will be protected by the fuses or CB protecting the feeder.
The more important circuit arrangement permitted by the preceding rule is shown in Fig. 430-9. Instead of placing the fuses or other branch-circuit protective device at the point where the connections are made to the feeder, conductors having at least one-third the ampacity of the feeder are tapped solidly to the feeder and may be run a distance not exceeding 25 ft (7.5 m) to the branch-circuit protective device. From this point on to the motor-running protective device and thence to the motor, conductors are run having the standard ampacity, that is, 125 percent of the full-load motor current, as specified in 430.22. If the tap conductors shown did not have an ampacity at least equal to one-third of that of the feeder conductors, then the tap conductors must not be over 10 ft (3.0 m) long.
Fig. 430-9. Feeder tap may be sized as provided by 240.21(B)(2). (Sec. 430.28.)
Note that this rule actually modifies the requirements of 240.21 for taps to motor loads. 240.21(B)(1) literally calls for 10-ft (3.0-m) tap conductors to have ampacity at least equal to the rating or setting of the fuses or CB (whichever is used) at the load end of the tap. And such protection may be rated up to 4 times motor full-load current. But condition (1) of this Code section requires sizing of the 10-ft (3.0-m) tap conductors to be at least one-tenth the rating of the over-current device, protecting the feeder from which the tap conductors are supplied. And condition (2) does not require a 25-ft (7.5-m) tap to terminate in a protective device rated to protect the conductors at their ampacity (Fig. 430-10).
example A 15-hp 230-V 3-phase motor with autotransformer starter is to be supplied by a tap made to a 250-kcmil feeder. All conductors are to be Type THW.
The feeder has an ampacity of 255 A; one-third of 255 A equals 85 A. Therefore the tap cannot be smaller than No. 4, which has an ampacity of 85 A for 75°C ratings.
The full-load current of the motor is 40 A and, according to part (IV) of Art. 430, assuming that the motor is not marked with a Code letter, the branch-circuit fuses should be rated at not more than 300 percent of 40 A, or 120 A, which calls for 125-A fuses (430.52) or less. With the motor-running protection set at 50 A (125 percent × 40 A), the tap conductors are well protected from overload.
The conductors tapped solidly to the feeder must never be smaller than the size of branch-circuit conductors required by 430.22.
The exception in this rule notes that a branch-circuit or subfeeder tap up to 100 ft (30.0 m) long may be made from a feeder to supply motor loads. The specific conditions are given for making a tap that is over 25 ft (7.5 m) long and up to 100 ft (30.0 m) long—where no protection is provided at the point of tap from the feeder conductors. This is a motor-circuit adaptation of the 100-ft (30.0-m) tap permission, which is fully described under 240.21(B)(4).
Fig. 430-10. Tap conductors may terminate in protective device rated above their ampacity. (Sec. 430.28.)
430.29. Constant-Voltage DC Motors—Power Resistors. These rules cover sizing of conductors from a DC motor controller to separate resistors for power accelerating and dynamic braking. This section, with its table of conductor ampacity percentages, ensures proper application of DC constant-potential motor controls and power resistors.
430.31. General. Detailed requirements for the installation of fire pumps are included in the National Electrical Code in Art. 695. Although electrical concerns are now covered in the NEC, more in-depth coverage of other concerns is given in NFPA 20.
As intended by 430.52, the motor branch-circuit protective device provides short-circuit protection for the circuit conductors. In order to carry the starting current of the motor, this device must commonly have a rating or setting so high that it cannot protect the motor against overload.
For a squirrel-cage induction motor, overload protection must be of the inverse time type with a setting of not over 20 s at 600 percent of the motor full-load current. It is the intent that the fire-pump motor be permitted to run under any condition of loading, even to complete failure, and not be automatically disconnected by an overload protection device. However, should a ground fault or short circuit develop in its conductors, it is also the intent to clear those faults before they result in another task for the fire pump to address. Refer to the extensive coverage of Art. 695 in this book for complete information.
Except where time-delay fuses provide both running overload protection and short-circuit protection as described in 430.55, in practically all cases where motor-running overload protection is provided the motor controller consists of two parts: (1) a switch or contactor to control the circuit to the motor, and (2) the motor-running protective device. Most of the protective devices make use of a heater coil, usually consisting of a few turns of high-resistance metal, though the heater may be of other form.
430.32. Continuous-Duty Motors. The Code makes specific requirements on motor running overload protection intended to protect the elements of the branch circuit—the motor itself, the motor control apparatus, and the branch-circuit conductors—against excessive heating due to motor overloads. Overload protection may be provided by fuses, CBs, or specific overload devices like OL relays.
Overload is considered to be operating overload up to and including stalled-rotor current. When overload persists for a sufficient length of time, it will cause damage or dangerous overheating of the apparatus. Overload does not include fault current due to shorts or grounds.
Typical overload devices include:
1. Heaters in series with line conductors acting upon thermal bimetallic overload relays
2. Overload devices using resistance or induction heaters and operating on the solder-ratchet principle (Fig. 430-11)
3. Magnetic relays with adjustable instantaneous setting or adjustable time-delay setting
4. Microprocessor support in an adjustable speed drive or in other controllers capable of electronic settings
Of course, the provisions for overload protection are integrated in the enclosure of the controller.
Overload protective devices of the straight thermal type are available with varying tripping and time-delay characteristics. In such devices, the heater coils are made in many sizes and are interchangeable to permit use of the required heater sizes to provide running protection for different motor full-load current ratings. In some units, the heater coil can be adjusted to exact current values. Individual covers are used on the heating elements in some starters to isolate the relay from possible effects on its operation because of the temperature of surrounding air.
In general, it is required that every motor shall be provided with a running protective device that will open the circuit on any current exceeding prescribed percentages of the full-load motor current, the percentage depending on the type of motor. The running protective device is intended primarily to protect the windings of the motor; but by providing that the circuit conductors shall have an ampacity not less than 125 percent of the full-load motor current, it is obvious that these conductors are reasonably protected by the running protective device against any overcurrent caused by an overload on the motor.
Part (A) covers application for motors of more than 1 hp. If such a motor is used for continuous duty, running overload protection must be provided. This may be an external overcurrent device actuated by the motor running current and set to open at not more than 125 percent of the motor full-load current for motors marked with a service factor of not less than 1.15 and for motors with a temperature rise not over 40°C. See examples in Fig. 430-4. Sealed (hermetic-type) refrigeration compressor motors must be protected against overload and failure to start, as specified in 440.52. The overload device must be rated or set to trip at not more than 115 percent of the motor full-load current for all other motors, such as motors with a 1.0 service factor or a 55°C rise (Fig. 430-12).
Fig. 430-11. Overload relay devices are made in various operating types. (Sec. 430.32.)
Fig. 430-12. Specific rules apply to continuous-duty motors rated over 1 hp. (Sec. 430.32.)
Be careful when looking at overload element tables provided with controllers. As a general rule, those tables are intended to be directly read out. That is, the 125 percent has already been factored into the selection table. Look up the nameplate FLA, and read out the relay element. If you take the nameplate FLA, multiply by 125 percent, and then go to the table you will be selecting an overload based on 156 percent of the motor nameplate FLA (125 percent of 125 percent). The result will protect neither the motor nor the conductors from sustained overload should the motor malfunction.
The term rating, or setting, as used here means the current at which the device will open the circuit if this current continues for a considerable length of time.
Note: Refer to 460.9, which discusses the need to correct the sizing of running overload protection when power-factor capacitors are installed on the load side of the controller.
A motor having a temperature rise of 40°C when operated continuously at full load can carry a 25 percent overload for some time without injury to the motor. Other types of motors, such as enclosed types, do not have so high an overload capacity, and the running protective device should therefore open the circuit on a prolonged overload which causes the motor to draw 115 percent of its rated full-load current.
Basic Code requirements are concerned with the rating or setting of overcurrent devices separate from motors. However, the Code permits the use of integral protection. Paragraph (2) of part (A) covers use of running overload protective devices within the motor assembly rather than in the motor starter. A protective device integral with the motor as used for the protection of motors is shown in Fig. 430-13. This device is placed inside the motor frame and is connected in series with the motor winding. It contains a bimetallic disk carrying two contacts, through which the circuit is normally closed. If the motor is overloaded and its temperature is raised to a certain limiting value, the disk snaps to the open position and opens the circuit. The device also includes a heating coil in series with the motor windings which causes the disk to become heated more rapidly in case of a sudden heavy overload.
Fig. 430-13. Running overload protection may be built into the motor. (Sec. 430.32.)
Where the circuit-interrupting device is separate from the motor and is actuated by a device integral with the motor, the two devices must be so designed and connected that any accidental opening of the control circuit will stop the motor; otherwise, the motor would be left operating without any overcurrent protection.
There is special need for running protection on an automatically started motor because, if the motor is stalled when the starter operates, the motor will probably burn out if it has no running protection.
Part (B) of this section applies to smaller motors (1 hp or less) that are automatically started. Automatically started motors of 1 hp or less must be protected against running overload in the same way as motors rated over 1 hp—as noted in part (B). That is, a separate or integral overload device must be used.
There are alternatives to the specific overload protection rules of parts (A) and (B). Under certain conditions, no specific running overload protection need be used: The motor is considered to be properly protected if it is part of an approved assembly which does not normally subject the motor to overloads and which has controls to protect against a stalled rotor. Or if the impedance of the motor windings is sufficient to prevent overheating due to failure to start, the branch-circuit protection is considered adequate.
In part (C), the Code covers the procedure for “Selection of Overload Relay.” This rule sets the absolute maximum permitted rating of an overload relay where values are higher than the 125 or 115 percent trip ratings of 430.32(A)(1) and (B). Motors with a marked service factor not less than 1.15 and 40°C-rise motors may, if necessary to enable the motor to start or carry its load, be protected by overload relays with trip settings up to 140 percent of motor full-load current. Motors with a 1.0 service factor and motors with a temperature rise over 40°C (such as 55°C-rise motors) must have their relay trip setting at not over 130 percent of motor full-load current.
BUT WATCH OUT! The maximum settings of 140 or 130 percent apply only to OL relays, such as used in motor starters. Use of this option is discouraged, and a fine-print note advises that instead of raising the trip setting, a better option might be to change the class of the overload relay element. The usual element, a Class 20, will carry 6 times its rated current for 20 s before opening. Class 10 elements will carry the same current for 10 s, and Class 30 elements for 30 s. Changing from a Class 20 to a Class 30 increases the hold-in time by 50 percent without varying the basic trip current, and this should be enough to solve legitimate problems of failure to start.
Fuses or CBs may be used for running overload protection but may not be rated or set up to the 140 or 130 percent values. Fuses and breakers must have a maximum rating as shown in 430.32(A) and (B). If the value determined as indicated there does not correspond to a standard rating of fuse or CB, the next smaller size must be used. A rating of 125 percent of full-load current is the absolute maximum for fuses or breakers.
Part (D) covers motors of 1 hp or less that are manually started are considered. They are considered to be protected against overload by the branch-circuit protection if the motor is within sight from the starter and the motor is not permanently installed (Fig. 430-14). Running overload devices are not required in such cases. A distance of over 50 ft (15.0 m) is considered out of sight. If the motor is out of sight of its controller, or is permanently installed, then the usual rules in 430.32(B) will apply.
430.33. Intermittent and Similar Duty. A motor used for a condition of service which is inherently short-time, intermittent, periodic, or varying duty does not require protection by overload relays, fuses, or other devices required by Sec. 430.32, but, instead, is considered as protected against overcurrent by the branch-circuit over-current device (CB or fuses rated in accordance with 430.52). Motors are considered to be for continuous duty unless the motor is completely incapable of operating continuously with load under any condition of use. One classic example is an elevator motor in a building of finite height.
Fig. 430-14. The rules for automatic-start motors are different. (Sec. 430.32.)
430.35. Shunting During Starting Period. As covered in part (A) for motors that are not automatically started, where fuses are used as the motor-running protection, they may be cut out of the circuit during the starting period. This leaves the motor protected only by the branch-circuit fuses, but the rating of these fuses will always be well within the 400 percent limit specified in the rule. If the branch-circuit fuses are omitted, as allowed by the rule in 430.53(D), it is not permitted to use a starter that cuts out the motor fuses during the starting period unless the protection of the feeder is within the limits set by this rule. As shown in Fig. 430-15, a double-throw switch is arranged for across-the-line starting. The switch is thrown to the right to start the motor, thus cutting the running fuses out of the circuit. The switch must be so made that it cannot be left in the starting position.
Fig. 430-15. Motor OL fuses may be shunted out for starting. (Sec. 430.35.)
In the exception to part (B), conditions are given for shunting out overload protection of a motor that is automatically started. In previous Code editions, any motor that was automatically started was not permitted to have its overload protection shunted or cut out during the starting period. This exception now accommodates those motor-and-load applications that have a long accelerating time and would otherwise require an overload device with such a long trip time that the motor would not be protected if it stalled while running.
430.36. Fuses—In Which Conductor. This rule is listed in 240.22 as subpart (2) to the rule that prohibits use of an overcurrent device in an intentionally grounded conductor. When fuses are used for protection of service, feeder, or branch-circuit conductors, a fuse must never be used in a grounded conductor, such as the grounded leg of a 3-phase, 3-wire corner-grounded delta system. But, if fuses are used for OL protection for a 3-phase motor connected on such a system, a fuse must be used in all three phase legs—EVEN THE GROUNDED LEG. Figure 430-16 shows two conditions of such fuse application for OL protection for a motor.
430.37. Devices Other than Fuses—In Which Conductor. Complete data on the number and location of overcurrent devices are given in Code Table 430.37.
Table 430.37 requires three running overload devices (trip coils, relays, thermal cutouts, etc.) for all 3-phase motors unless protected by other approved means, such as specifically designed embedded detectors with or without supplementary external protective devices.
Fig. 430-16. A fuse for OL protection must be used in each phase leg of circuit. (Sec. 430.36.)
Figure 430-17 points out this requirement.
If fuses are used as the running protective device, 430.36 requires a fuse in each ungrounded conductor. If the protective device consists of an automatically operated contactor or CB, the device must open a sufficient number of conductors to stop the current flow to the motor and must be equipped with the number of overload units specified in Table 430.37.
430.42. Motors on General-Purpose Branch Circuits. Refer to Fig. 430-18, Type 3.
Branch circuits supplying lamps are usually 120-V single-phase circuits, and on such circuits the effect of subparagraphs (A) and (B) is that any motor larger than 6 A must be provided with a starter that is approved for group operation.
It is provided in 210.24 that receptacles on a 20-A branch circuit may have a rating of 20 A, and in such case subparagraph (C) requires that any motor or motor-driven appliance connected through a plug and receptacle must have running overcurrent protection. If the motor rating exceeds 1 hp or 6 A, the protective device must be permanently attached to the motor and subparagraph (B) must be complied with.
Fig. 430-17. Three OL units are required for 3-phase motors. (Sec. 430.37.)
Fig. 430-18. Motor branch-circuit protection is used in various types of layouts. (Sec. 430.51.)
The requirements of 430.32 for the running overcurrent protection of motors must be complied with in all cases, regardless of the type of branch circuit by which the motor is supplied and regardless of the number of motors connected to the circuit.
430.43. Automatic Restarting. As noted in the comments to 430.32, an integral motor-running protective device may be of the type which will automatically restart, or it may be so constructed that after tripping out it must be closed by means of a reset button. However, an automatic restart type of protective device must not be used if an automatic restart could injure someone.
430.44. Orderly Shutdown. Although the NE Code has all those requirements on use of running overload protection of motors, this section recognizes that there are cases when automatic opening of a motor circuit due to overload may be objectionable from a safety standpoint. In recognition of the needs of many industrial applications the rule here permits alternatives to automatic opening of a circuit in the event of overload. This permission for elimination of overload protection is similar to the permission given in 240.12 to eliminate overload protection when automatic opening of the circuit on an overload would constitute a more serious hazard than the overload itself. However, it is necessary that the circuit be provided with a motor overload sensing device conforming to the Code requirement on overload protection to indicate by means of a supervised alarm the presence of the overload (Fig. 430-19). Overload indication instead of automatic opening will alert personnel to the objectionable condition and will permit corrective action, either immediately or at some more convenient time, for an orderly shutdown to resolve the difficulty. But, as is required in 240.4(A), short-circuit protection on the motor branch circuit must be provided to take care of those high-level ground faults and short circuits that would be more serious in their hazardous implications than simple overload.
Fig. 430-19. This type of hookup may be used to warn of, but not open, an overload. (Sec. 430.44.)
Note: 445.12 also has an exception that permits this same use of an alarm instead of overcurrent protection where it is better to have a generator fail than stop operating.
430.51. General. This section indicates the coverage of part IV, which requires “Motor Branch-Circuit Short-Circuit and Ground-Fault Protection.” Although the phrase “ground-fault protection” is used in several of the sections of part IV, it should be noted that it refers to the protection against ground fault that is provided by the set of fuses or CB that is used to provide short-circuit protection. The single CB or set of fuses is referred to as a “short-circuit and ground-fault protective device.” The rule is not intended to require the type of ground-fault protective hookup required by 230.95 on service disconnects (such as a zero-sequence transformer and relay hookup).
Motor branch circuits are commonly laid out in a number of ways. With respect to branch-circuit protection location and type, the layouts shown in Fig. 430-18 are as follows:
Type 1
An individual branch circuit leads to each motor from a distribution center. This type of layout can be used under any conditions and is the one most commonly used.
Type 2
A feeder or subfeeder with branch circuits tapped on at convenient points. This is the same as Type 1 except that the branch-circuit overcurrent protective devices are mounted individually at the points where taps are made to the sub-feeder, instead of being assembled at one location in the form of a branch-circuit distribution center. Under certain conditions, the branch-circuit protective devices may be located at any point not more than 25 ft (7.5 m) distant from the point where the branch circuit is tapped to the feeder.
Type 3
Small motors, lamps, and appliances may be supplied by a 15- or 20-A circuit as described in Art. 210. Motors connected to these circuits must be provided with running overcurrent protective devices in most cases. See 430.42.
Figure 430-20 shows the typical elements of a motor branch circuit in their relation to branch-circuit protection, so that the protection is effective for the circuit conductors, the control and disconnect means, and the motor. Motor controllers provide protection for the motors they control against all ordinary overloads but are not intended to open short circuits. Fuses, CBs, or motor short-circuit protectors used as the branch-circuit protective device will open short circuits and therefore provide short-circuit protection for both the motor and the running protective device. Where a motor is supplied by an individual branch circuit having branch-circuit protection, the circuit protective devices may be either fuses or a CB, and the rating or setting of these devices must not exceed the values specified in 430.52. In Fig. 430-20, the fuses or CB at the panelboard must carry the starting current of the motor, and in order to carry this current the fuse rating or CB setting may be rated up to 300 or 400 percent of the running current of the motor, depending on the size and type of motor. It is evident that to install motor circuit conductors having an ampacity up to that percent of the motor full-load current would be unnecessary.
Fig. 430-20. Branch-circuit protection is on the line side of other components. (Sec. 430.51.)
There are three possible causes of excess current in the conductors between the panelboard and the motor controller—a short circuit between two of these conductors, a ground fault on one conductor that forms a short circuit, and an overload on the motor. A short circuit would draw so heavy a current that the fuses or breaker at the panelboard would immediately open the circuit, even though the rating or setting is in excess of the conductor ampacity. Any excess current due to an overload on the motor must pass through the protective device at the motor controller, causing this device to open the circuit. Therefore, with circuit conductors having an ampacity equal to 125 percent of the motor-running current and with the motor-protective device set to operate at near the same current, the conductors are reasonably protected.
430.52. Rating or Setting for Individual Motor Circuit. The Code requires that branch-circuit protection for motor circuits must protect the circuit conductors, the control apparatus, and the motor itself against overcurrent due to short circuits or ground (430.51 through 430.58).
The first, and obviously necessary, rule is that the branch-circuit protective device for an individual branch circuit to a motor must be capable of carrying the starting current of the motor without opening the circuit. Then the Code proceeds to place maximum values on the ratings or settings of such overcurrent devices. It says that such devices must not be rated in excess of the values given in Table 430.52.
In case the values for branch-circuit protective devices determined by Table 430.52 do not correspond to the standard sizes or ratings of fuses, nonadjustable CBs, or thermal devices, or possible settings of adjustable CBs adequate to carry the load, the next higher size, rating, or setting may be used.
Under exceptionally severe starting conditions where the nature of the load is such that an unusually long time is required for the motor to accelerate to full speed, the fuse or CB rating or setting recommended in Table 430.52 may not be high enough to allow the motor to start. It is desirable to keep the branch-circuit protection at as low a rating as possible, but in unusual cases, it is permissible to use a higher rating or setting. Where absolutely necessary in order to permit motor starting, the device may be rated at other maximum values, as follows:
1. The rating of a fuse that is not a dual-element time-delay fuse (or a time-delay Class CC fuse) and is rated not over 600 A may be increased above the Code table value but must never exceed 400 percent of the full-load current.
2. The rating of a time-delay (dual-element) fuse may be increased but must never exceed 225 percent of full-load current.
3. The setting of an instantaneous trip CB that is part of a listed combination starter (which contains a magnetic short-circuit trip element, without time delay, and independent overload device) may be increased but never over 1300 percent of the motor full-load current, unless supplying a Design B energy-efficient motor, in which the setting may be increased to not more than 1700 percent of motor full-load current. In this category, it is not necessary to actually experiment with a lower-rated breaker; the need can be established by engineering evaluation. And it is never necessary to decrease the protection below 15 A. Motor short-circuit protectors (also where part of a listed combination controller) and listed self-protected combination controllers are permitted to used the same parameters for protection.
4. The rating of an inverse time CB (a typical thermal-magnetic CB with a time-delay and instantaneous trip characteristic) may be increased but must not exceed 400 percent for full-load currents of 100 A or less and must not exceed 300 percent for currents over 100 A.
5. A fuse rated 601 to 6000 A may be increased but must not exceed 300 percent of full-load current.
6. Torque motors must be protected at the motor nameplate current rating, and if a standard overcurrent device is not made in that rating, the next higher standard rating of protective device may be used.
7. Multispeed motors may have a single short-circuit and ground-fault protective device if it meets the multipliers established in Table 430.52. As an alternative, the protection can be set based on the needs of the highest current winding, provided there is running overload protection for both speed windings, and both the controllers and the circuit conductors for every speed winding are sized in accordance with the comparable components of the highest current winding.
8. There are two global provisions to be considered before leaving this topic. The first is the issue of “Power Electronic Devices” that are special fuses that can be substituted for any of the short-circuit and ground-fault protective devices considered so far. Their use is limited to solid-state motor controller systems, and there must be a marking for replacement fuses provided adjacent to the fuseholders.
The second is 430.52(C)(2), which is a very important and widely overlooked item. This provision says that if the manufacturer of the overload relay components chooses to specify a maximum setting of short-circuit and ground-fault protective devices that can be used ahead of his equipment, and posts it as part of his relay table (or otherwise on the equipment), that number absolutely trumps any calculations made in accordance with multipliers in Table 430.52. That requirement is also specified in UL regulations which regulate the exposure of motor controllers to short-circuit currents to protect internal components, such as overload relays and contacts, from damage or destruction. Those rules state:
Motor controllers incorporating thermal cutouts, thermal overload relays, or other devices for motor-running overcurrent protection are considered to be suitably protected against overcurrent due to short circuits or grounds by motor branch circuit, short circuit, and ground-fault protective devices selected in accordance with the National Electrical Code and any additional information marked on the product. Motor controllers may specify that protection is to be provided by fuses or by an inverse time circuit breaker. If there is no marking of protective device type, controllers are considered suitably protected by either type of device. Motor controllers may specify a maximum rating of protective device. If not marked with a rating, the controllers are considered suitably protected by a protective device of the maximum rating permitted by the National Electrical Code.
The rules of this section establish maximum values for branch-circuit protection, setting the limit of safe applications. However, use of smaller sizes of branch-circuit protective devices is obviously permitted by the Code and does offer opportunities for substantial economies in selection of CBs, fuses and the switches used with them, panelboards, and so forth. In any application, it is only necessary that the branch-circuit device which is smaller than the maximum permitted rating must have sufficient time delay in its operation to permit the motor starting current to flow without opening the circuit.
But a CB for branch-circuit protection must have a continuous current rating of not less than 115 percent of the motor full-load current, as required by 430.58.
Unless otherwise marked, motor controllers incorporating thermal cutouts or overload relays are considered suitable for use on circuits having available fault currents not greater than (refer to Fig. 430-21):
Typical application of the basic rule of 430.52 on short-circuit protection for motor circuits is shown in Fig. 430-4. Overcurrent (branch-circuit) protection (from Table 430.52 and 430.52) using nontime-delay fuses is calculated as follows:
1. The 50-hp squirrel-cage motor must be protected at not more than 200 A (65 A × 300 percent, next higher standard size device allowed).
2. The 30-hp wound-rotor motor must be protected at not more than 60 A (40 A × 150 percent).
3. Each 10-hp motor must be protected at not more than 45 A (14 A × 300 percent, next higher standard size device allowed).
Fig. 430-21. UL specifies maximum short-circuit withstand ratings for controllers. (Sec. 430.52.)
As shown in Code Table 430.52, if thermal-magnetic CBs were used, instead of the fuses, for branch-circuit protection, the maximum ratings that are permitted by the basic rule are:
1. For the 50-hp motor—65 A × 250 percent, or 162.5 A, with the next higher standard CB rating of 175 A permitted.
2. For the 30-hp wound-rotor motor—40 A × 150 percent, or 60 A, calling for a 60-A CB.
3. For each 10-hp motor—14 A × 250 percent, or 35 A, calling for a 35-A CB.
Instantaneous Trip CBs
The NE Code recognizes the use of an instantaneous trip CB (without time delay) for short-circuit protection of motor circuits. Such breakers—also called magnetic-only breakers—may be used only if they are adjustable and if combined with motor starters in combination assemblies. An instantaneous-trip CB or a motor short-circuit protector (MSCP) may be used only as part of a listed (such as by UL) combination motor controller. A combination motor starter using an instantaneous trip breaker must have running overload protection in each conductor (Fig. 430-22). Such a combination starter offers use of a smaller CB than would be possible if a standard thermal-magnetic CB were used. And the smaller CB offers faster operation for greater protection against grounds and short circuits—in addition to offering greater economy. Note that because these devices are only permitted as components in listed combination controllers, they are recognized components of listed equipment, but they are not and can never be listed.
A combination motor starter, as shown in Fig. 430-22, is based on the characteristics of the instantaneous-trip CB, which is covered by the third percent column from the left in Code Table 430.52. Molded-case CBs with only magnetic instantaneous-trip elements in them are available in almost all sizes. Use of such a device requires careful accounting for the absence of overload protection in the CB, up to the short-circuit trip setting. Such a CB is designed for use as shown in Fig. 430-22. The circuit conductors are sized for at least 125 percent of motor current. The thermal overload relays in the starter protect the entire circuit and all equipment against operating overloads up to and including stalled rotor current. They are commonly set at 125 percent of motor current. In such a circuit, a CB with an adjustable magnetic trip element can be set to take over the interrupting task at currents above stalled rotor and up to the short-circuit duty of the supply system at that point of installation. The magnetic trip in a typical unit might be adjustable from 3 to 17 times the breaker current rating; that is, a 100-A trip can be adjusted to trip anywhere between 300 and 1700 A. Thus the CB serves as motor circuit disconnect and short-circuit protection.
Fig. 430-22. Section 430.52 accepts use of a magnetic-only circuit breaker if it is part of a “listed” assembly of a combination starter. (Sec. 430.52.)
Selection of such a listed assembly with an instantaneous-only CB is based on choosing a nominal CB size with a current rating at least equal to 115 percent of the motor full-load current to carry the motor current and to qualify under 430.58 and 430.110(A) as a disconnect means. Then the adjustable magnetic trip is set to provide the short-circuit protection—the value of current at which instantaneous circuit opening takes place, which should be just above the starting current of the motor involved—using a multiplier of something like 1.5 on locked-rotor current to account for asymmetry in starting current. Asymmetry can occur when the circuit to the motor is closed at that point on the alternating voltage wave where the inrush starting current is going through the negative maximum value of its alternating wave. That is the same concept as asymmetry in the initiation of a short-circuit current. Where supplying Design B energy-efficient motors, a greater inrush can be anticipated on start-up. As a result, higher initial and maximum settings are recognized.
Listed equipment using an instantaneous CB type is available with very simple instructions by the manufacturer to make proper selection and adjustment of the instantaneous-trip CB combination starter a quick, easy matter. The following describes the concept behind the application of listed combination starters with instantaneous-only CBs.
Given: A 30-hp, 230-V, 3-phase, squirrel-cage motor marked with the code letter M, indicating that the motor has a locked-rotor current of 10 to 11.19 kVA per horsepower, from Code Table 430.7(B). A full-voltage controller is combined with the CB, with running overload protection in the controller to protect the motor within its heating damage curve on overload in a listed unit.
Required: Select the maximum setting and minimum rating for the CB which will provide short-circuit protection and will qualify as the motor circuit disconnect means.
Solution: The motor has a full-load current of 80 A (Code Table 430.250). A CB suitable for use as disconnect must have a current rating at least 115 percent of 80 A. As covered in 430.52(C)(3), for instantaneous-trip CBs, the initial setting from Table 430.52 would be limited to 800 percent of the 80-A full-load current. The maximum setting—for other than the high-efficiency Design B energy-efficient motors—is 1300 percent. For Design B energy-efficient motors, the initial setting may be 1100 percent of motor full-load current with a maximum setting of 1700 percent of the motor full-load current.
It should be noted that settings above 800 or 1100 percent of the motor’s full-load current are permitted only if nuisance tripping occurs on starting or if evaluation of the motor’s starting characteristics and the time-current trip curve of the breaker indicates that a greater setting is needed. Although not completely clear, the trip value established through the engineering evaluation should be considered as the maximum setting.
Because the use of a magnetic-only CB does not protect against low-level grounds and shorts in the circuit conductors on the line side of the starter running overload relays, the NE Code rule permits such application only where the CB and starter are part of a listed combination starter in a single enclosure.
MSCPs
A motor short-circuit protector (MSCP), as referred to in part (7) of 430.52, is a fuse-like device designed for use only in its own type of fusible-switch combination motor starter. The combination offers short-circuit protection, running overload protection, disconnect means, and motor control—all with assured coordination between the short-circuit interrupter (the motor short-circuit protector) and the running OL devices. It involves the simplest method of selection of the correct MSCP for a given motor circuit. This packaged assembly is a third type of combination motor starter—added to the conventional fusible-switch and CB types.
The NE Code recognizes motor short-circuit protectors in 430.40 and 430.52, provided the combination is a listed assembly. This means a combination starter equipped with motor short-circuit protectors and listed by Underwriters Laboratories, Inc., or another nationally recognized testing lab, as a package called an MSCP starter.
430.53. Several Motors or Loads on One Branch Circuit. A single branch circuit may be used to supply two or more motors as follows:
Part (A): Two or more motors, each rated not more than 1 hp and each drawing not over 6 A full-load current, may be used on a branch circuit protected at not more than 20 A at 125 V or less, or 15 A at 600 V or less. And the rating of the branch-circuit protective device marked on any of the controllers must not be exceeded. That is also a UL requirement.
Individual running overload protection is necessary in such circuits, unless: The motor is not permanently installed, is manually started, and is within sight from the controller location; or the motor has sufficient winding impedance to prevent overheating due to stalled rotor current; or the motor is part of an approved assembly that does not subject the motor to overloads and that incorporates protection for the motor against stalled rotor; or the motor cannot operate continuously under load.
Part (B): Two or more motors of any rating, each having individual running overload protection, may be connected to a branch circuit which is protected by a short-circuit protective device selected in accordance with the maximum rating or setting of a device which could protect an individual circuit to the motor of the smallest rating. This may be done only where it can be determined that the branch-circuit device so selected will not open under the most severe normal conditions of service which might be encountered.
This permission of part (B) offers wide application of more than one motor on a single circuit, particularly in the use of small integral-horsepower motors installed on 460-V, 3-phase systems. This application primarily concerns use of small integral-horsepower 3-phase motors as used in 208-, 220-, 460-, and 575-V industrial and commercial systems. Only such 3-phase motors have full-load operating currents low enough to permit more than one motor on circuits fed from 15-A protective devices.
There are a number of ways of connecting several motors on a single branch circuit, as follows:
In case I, Fig. 430-23, using a three-pole CB for branch-circuit protective device, application is made in accordance with part (B) as follows:
1. The full-load current for each motor is taken from NE Code Table 430.250 [as required by 430.6(A)].
2. Choosing to use a CB instead of fuses for branch-circuit protection, the rating of the branch-circuit protective device, 15 A, does not exceed the maximum value of short-circuit protection required by 430.52 and Table 430.52 for the smallest motor of the group—which is the 1½-hp motor. Although 15 A is greater than the maximum value of 250 percent times motor full-load current (2.5 × 3.0 A = 7.5 A) set by Table 430.52 (under the column “Inverse Time Breaker” opposite “polyphase squirrel-cage” motors), the 15-A breaker is the “next higher size, rating, or setting” for a standard CB—as permitted in 430.52. A 15-A CB is the smallest standard rating recognized by 240.6.
3. The total load of motor currents is:
4.8 A + 3.4 A + 3.0 A = 11.2 A
Fig. 430-23. Three integral-horsepower motors may be supplied by this circuit makeup. (Sec. 430.53.)
This is well within the 15-A CB rating, which has sufficient time delay in its operation to permit starting of any one of these motors with the other two already operating. Torque characteristics of the loads on starting are not high. It was therefore determined that the CB will not open under the most severe normal service.
4. Each motor has individual running overload protection in its starter.
5. The branch-circuit conductors are sized in accordance with 430.24:
4.8 A + 3.4 A + 2.6 A + (25 percent of 4.8 A) = 12.4 A
Conductors must have an ampacity at least equal to 12 A. No. 14 THW, TW, RHW, RHH, THHN, or XHHW conductors will fully satisfy this application.
In case II, Fig. 430-24, a similar hookup is used to supply three motors—also with a CB for branch-circuit protection.
1. Section 430.53(B) requires branch-circuit protection to be not higher than the maximum amps set by 430.52 for the lowest rated of the motors.
2. From 430.52 and Table 430.52, that maximum protection rating for a CB is 250 percent × 1.1 A (the lowest rated motor), or 2.5 A. But, 2.8 A is not a “standard rating” of CB from 240.6; and the first exception to 430.52(C)(1) permits use of the “next higher size, rating, or setting” of standard protective device.
3. Because 15 A is the lowest standard rating of CB, it is the “next higher” device rating above 2.5 A and satisfies Code rules on the rating of the branch-circuit protection.
The applications shown in cases I and II permit use of several motors up to circuit capacity, based on 430.24 and 430.53(B) and on starting torque characteristics, operating duty cycles of the motors and their loads, and the time delay of the CB. Such applications greatly reduce the number of CB poles, the number of panels, and the amount of wire used in the total system. One limitation, however, is placed on this practice in 430.52(C)(2), as noted previously. Where more than one fractional- or small-integral-horsepower motor is used on a single branch circuit of 15-A rating in accordance with NE Code 430.53(A) or (B), care must be taken to observe all markings on controllers that indicate a maximum rating of short-circuit protection ahead of the controller (Fig. 430-25).
Fig. 430-24. Fractional-horsepower and integral-horsepower motors may be supplied by the same circuit. (Sec. 430.53.)
In case III, Fig. 430-26, the same three motors shown in case II would be subject to different hookup to comply with the rules of 430.53(B) when fuses, instead of a CB, are used for branch-circuit protection, as follows:
1. To comply with 430.53(B), fuses used as branch-circuit protection must have a rating not in excess of the value permitted by 430.52 and Table 430.52 for the smallest motor of the group—one of the ½-hp motors.
2. Table 430.52 shows that the maximum permitted rating of nontime-delay type fuses is 300 percent of full-load current for 3-phase squirrel-cage motors. Applying that to one of the ½-hp motors gives a maximum fuse rating of
300 percent × 1.1 A = 3.3 A
3. The fuse protection will need to be set at 6 A—BECAUSE 6 A IS A “STANDARD” RATING OF FUSE (but not a standard rating of CB). 240.6 considers fuses rated at 1, 3, 6, and 10 A to be “standard” ratings.
4. The maximum branch-circuit fuse permitted by 430.53(B) for a ½-hp motor is 6 A (next higher standard size).
Fig. 430-25. Branch-circuit protection must not exceed marked maximum value. (Sec. 430.53.)
Fig. 430-26. Fuse protection may require different circuiting for several motors. (Sec. 430.53.)
5. The two ½-hp motors may be fed from a single branch circuit with three 6-A fuses in a three-pole switch.
6. Following the same Code rules, the 2-hp motor would require fuse protection rated not over 10 A (300 percent × 3.4 A = 10.2 A).
Note: Because the standard fuse ratings below 15 A place fuses in a different relationship to the applicable Code rules, it may require interpretation of the Code rules to resolve the question of acceptable application in case II versus case III. Some jurisdictions may attempt to exclude CBs as circuit protection in these cases where use of fuses, in accordance with the precise wording of the Code, provides lower-rated protection than CBs—when applying the rule of the third paragraph of 430.52. However, there is no support in the current Code text for an enforcement position that would compel a change of overcurrent design from CBs to fuses in this case, with one exception that tends to make the general point by virtue of its clearly limited effect. Review the discussion and UL data in 430.52(C)(2) in this book. If the manufacturer elects to specify the type and size of protection for his overloads, then and only then would a design change become enforceable.
Figure 430-27 shows one way of combining cases II and III to satisfy 430.53(B), 430.52, and 240.6; but the 15-A CB would then technically be feeder protection, because the fuses would be serving as the ”branch-circuit protective devices” as required by 430.53(B). Those fuses might be acceptable in each starter, without a disconnect switch, in accordance with 240.40—which allows use of cartridge fuses at any voltage without an individual disconnect for each set of fuses, provided only qualified persons have access to the fuses. But 430.112 would have to be satisfied to use the single CB as a disconnect for the group of motors. And part (B) of that exception recognizes one common disconnect in accordance with 430.53(A) but not 430.53(B). Certainly, the use of a fusible-switch-type combination starter for each motor would fully satisfy all rules.
Fig. 430-27. Multimotor circuit may be acceptable with fused starters. (Sec. 430.53.)
Figure 430-28 shows another hookup that might be required to supply the three motors of Fig. 430-23.
Figure 430-29 shows another hookup of several motors on one branch circuit—an actual job installation which was based on application of 430.53(B). The installation was studied as follows:
Problem: A factory has 100 1½-hp, 3-phase motors, with individual motor starters incorporating overcurrent protection, rated for 460 V. Provide circuits.
Fig. 430-28. This hookup might be required to satisfy literal Code wording. (Sec. 430.53.)
Fig. 430-29. Multimotor circuits offer economical supply to small integral-horsepower motors. (Sec. 430.53.)
Solution: Prior to 1965, the NE Code would not permit several integral-horsepower motors on one branch circuit fed from a three-pole CB in a panel. Each of the 100 motors would have had to have its own individual 3-phase circuit fed from a 15-A, 3-pole CB in a panel. As a result, a total of 300 CB poles would have been required, calling for seven panels of 42 circuits each plus a smaller panel (or special panels of greater numbers than 42 poles per panel).
Under the present Code, depending on the starting torque characteristics and operating duty of the motors and their loads, with each motor rated for 3.0 A, three or four motors could be connected on each 3-phase, 15-A circuit—greatly reducing the number of panelboards and overcurrent devices and the amount of wire involved in the total system. Time delay of CB influences number of motors on each circuit.
BUT, an extremely important point that must be strictly observed is the requirement that the rating of branch-circuit protection must not exceed any maximum value that might be marked on the starters used with the motors.
Part (C): In selecting the wording for part (C), it was the intent of the Code-making panel to clarify the intent that several motors should not be connected to one branch circuit unless careful engineering is exercised by qualified persons to determine that all components of the branch circuit are selected and specified to meet the present requirements and to function together. The intent is to allow:
a. Completely factory-assembled equipment, or
b. A factory-assembled unit with a separate branch-circuit short-circuit and ground-fault protective device of a type and rating specified, or
c. Separately mounted components which are listed for use together and are specified for such use together by manufacturer’s instructions and/or nameplate markings. Note that the circuit breaker need only be a listed inverse-time breaker, and need not be specifically evaluated for group installations. This was the procedure that generated the “HACR” label; it was established that all circuit breakers of the relevant sizes could be marked in that way because the product standard required the same testing anyway, whether or not the HACR label was applied, and therefore the group installations qualification was withdrawn.
It is not the intent to change requirements for supplemental overcurrent protection such as in 422.11(F) or 424.22(C).
Two or more motors of any rating may be connected to one branch circuit if each motor has running overload protection, if the overload devices and controllers are approved for group installation, and if the branch-circuit fuse or time-delay CB rating is in accordance with 430.52 for the largest motor plus the sum of the full-load current ratings of the other motors (Fig. 430-30). The branch-circuit fuses or CB must not be larger than the rating or setting of short-circuit protection permitted by 430.52 for the smallest motor of the group, unless the thermal device is approved for group installation with a given maximum size of fuse or time-delay CB for short-circuit protective device. (See 430.40.) Underwriters Laboratories notes that motor controllers for group installation are marked with a maximum rating of fuse required to suitably protect the controller. 430.8, however, calls for a group installation controller to be marked for the rating of fuse or CB ahead of it.
Fig. 430-30. Motors of any horsepower rating require circuit equipment for group installation. (Sec. 430.53.) This is no longer a requirement for the circuit breaker, which now need only be listed and of the inverse-time type.
Part (D): For installations of groups of motors as covered in part (C), tap conductors run from the branch-circuit conductors to supply individual motors must be sized properly. Such tap conductors would, of course, be acceptable where they are the same size as the branch-circuit conductors themselves. However, tap conductors to a single motor may be smaller than the main branch-circuit conductors provided that: They have an ampacity at least one-third that of the branch-circuit conductors, their ampacity is not less than 125 percent of the motor full-load current, they are not over 25 ft (7.5 m) long, and they are in a raceway or are otherwise protected from physical damage (Fig. 430-31).
Fig. 430-31. Overcurrent protection not required for taps to single motors of a group. (Sec. 430.53.)
An additional option is to use even smaller tap conductors based on not less than 10 percent of the branch-circuit short-circuit and ground-fault protective device [but still in accordance with 430.22(A)] that run in a raceway or other suitably sheltered location and not over 3 m (10 ft) in length, ending at a listed manual motor controller that is marked “Suitable for Tap Conductor Protection in Group Installations.” If the tap conductors are fully sized to be equal to the branch-circuit conductors, then the length and enhanced protection requirements don’t apply. This takes the principles of 430.28 and translates them here, with the terminating branch circuit device in 430.28 replaced by a special issue manual motor controller that has an instantaneous-trip component in it, according to the original substantiation.
The principle applied here is that, since the conductors are short and protected from physical damage, it is unlikely that trouble will occur in the run between the mains and the motor protection which will cause the conductors to be overloaded, except some accident resulting in an actual short circuit. A short circuit will blow the fuses or trip the CB protecting the mains. An overload on the conductors caused by overloading the motor or trouble in the motor itself will cause the motor protective device to operate and so protect the conductors.
430.55. Combined Overcurrent Protection. A CB or set of fuses may provide both short-circuit protection and running overload protection for a motor circuit. For instance, a CB or dual-element time-delay fuse sized at not over 125 percent of motor full-load current (430.32) for a 40°C-rise continuous-duty motor would be acceptable protection for the branch circuit and the motor against shorts, ground faults, and operating overloads on the motor. See the bottom of Fig. 430-16 for a typical fuse application.
Figure 430-32 shows a CB used to fulfill four Code requirements simultaneously. For the continuous-duty, 40°C-rise motor shown, the CB may provide running overload protection if it is rated not over 125 percent of the motor’s full-load running current. Therefore, 28 A × 1.25 = 35 A, which satisfies 430.32(A). Because the rating of the thermal-magnetic CB is not over 250 percent times the full-load current (from Table 430.52), the 35-A CB satisfies 430.52 and 430.58 as short-circuit and ground-fault protection. The CB may serve both those functions, as noted in 430.55. The CB may serve as the motor controller, as permitted by 430.83(A)(2). The CB also satisfies as the required disconnect means in accordance with 430.111 and has the rating “of at least 115 percent of the full-load current rating of the motor,” as required by 430.110(A). And because it satisfies 430.110(A) on the disconnect minimum rating, it therefore satisfies 430.58, which sets the same minimum rating for a CB used as branch-circuit protection.
Fig. 430-32. Overcurrent functions may be combined in a single CB or set of fuses. (Sec. 430.55.)
430.56. Branch-Circuit Protective Devices—In Which Conductor. Motor branch circuits are to be protected in the same way as other circuits with regard to the number of fuses and the number of poles and overcurrent units of CBs. If fuses are used, a fuse is required in each ungrounded conductor. If a CB is used, there must be an overcurrent unit in each ungrounded conductor (See 240.15).
430.57. Size of Fuseholder. The basic rule of this section covers sizing of fuse-holders for standard nontime-delay fuses used as motor branch-circuit protection. The exception recognizes that time-delay fuses permit use of smaller switches and lower-rated fuseholders.
A fusible switch can take either standard NE Code fuses or time-delay fuses—up to the rating of the switch. Because a given size of time-delay fuse can hold on the starting current of a motor larger than that which could be used with a standard fuse of the same rating, fusible switches are given two horsepower ratings—one for use with standard fuses, the other for use with time-delay fuses. For example, a 3-pole, 30-A, 240-V fused switch has a rating of 3 hp for a 3-phase motor if standard fuses without time-delay characteristics are used. If time-delay fuses are used, the rating is raised to 7½ hp.
Consider a 7½-hp, 230-V, 3-phase motor (full-voltage starting, without code letters, or with code letters F to V), with a full-load current of 22 A. NE Code Table 430.52 shows that such a motor may be protected by nontime-delay fuses with a maximum rating equal to 300 percent of the full-load current (66 A), or time-delay fuses with a maximum rating equal to 175 percent of the full-load current (38.5 A).
If standard, nontime-delay fuses were used, the maximum size permitted would be 70 A (the next standard size larger than 66 A). From the table, this would require a 100-A, 15-hp switch, which would have fuseholders that could accommodate the fuses, as required by the basic rule. Or, a 60-A, 7½-hp switch might be used with standard fuses rated 60 A maximum. But such a switch would be required by the basic rule to have fuseholders that could accommodate 70-A fuses. Because such a fuse has knife-blade terminals instead of end ferrules and is larger than a 60-A fuse, fuseholders in the 60-A switch could be held in conflict with the Code rule, even though the level of protection would be better with 60-A fuses in the 60-A switch. This is the reason for the exception that now clearly establishes that a switch designed for the smaller time-delay fuses can be used.
430.58. Rating of Circuit Breaker. This rule sets a maximum and minimum rating for a CB as branch-circuit protection. Refer to 430.55.
In the case of a CB having an adjustable trip point, this rule refers to the capacity of the CB to carry current without overheating and has nothing to do with the setting of the breaker. The breaker most commonly used as a motor branch-circuit protective device is the nonadjustable CB (see 240.6), and any breaker of this type having a rating in conformity with the requirements of 430.52 will have an ampacity considerably in excess of 115 percent of the full-load motor current.
430.62. Rating or Setting—Motor Load. Overcurrent protection for a feeder to several motors must have a rating or setting not greater than the largest rating or setting of the branch-circuit protective device for any motor of the group plus the sum of the full-load currents of the other motors supplied by the feeder.
The second paragraph notes that there are cases where two or more motors fed by a feeder will have the same rating as the branch-circuit device. And that can happen where the motors are of the same or different horsepower ratings. It is possible for motors of different horsepower ratings to have the same rating as the branch-circuit protective device, depending on the type of motor and the type of protective device. If two or more motors in the group are of different horsepower rating but the rating or setting of the branch-circuit protective device is the same for both motors, then one of the protective devices should be considered as the largest for the calculation of feeder overcurrent protection.
And because Table 430.52 recognizes many different ratings of branch-circuit protective devices (based on use of fuses or CBs and depending on the particular type of motor), it is possible for two motors of equal horsepower rating to have widely different ratings of branch-circuit protection. If, for instance, a 25-hp motor was protected by nontime-delay fuses, Table 430.52 gives 300 percent of the full-load motor current as the maximum rating or setting of the branch-circuit device. Thus, 250-A fuses would be used for a motor that had a 78-A full-load rating. But another motor of the same horsepower and even of the same type, if protected by time-delay fuses, must use fuses rated at only 175 percent of 78 A, which would be 150-A fuses, as shown in Fig. 430-33. If the two 25-hp motors were of different types, one being a wound-rotor motor, it would still be necessary to base selection of the feeder protection on the largest rating or setting of a branch-circuit protective device, regardless of the horsepower rating of the motor.
Fig. 430-33. Feeder protection is based on largest branch-circuit protection, not on motor horsepower ratings. (Sec. 430.62.)
Figure 430-34 shows a typical motor feeder calculation, as follows:
The four motors supplied by the 3-phase, 460-V, 60-cycle feeder, which are not marked with a code letter (see Table 430.52), are as follows:
One 50-hp squirrel-cage induction motor (full-voltage starting)
One 30-hp wound-rotor induction motor
Two 10-hp squirrel-cage induction motors (full-voltage starting)
Fig. 430-34. Rating of feeder protection is based on branch protection and motor currents. (Sec. 430.62.)
Step 1. Branch-Circuit Loads
From Table 430.250, the motors have full-load current ratings as follows:
50-hp motor—65 A
30-hp motor—40 A
10-hp motor—14 A
Step 2. Conductors
The feeder conductors must have a carrying capacity as follows (see 430.24):
1.25 × 65 = 81 A
81 + 40 + (2 × 14) = 149 A
The feeder conductors must be at least No. 3/0 TW, 1/0 THW, or 1/0 RHH or THHN (copper).
Step 3. Branch-Circuit Protection
Overcurrent (branch-circuit) protection (from Table 430.52 and 430.52) using nontime-delay fuses is as follows:
1. The 50-hp motor must be protected at not more than 200 A (65 A × 300 percent).
2. The 30-hp motor must be protected at not more than 60 A (40 A × 150 percent).
3. Each 10-hp motor must be protected at not more than 45 A (14 A × 300 percent).
Step 4. Feeder Protection
As covered in 430.62, the maximum rating or setting for the overcurrent device protecting such a feeder must not be greater than the largest rating or setting of branch-circuit protective device for one of the motors of the group plus the sum of the full-load currents of the other motors. From the preceding, then, the maximum allowable size of feeder fuses is 200 + 40 + 14 + 14 = 268 A.
This calls for a maximum standard rating of 250 A for the motor feeder fuses, which is the nearest standard fuse rating that does not exceed the maximum permitted value of 268 A.
Exception No. 1 to part (A) addresses those installations where instantaneous-trip CBs and/or MSCPs are used as short-circuit and ground-fault protection for the largest motor supplied by the feeder to be protected. Under certain conditions those devices may set or rated to trip at 13 times or, for Design B energy-efficient motors, even as high as 17 times the motor full-load current. To prevent the feeder conductors from being underprotected, this exception requires that the rating or setting of the feeder protective device be based on the type of device used. That is, if nontime delay fuses are used for feeder protection, the rating of those fuses must be based on the rating of fuse that would be permitted to protect the motor branch circuit if a fuse were used as branch-circuit protection instead of an instantaneous-trip CB or MSCP. For example, consider a 460-V, 3-phase, 100-hp motor, which draws approximately 124 A. If an instantaneous-trip breaker were used for branch-circuit short-circuit and ground-fault protection, it could be rated at over 1600 A (13 × 124 A).
When establishing the rating of protection for the feeder supplying a motor branch-circuit so protected, if nontime delay fuses are used, the value of current that is summed with the ratings of the other type branch-circuit protective devices must be no more than that which would be permitted if nontime delay fuses were used as branch-circuit protection instead of an instantaneous-trip CB or MSCP. In this example, Table 430.52 would permit a nontime delay fuse to be 300 percent of the motor full-load current. If the branch-circuit in question is protected by nontime delay fuses, they would be rated at 350 A (3 × 125 A; rounded down because the exception gives no allowance to modify the Table 430.52 results.). And 350 A, not 1300 A, would be used to calculate the maximum rating of nontime delay fuses that are permitted to protect the feeder conductors.
Note: There is no provision in 430.62 whatsoever that would permit the use of “the next higher size, rating, or setting” of the protective device, for a motor feeder when the calculated maximum rating does not correspond to a standard size of device. The feeder calculations are rounded down, not up.
According to part (B) of this section, in large-capacity installations where extra feeder capacity is provided for load growth or future changes, the feeder overcurrent protection may be calculated on the basis of the rated current-carrying capacity of the feeder conductors. In some cases, such as where two or more motors on a feeder may be started simultaneously, feeder conductors may have to be larger than usually required for feeders to several motors.
In selecting the size of a feeder overcurrent protective device, the NE Code calculation is concerned with establishing a maximum value for the fuse or CB. If a lower value of protection is suitable, it may be used.
430.63. Rating or Setting—Power and Light Loads. Protection for a feeder to both motor loads and a lighting and/or appliance load must be rated on the basis of both of these loads. The rating or setting of the overcurrent device must be sufficient to carry the lighting and/or appliance load plus the rating or setting of the motor branch-circuit protective device if only one motor is supplied, or plus the highest rating or setting of branch-circuit protective device for any one motor plus the sum of the full-load currents of the other motors, if more than one motor is supplied.
Figure 430-35 presents basic NE Code calculations for arriving at minimum requirements on wire sizes and overcurrent protection for a combination power and lighting load as follows:
Fig. 430-35. Feeder protection for combination load must properly add both loads. (Sec. 430.63.)
Step 1. Total Load
430.25(A) says that conductors supplying a lighting load and a motor must have capacity for both loads, as follows:
Motor load = 65 A + 40 A + 14 A + 14 A + (0.25 × 65 A) = 149 A per phase
Lighting load = 120 A per phase × 1.25 = 150 A
Total load = 149 + 150 = 299 A per phase leg
Step 2. Conductors
Table 310.16 shows that a load of 299 A can be served by the following copper conductors:
500-kcmil TW
350-kcmil THW
Table 310.16 shows that this same load can be served by the following aluminum or copper-clad aluminum conductors:
700-kcmil TW
500-kcmil THW, RHH, or THHN
Step 3. Protective Devices
430.63 says, in effect, that the protective device for a feeder supplying a combined motor load and lighting load may have a rating not greater than the sum of the maximum rating of the motor feeder protective device and the lighting load, as follows:
1. Motor feeder protective device = rating or setting of the largest branch-circuit device for any motor of the group being served plus the sum of the full-load currents of the other motors:
200 A (50-hp motor) + 40 + 14 + 14 = 268 A maximum
This calls for a maximum standard rating of 250 A for the motor feeder fuses, which is the nearest standard fuse rating that does not exceed the maximum permitted value of 268 A.
2. Lighting load = 120 A × 1.25 = 150 A
Rating of CB for combined load = 268 + 150 = 418 A maximum
This calls for a 400-A CB, the nearest standard rating that does not exceed the 418-A maximum.
Again: There is no provision in 430.63 which permits the use of “the next higher size, rating, or setting” of the protective device for a motor feeder when the calculated maximum rating does not correspond to a standard size of device.
Such considerations as voltage drop, I2R loss, spare capacity, lamp dimming on motor starting, and so forth would have to be made to arrive at actual sizes to use for the job. But, the circuiting as shown would be safe—although maybe not efficient or effective for the particular job requirements.
430.71. General. Figure 430-36 shows the motor control circuit part of a motor branch circuit, as defined in the second paragraph of this section. A control circuit, as discussed here, is any circuit which has as its load device the operating coil of a magnetic motor starter, a magnetic contactor, or a relay. Strictly speaking, it is a circuit which exercises control over one or more other circuits. And these other circuits controlled by the control circuit may themselves be control circuits or they may be load circuits—carrying utilization current to a lighting, heating, power, or signal device.
Fig. 430-36. A control circuit governs the operating coil that switches the load circuit. (Sec. 430.71.)
The elements of a control circuit include all the equipment and devices concerned with the function of the circuit: conductors, raceway, contactor operating coil, source of energy supply to the circuit, overcurrent protective devices, and all switching devices which govern energization of the operating coil.
The NE Code covers application of control circuits in Art. 725 and in 240.4 and 430.71 through 430.74. Design and installation of control circuits are basically divided into three classes (in Art. 725) according to the energy available in the circuit. Class 2 and 3 control circuits have low energy-handling capabilities; and any circuit, to qualify as a class 2 or 3 control circuit, must have its open-circuit voltage and overcurrent protection limited to conditions given in 725.121.
Most control circuits for magnetic starters and contactors could not qualify as class 2 or 3 circuits because of the relatively high energy required for operating coils. And any control circuit rated over 150 V (such as 230- or 460-V coil circuits) can never qualify, regardless of energy.
Class 1 control circuits include all operating coil circuits for magnetic starters which do not meet the requirements for class 2 or 3 circuits. Class 1 circuits must be wired in accordance with 725.41 to 725.52.
430.72. Overcurrent Protection. Part (A) tells the basic idea behind protection of the operating coil circuit of a magnetic motor starter, as distinguished from a manual (mechanically operated) starter:
1. 430.72 covers motor control circuits that are derived within a motor starter from the power circuit which connects to the line terminals of the starter. The rule here refers to such a control circuit as one “tapped from the load side” of the fuses or circuit breaker that provides branch-circuit protection for the conductors which supply the starter. See the top of Fig. 430-37.
2. The control circuit that is tapped from the line terminals within a starter is not a branch circuit itself.
3. Depending on other conditions set in 430.72, the conductors of the control circuit will be considered as protected by either the branch-circuit protective device ahead of the starter or the supplementary protection (usually fuses) installed in the starter enclosure.
4. Any motor control circuit that is not tapped from the line terminals within a starter must be protected against overcurrent in accordance with 725.43. Such control circuits would be those that are derived from a panelboard or a control transformer—as where, say, 120-V circuits are derived external to the starters and are typically run to provide lower-voltage control for 230-, 460-, or 575-V motors. See the bottom of Fig. 430-37.
Part (B) applies to overcurrent protection of conductors used to make up the control circuits of magnetic motor starters. Such overcurrent protection must be sized in accordance with the amp values shown in Table 430.72(B). And where that table makes reference to amp values specified in 310.15, as applicable, it does not specify that 310.15(B)(2) must be observed by derating conductor ampacity where more than three current-carrying conductors are used in a conduit. Previously, the rule in part (B) of this section specifically recognized the use of control-circuit wires in raceway “without derating factors.” 725.51(A), however, does require class 1 remote-control wires to have their ampacity derated in accordance with 310.15(B)(2), based on the number of conductors, when the conductors “carry continuous loads” in excess of 10 percent of each conductor’s ampacity.
Fig. 430-37. Source of power supply to the control circuit determines which Code section applies to the coil circuit. (Sec. 430.72.)
That makes the application in Fig. 430-38 compliant (almost, but not completely certain). The reason for the rule in 725.51(A) is that heat given off from a wire is a function of I2R losses. A wire carrying 10 percent of its ampacity is putting out 1 pecent (0.12) of the heat generated when fully loaded. That is a negligible amount. Further, the realities of most control circuits are that they seldom carry much in the way of actual current. Most contactor coils and other such devices take a fraction of an ampere over time, and in order to trip over 310.15(B)(2)(a) the 10 percent limit not only has to be exceeded, it must be exceeded on a continuous basis, not just for the fraction of a second that a coil is actually pulling in. So, unless the control circuits are very unusual, the wires are as a practical matter not subject to derating for mutual conductor heating.
Fig. 430-38. Derating of control-wire ampacity is seldom necessary when more than three conductors are run within the same raceway. (Sec. 430.72.)
The basic rule of part (B)(1) requires coil-circuit conductors to have overcurrent protection rated in accordance with the maximum values given in Column A of Table 430.72(B). That table shows 7 A as the maximum rating of protection for 18 AWG copper wire and 10 A for 16 AWG wire and refers to Table 310.16 for larger wires—20 A for 14 AWG copper, 25 A for No. 12, and so forth. These values are taken from the 60°C column to be consistent with other calculations in this table, but purposely not using the 240.4(D) special ampacities because the work here is governed by 240.4(G). The basic rules in part (B)(2) cover conditions under which other ratings of protection may be used, as follows:
The first sentence in part (B)(2) covers protection of control wires for magnetic starters that have their START-STOP buttons in the cover of the starter enclosure.
In part (B)(2), the value of branch-circuit protection must be compared to the ampacity of the control-circuit wires that are factory-installed in the starter and connected to the START-STOP buttons in the cover. If the rating of the branch-circuit fuse or CB does not exceed the value of the current shown in Column B of Table 430.72(B) for the particular size of either copper or aluminum wire used to wire the coil circuit within the starter, then other protection is not required to be installed within the starter (Fig. 430-39). If the rating of branch-circuit protection does exceed the value shown in Column B for the size of coil-circuit wire, then separate protection must be provided within the starter, and it must be rated not greater than the value shown for that size of wire in Column A of Table 430.72(B). For instance, if the internal coil circuit of a starter is wired with No. 16 copper wire and the branch-circuit device supplying the starter is rated over the 40-A value shown for 16 AWG copper wire in Column B of Table 430.72(B), then protection must be provided in the starter for the 16 AWG wire and the protective device(s) must be rated not over the 10-A value shown for 16 AWG copper wire in Column A of Table 430.72(B).
Fig. 430-39. This is the rule of part (B)(1) of Sec. 430.72.
Because most starters are the smaller ones using 18 and 16 AWG wires for their coil circuits, part (B)(2) and its reference to Column B are particularly applicable to those wire sizes. For 16 AWG control wires, branch-circuit protection rated up to 40 A would eliminate any need for a separate control-circuit fuse in the starter. And for 18 AWG control wires, separate coil-circuit protection is not needed for a starter with branch-circuit protection rated not over 25 A. For 14, 12, and 10 AWG copper control wires, maximum protective-device ratings are given in Column B as 100, 120, and 160 A, respectively. For conductors larger than No. 10, the protection may be rated up to 400 percent of (or 4 times) the free-air ampacity of the size of conductor from Table 310.17.
The third sentence in part (B)(2) covers protection of control wires that run from a starter to a remote-control device (pushbutton station, float switch, limit switch, etc.). Such control wires may be protected by the branch-circuit protective device—without need for separate protection within the starter—if the branch-circuit device has a rating not over the value shown for the particular size of copper or aluminum control wire in Column C of Table 430.72(B) (Fig. 430-40). Note that the maximum ratings of 7 A for 18 AWG and 10 A for 16 AWG require that fuse protection at those ratings must always be used to protect those sizes of control-circuit wires connected to motor starters supplied by CB branch-circuit protection, because 15 A is the lowest available standard rating of CB. But branch-circuit fuses of 7- or 10-A rating could eliminate the need for protection in the starter where 18 AWG or 16 AWG control wires are used. Figure 430-41 shows an application that was permitted for many years under previous wording of the Code rule but is now contrary to the letter and intent of the rule.
Fig. 430-40. This is covered by part (B)(2) of Sec. 430.72.
For any size of control wire, if the branch-circuit protection ahead of the starter has a rating greater than the value shown in Column C of Table 430.72(B), then the control wire must be protected by a device(s) rated not over the amp value shown for that size of wire in Column A of Table 430.72(B). For instance, if 14 AWG copper wire is used for the control circuit from a starter to a remote pushbutton station and the branch-circuit protection ahead of the starter is rated at 40 A, then the branch-circuit device is not over the value of 45 A shown in Column C, and separate control protection is not required within the starter. But if the branch-circuit protection were, say, 100 A, then 14 AWG control wire would have to be protected at 15 A because Column A shows that 14 AWG must have maximum protection rating from Note 1—which refers to Table 310.16 where 14 AWG wire in conduit is shown as 20 A. As noted previously, values are taken from the 60°C column to be consistent with other calculations in this table, but purposely not using the 240.4(D) special ampacities because the work here is governed by 240.4(G).
Fig. 430-41. This was permitted by previous NEC editions but is now a violation of part (B)(2). (Sec. 430.72.)
It should be noted that Column A gives the values to be used for overcurrent protection placed within the starter to protect control-circuit wires in any case where the rating of branch-circuit protection exceeds the value shown in either Column B (for starters with no external control wires) or Column C (for control wires run from a starter to a remote pilot control device).
Part (C) permits protection on the primary side of a control transformer to protect the transformer in accordance with 450.3 and the secondary conductors in accordance with the amp value shown in Table 430.72(B) for the particular size of the control wires fed by the secondary. This use is limited to transformers with 2-wire secondaries (Fig. 430-42). Because 430.72(A) notes that the rules of 430.72 apply to control circuits tapped from the motor branch circuit, the rule of part (C) must be taken as applying to a control transformer installed within the starter enclosure—although the general application may be used for any transformer because it conforms to 240.4(F), and to 450.3.
Fig. 430-42. Exception No. 2 to part (B) of 430.72 permits the secondary wires of the coil circuit to be protected by primary-side overcurrent protection. (Sec. 430.72.)
The exception eliminates any need for control-circuit protection where opening of the circuit would be objectionable, as for a fire-pump motor or other essential or safety-related operation.
Part (C) covers the use of control transformers and requires protection on the primary side. And, again, it must be taken to apply specifically to such transformers used in motor control equipment enclosures. The basic rule in part (C)(2) calls for each control transformer to be protected in accordance with 450.3 (usually by a primary-side protective device rated not over 125 or 167 percent of primary current), as shown in Fig. 430-42. But other options are given.
Part (C)(3) eliminates any need for protection of any control transformer rated less than 50 VA, provided it is part of the starter and within its enclosure.
Part (C)(4) permits a control transformer with a rated primary current of less than 2 A to be protected at up to 500 percent of rated primary current by a protective device in each ungrounded conductor of the supply circuit to the transformer primary, as shown in Fig. 430-43.
Fig. 430-43. A control circuit fed by a transformer within the starter enclosure may have overcurrent protection in the primary rated up to 500 percent of the rated primary current of a small transformer. (Sec. 430.72.)
In the majority of magnetic motor controllers and contactors, the voltage of the operating coil is the voltage provided between two of the conductors supplying the load, or one conductor and the neutral. Conventional starters are factory wired with coils of the same voltage rating as the phase voltage to the motor. However, there are many cases in which it is desirable or necessary to use control circuits and devices of lower voltage rating than the motor. Such could be the case with high-voltage (over 600 V) controllers, for instance, in which it is necessary to provide a source of low voltage for practical operation of magnetic coils. And even in many cases of motor controllers and contactors for use under 600 V, safety requirements dictate the use of control circuits of lower voltage than the load circuit.
Although contactor coils and pilot devices are available and effectively used for motor controllers with up to 600-V control circuits, such practice has been prohibited in applications where atmospheric and other working conditions make it dangerous for operating personnel to use control circuits of such voltage. And certain OSHA regulations require 120- or 240-V coil circuits for the 460-V motors. In such cases, control transformers are used to step the voltage down to permit the use of lower-voltage coil circuits.
430.73. Mechanical Protection of Conductor. The condition under which physical protection of the control circuit conductor becomes necessary is where damage to the conductors would constitute either a fire or an accident hazard. Damage to the control circuit conductors resulting in short-circuiting two or more of the conductors or breaking one of the conductors would either cause the device to operate or render it inoperative, and in some cases either condition would constitute a hazard either to persons or to property; hence, in such cases the conductors should be installed in rigid or other metal conduit. On the other hand, damage to the conductors of the low-voltage control circuit of a domestic oil burner or automatic stoker does not constitute a hazard, because the boiler or furnace is equipped with an automatic safety control.
430.74. Electrical Arrangement of Control Circuits. This section focuses on the hazard of accidental starting of a motor. Figure 430-44 shows an example of a control circuit installation that should be carefully designed and is required to be observed for any control circuit which has one leg grounded. Whenever the coil is fed from a circuit made up of a hot conductor and a grounded conductor (as when the coil is fed from a panelboard or separate control transformer, instead of from the supply conductors to the motor), care must be taken to place the pushbutton station or other switching control device in the hot leg to the coil and not in the grounded leg to the coil. By switching in the hot leg, the starting of the motor by accidental ground fault can be effectively eliminated.
Fig. 430-44. Control hookup must prevent accidental starting. (Sec. 430.73.)
Note the words “remote from the motor controller.” This means the rule does not require that the running overload relay contacts be wired on the ungrounded side of the control circuit. Very few are done this way and not much can go wrong to short leads that never leave their enclosure, but it is certainly permissible, if seldom done, to place them on the ungrounded side.
Combinations of ground faults can develop to short the pilot starting device—pushbutton, limit switch, pressure switch, and so forth—accidentally starting the motor even though the pilot device is in the OFF position. And because many remote-control circuits are long, possible faults have many points at which they might occur. Insulation breakdowns, contact shorts due to accumulation of foreign matter or moisture, and grounds to conduit are common fault conditions responsible for accidental operation of motor controllers.
Although not specifically covered by Code rules, there are many types of ground-fault conditions that affect motor starting and should be avoided.
As shown in Fig. 430-45, any magnetic motor controller used on a 3-phase, 3-wire ungrounded system always presents the possibility of accidental starting of the motor. If, for instance, an undetected ground fault exists on one phase of the 3-phase system—even if this system ground fault is a long distance from the controller—a second ground fault in the remote-control circuit for the operating coil of the starter can start the motor.
Fig. 430-45. Accidental motor starting can be hazardous and contrary to Code rule. (Sec. 430.73.)
Figure 430-46 shows the use of a control transformer to isolate the control circuit from responding to the combination of ground faults shown in Fig. 430-47. This transformer may be a one-to-one isolating transformer, with the same primary and secondary voltage, or the transformer can step the motor circuit voltage down to a lower level for the control circuit.
In the hookup shown in Fig. 430-47, a 2-pole START button is used in conjunction with two sets of holding contacts in the motor starter. This hookup protects against accidental starting of the motor under the fault conditions shown in Fig. 430-45. The hookup also protects against accidental starting due to two ground faults in the control circuit simply shorting out the START button and energizing the operating coil. This could happen in the circuit of Fig. 430-45 or the circuit of Fig. 430-46.
Fig. 430-46. Control transformer can isolate control circuit from accidental starting. (Sec. 430.73.)
Another type of motor control circuit fault can produce a current path through the coil of a closed contactor to hold it closed regardless of the operation of the pilot device for opening the coil circuit. Again this can be done by a combination of ground faults which short the STOP device. Failure to open can do serious damage to motors in some applications and can be a hazard to personnel. The operating characteristics of contactor coils contribute to the possible failure of a controller to respond to the opening of the STOP contacts. It takes about 85 percent of rated coil voltage to operate the armature associated with the coil; but it takes only about 50 percent of the rated value to enable the coil to hold the contactor closed once it is closed. Under such conditions, even partial grounds and shorts on control contact assemblies can produce paths for sufficient current flow to cause shorting of the stop position of pilot devices. And faults can short out running overload relays, eliminating overcurrent protection of the motor, its associated control equipment, and conductors.
Fig. 430-47. Use of 2-pole start button can prevent accidental starting. (Sec. 430.73.)
Figure 430-48 is a modification of the circuit of Fig. 430-47, using a 2-pole START button and a 2-pole STOP button—protecting against both accidental starting and accidental failure to stop when the STOP button is pressed. Both effects of ground faults are eliminated.
Fig. 430-48. This circuit prevents accidental starting and assures stopping. (Sec. 430.73.)
430.75. Disconnection. The control circuit of a remote-control motor controller shall always be so connected that it will be cut off when the disconnecting means is opened, unless a separate disconnecting means is provided for the control circuit.
When the control circuit of a motor starter is tapped from the line terminals of the starter—in which case it is fed at line-to-line voltage of the circuit to the motor itself—opening of the required disconnect means ahead of the starter deenergizes the control circuit from its source of supply, as shown in Fig. 430-49. But, where voltage supply to the coil circuit is derived from outside the starter enclosure (as from a panelboard or from a separate control transformer), provision must be made to ensure that the control circuit is capable of being deenergized to permit safe maintenance of the starter. In such cases, the required power-circuit disconnect ahead of the starter can open the power circuit to the starter’s line terminals; but, unless some provision is made to open the externally derived control circuit voltage supply, a maintenance worker could be exposed to the unexpected shock hazard of the energized control circuit within the starter.
Fig. 430-49. Disconnect ahead of starter opens supply to line-voltage coil circuit. (Sec. 430.74.)
The disconnect for control voltage supply could be an extra pole or auxiliary contact in the switch or CB used as the main power disconnect ahead of the starter, as shown in Fig. 430-50. Or the control disconnect could be a separate switch (like a toggle switch), provided this separate switch is installed “immediately adjacent” to the power disconnect—so it is clear to maintenance people that both disconnects must be opened to kill all energized circuits within the starter. Control circuits operating contactor coils, and so forth, within controllers present a shock hazard if they are allowed to remain energized when the disconnect is in the OFF position. Therefore, the control circuit either must be designed in such a way that it is disconnected from the source of supply by the controller disconnecting means or must be equipped with a separate disconnect immediately adjacent to the controller disconnect for opening of both disconnects. [For grounding of the control transformer secondary in Fig. 430-50, refer to 250.20(B).]
Exception No. 1 of part (A) is aimed at industrial-type motor control hookups which involve extensive interlocking of control circuits for multi-motor process operations or machine sequences. In recognition of the unusual and complex control conditions that exist in many industrial applications—particularly process industries and manufacturing facilities—Exception No. 1 alters the basic rule that disconnecting means for control circuits must be located “immediately adjacent one to each other” (Fig. 430-51). When a piece of motor control equipment has more than 12 motor control conductors associated with it, remote locating of the disconnect means is permitted under the conditions given in Exception No. 1. As shown in Fig. 430-52, this permission is applicable only where qualified persons have access to the live parts and sufficient warning signs are used on the equipment to locate and identify the various disconnects associated with the control circuit conductors.
Fig. 430-50. Control disconnect means must supplement power-circuit disconnect. (Sec. 430.74.)
Fig. 430-51. Industrial control layouts with more than 12 control circuit conductors for interlocking of controllers and operating stations (arrow) do not require control disconnects to be “immediately adjacent” to power disconnects. (Sec. 430.74.)
Fig. 430-52. For extensively interlocked control circuits, control disconnects do not have to be adjacent to power disconnects. (Sec. 430.74.)
Exception No. 2 presents another instance in which control circuit disconnects may be mounted other than immediately adjacent to each other. It notes that where the opening of one or more motor control circuit disconnects might result in hazard to personnel or property, remote mounting may be used where the conditions specified in Exception No. 1 exist, that is, that access is limited to qualified persons and that a warning sign is located on the outside of the equipment to indicate the location and the identification of each remote control circuit disconnect.
The requirement of part (B) of this section is shown in Fig. 430-53. When a control transformer is in the starter enclosure, the power disconnect means is on the line side and can de-energize the transformer control circuit. Grounding of the control circuit is not always necessary, as noted in 250.21(C), although there must be a showing of a need for power continuity and ground detectors must be installed when this option is used. Overcurrent protection must be provided for the control circuit when a control circuit transformer is used, as covered in 430.72(B). Such protection may be on the primary or secondary side of the transformer, as described. In 450.1, Exception No. 2 notes that the rules of Art. 450 do not apply to “dry-type transformers that constitute a component part of other apparatus. . . .” A control transformer supplied as a factory-installed component in a starter would therefore be exempt from the rules of 450.3(B), covering overcurrent protection for transformers, but would have to comply with 430.72(C).
430.81. General. As used in Art. 430, the term controller includes any switch or device normally used to start and stop a motor, in addition to motor starters and controllers as such. As noted, the branch-circuit fuse or CBs are considered an acceptable control device for stationary motors not over 
hp where the motor has sufficient winding impedance to prevent damage to the motor with its rotor continuously at standstill. And a plug and receptacle connection may serve as the controller for portable motors up to 
hp.
Fig. 430-53. Control transformer in starter must be on load side of disconnect. (Sec. 430.74.)
As described in the definition in 430.2, a controller is a device that starts and stops a motor by “making and breaking the motor circuit current”—that is, the power current flow to the motor windings. A pushbutton station, a limit switch, a float switch, or any other pilot control device that “carries the electric signals directing the performance of the controller” (see the definition of Motor Control Circuit in 430.71) is not the controller where such a device is used to carry only the current to the operating coil of a magnetic motor controller. For purposes of Code application, the contactor mechanism is the motor “controller.”
430.82. Controller Design. Every controller must be capable of starting and stopping the motor which it controls, must be able to interrupt the stalled-rotor current of the motor, and must have a horsepower rating not lower than the rating of the motor, except as permitted by 430.83.
430.83. Ratings. Figure 430-54 shows the basic requirements from part (A) for the required rating of a controller. Although the basic rule calls for a horsepower-rated switch or a horsepower-rated motor starter, there are acceptable alternative methods for specific applications as noted in 430.81 and as follows:
Fig. 430-54. Controller must be a horsepower-rated switch or CB—but other devices may satisfy. (Sec. 430.83.)
The wording of 430.83(A)(1) calls for controllers—other than inverse time CBs and molded-case switches—to have a horsepower rating at least equal to that of the motor at the application voltage.
A branch-circuit CB rated in amperes only, may be used as a controller. If the same CB is used as a controller and to provide overload protection for the motor circuit, it must be rated in accordance with 430.32.
A molded-case switch may be used as a controller.
A general-use switch rated at not less than twice the full-load motor current may be used as the controller for stationary motors up to 2 hp, rated 300 V or less. On AC circuits, a general-use snap switch suitable only for use on AC may be used to control a motor having a full-load current rating not over 80 percent of the ampere rating of the switch.
In the UL’s Electrical Construction Materials Directory, data are presented on use of switches in motor circuits, as follows:
1. Enclosed switches with horsepower ratings in addition to current ratings may be used for motor circuits as well as for general-purpose circuits. Enclosed switches with ampere-only ratings are intended for general use but may also be used for motor circuits (as controllers and/or disconnects) as permitted by NE Code 430.83(A)(1), 430.109, and 430.111.
2. A switch that is marked “MOTOR CIRCUIT SWITCH” is intended for use only in motor circuits.
3. For switches with dual-horsepower ratings, the higher horsepower rating is based on the use of time-delay fuses in the switch fuseholders to hold in on the inrush current of the higher-horsepower-rated motor.
4. Although 430.83 permits use of horsepower-rated switches as controllers and UL lists horsepower-rated switches up to 500 hp, UL does state in its Green Book that “enclosed switches rated higher than 100 hp are restricted to use as motor disconnect means and are not for use as motor controllers.” But a horsepower-rated switch up to 100 hp may be used as both a controller and disconnect if it breaks all ungrounded legs to the motor, as covered in 430.111.
Figure 430-55 covers two of those points.
For selection of a controller for a sealed (hermetic-type) refrigeration compressor motor, refer to 440.41.
430.84. Need Not Open All Conductors. It is interesting to note that the NE Code says that a controller need not open all conductors to a motor, except when the controller serves also as the required disconnecting means. For instance, a 2-pole starter of correct horsepower rating could be used for a 3-phase motor if running overload protection is provided in all three circuit legs by devices separate from the starter, such as by dual-element, time-delay fuses which are sized to provide running overload protection as well as short-circuit protection for the motor branch circuit. The controller must interrupt only enough conductors to be able to start and stop the motor.
However, when the controller is a manual (nonmagnetic) starter or is a manually operated switch or CB (as permitted by the Code), the controller itself also may serve as the disconnect means if it opens all ungrounded conductors to the motor, as covered in 430.111. This eliminates the need for another switch or CB to serve as the disconnecting means. But, it should be noted that only a manually operated switch or CB may serve such a dual function. A magnetic starter cannot also serve as the disconnecting means, even if it does open all ungrounded conductors to the motor.
Fig. 430-55. UL rules limit Code applications. (Sec. 430.83.)
Figure 430-56 shows typical applications in which the controller does not have to open all conductors but a separate disconnect switch or CB is required ahead of the controller. In the drawing, the word ungrounded refers to the condition that none of the circuit conductors is grounded. These may be the ungrounded conductors of grounded systems.
Generally, one conductor of a 115-V circuit is grounded, and on such a circuit a single-pole controller may be used connected in the ungrounded conductor, or a 2-pole controller is permitted if both poles are opened together. In a 230-V circuit there is usually no grounded conductor, but if one conductor is grounded, 430.85 permits a 2-pole controller.
430.85. In Grounded Conductors. This rule permits a 3-pole switch, CB, or motor starter to be used in a 3-phase motor circuit derived from a 3-phase, 3-wire, corner-grounded delta system—with the grounded phase leg switched along with the hot legs, as in 430.36.
430.87. Number of Motors Served by Each Controller. Generally, an individual motor controller is required for each motor. However, for motors rated not over 600 V, a single controller rated at not less than the sum of the horsepower ratings of all the motors of the group may be used with a group of motors if any one of the conditions specified is met. Where a single controller is used for more than one motor connected on a single branch circuit as permitted under condition b, it should be noted that the reference is to part (A) of 430.53. That use of a single controller applies only to cases involving motors of 1 hp or less and does not apply for several motors used on a single branch circuit in accordance with parts (B) and (C) of 430.53—unless the several motors satisfy conditions a or c of this section.
Fig. 430-56. Controller does not have to break all legs of motor supply circuit. (Sec. 430.84.)
See 430.112, where the same conditions are set for a single disconnect means to serve a group of motors. Note that the reference to 430.110(C)(1) requiring an equivalent horsepower calculation can add unexpected sizing. For example, suppose you want to group control a 10-hp, a 15-hp, and a 20-hp, 460-V motor. The total horsepower by simple summation would be 45 hp, but that is not the correct result. The ampere ratings from Table 430.250 and the locked-rotor current numbers from Table 430.251(B) have to be summed as well: 14 A + 21 A + 27 A = 62 A; and 81 A + 116 A + 145 A = 342 A. Making a reverse lookup in Tables 430.250 and 251(B) using these summations results in a requirement for a 50-hp-capable controller.
430.88. Adjustable-Speed Motors. Field weakening is quite commonly used as a method of controlling the speed of DC motors. If such a motor were started under a weakened field, the counter emf is reduced commensurately and the starting current would be excessive unless the motor is specially designed for starting in this manner.
430.89. Speed Limitation. A common example of a separately excited DC motor is found in a typical speed control system that is widely used for electric elevators, hoists, and other applications where smooth control of speed from standstill to full speed is necessary. In Fig. 430-57, G1 and G2 are two generators having their armatures mounted on a shaft which is driven by a motor, not shown in the diagram. M is a motor driving the elevator drum or other machine. The fields of generator G1 and motor M are excited by G1. By adjusting the rheostat R, the voltage generated by G2 is varied, and this in turn varies the speed of motor M. It is evident that if the field circuit of motor M should be accidentally opened while the motor is lightly loaded, the motor would reach an excessive speed. In many applications of this system the motor is always loaded and no speed-limiting device is required.
Fig. 430-57. Typical speed control hookup involving the rule of 430.89.(Sec. 430.89.)
The speed of a series motor depends on its load and will become excessive at no load or very light loads. Traction motors are commonly series motors, but such a motor is geared to the drive wheels of the car or locomotive and hence is always loaded.
Where a motor generator, consisting of a motor driving a compound-wound DC generator, is operated in parallel with a similar machine or is used to charge a storage battery, if the motor circuit is accidentally opened while the generator is still connected to the DC buses or battery, the generator will be driven as a motor and its speed may become dangerously high. A synchronous converter operating under similar conditions may also reach an excessive speed if the AC supply is accidentally cut off.
A safeguard against overspeed is provided by a centrifugal device on the shaft of the machine, arranged to close (or open) a contact at a predetermined speed, thus tripping a CB which cuts the machine off from the current supply.
430.90. Combination Fuseholder and Switch as Controller. The use of a fusible switch as a motor controller with fuses as motor-running protective devices is practicable when time-delay types of fuses are used. The rating of the fuses must not exceed 125 percent, or in some cases 115 percent, of the full-load motor current, and nontime-delay fuses of this rating would, in most cases, be blown by the starting current drawn by the motor, particularly where the motor turns on and off frequently. (See 430.35.)
It may be found that a switch having the required horsepower rating is not provided with fuse terminals of the size required to accommodate the branch-circuit fuses. For example, assume a 7½-hp, 230-V, 3-phase motor started at full-line voltage. A switch used as the disconnecting means for this motor must be rated at not less than 7½ hp, but this would probably be a 60-A switch and therefore, if fusible, would be equipped with terminals to receive 35- to 60-A fuses. 430.90 provides that fuse terminals must be installed that will receive fuses of 70-A rating. In such case a switch of the next higher rating must be provided, unless time-delay fuses are used.
430.92. General. This is the motor control center part of the article. Motor control centers are essentially switchboards adapted for this function, and many of the construction requirements came from Art. 408. It should be remembered that the clearances in 110.26 apply to this equipment generally, and 110.26(F), addressing what can go where in the dedicated wiring space above and below such equipment, specifically refers to motor control centers as a topic to which it applies.
430.94. Overcurrent Protection. Motor control centers require individual over-current protection, either in the unit or ahead of it, rated or set not to exceed the rating of the common power bus. This is a significant departure from switchboards that do not have this limitation. Note that this limitation may limit the application of 430.62 or 430.63 because feeder protection determined by those sections, particularly with a very large motor in the load group, may significantly exceed the ampacity of the feeder conductors. The common power bus of a motor control center is a feeder, and this rule means that a motor control center supplying the same large motor will require a common power bus that is larger than the equivalent conductors installed as a conventional feeder.
430.102. Location. Along with 430.101, this section specifically requires that a disconnecting means—basically, a motor-circuit switch rated in horsepower, or a CB—be provided in each motor circuit. Figure 430-58 shows the basic rule on “in-sight” location of the disconnect means. This applies always for all motor circuits rated up to 600 V—even if an “out-of-sight” disconnect can be locked in the open position.
Because the basic rule here requires a disconnecting means to be within sight from the “controller location,” the question arises, “Is the magnetic contactor the controller or is the pushbutton station the controller?” The NEC makes clear that the contactor of a magnetic motor starter is the controller for the motor, not the pushbuttons that actuate the coil of the contactor. The NEC establishes that identification by the definition of controller in Art. 100 and by the definition of a motor control circuit in 430.2, as follows:
Controller: A device or group of devices that serves to govern, in some predetermined manner, the electric power delivered to the apparatus to which it is connected. Note that this definition is modified “for the purposes of this article” in 430.2 by a definition that goes on to state that it is “any switch or device that is normally used to start and stop a motor by making and breaking the motor circuit current.”
Motor control circuit: The circuit of a control apparatus or system that carries the electric signals directing the performance of the controller, but does not carry the main power current.
Fig. 430-58. The required disconnect must be visible from the controller. (Sec. 430.102.)
In a magnetic motor starter hookup, it is the contactor that actually governs the electric power delivered to the motor to which it is connected. The motor connects to the contactor and not to the pushbuttons, which are in the control circuit that carries the electric signals directing the performance of the controller (i.e., the contactor). The pushbuttons do not carry “the main power current,” which is “delivered” to the motor by the contactor and which is, therefore, “the controller.” It is well established that the intent of the Code rule, as well as the letter of the rule, is to designate the contactor and not the pushbutton station as the controller, and the disconnect must be within sight from it and not from a pushbutton station or some other remotely located pilot control device that connects into the contactor.
There are three exceptions to this basic Code rule, requiring a disconnect switch or CB to be located in sight from the controller:
Exception No. 1 permits the disconnect for a medium-voltage (over 600 V) motor to be out of sight from the controller location, as shown in Fig. 430-59. But, such use of a lock-open type switch as an out-of-sight disconnect for a motor circuit rated 600 V or less is a clear Code violation.
Exception No. 2 is aimed at permitting practical, realistic disconnecting means for industrial applications of large and complex machinery utilizing a number of motors to power the various interrelated parts of the machine. The exception recognizes that a single common disconnect for a number of controllers (as permitted by part (A) of the Exception of 430.112) is often impossible to install “within sight” of all the controllers, even though the controllers are “adjacent one to each other.” On much industrial process equipment, the components of the overall structure obstruct the view of many controllers. Exception No. 2 permits the single disconnect to be technically out of sight from some or even all the controllers if the disconnect is simply adjacent to them—that is, nearby on the equipment structure, as shown in Fig. 430-60.
Fig. 430-59. An out-of-sight disconnect may be used for a high-voltage motor. (Sec. 430.102.)
Exception No. 3 is new in the 2008 edition of the NEC and addresses instances where a local disconnect for “valve actuator motors” would introduce additional hazards, and there is a label applied giving the location of a lock-open disconnect.
These motors, defined in 430.2, are common in industrial facilities, where they control process fluids. The actual motors are short-time duty with high torque. They generally have a controller built into the assembly, and the motor incorporates a self-resetting running overload protection. They are reversible for obvious reasons, and available in a range of voltages and either single or three-phase, and up to eight poles. The gearing differs from unit to unit, and therefore the typical rating is not in horsepower but in torque. The short-circuit and ground-fault protective device ratings for these motors are determined by Table 430.52 using the nameplate current and not current taken from the tables at the end of the article. This procedure is not new; as covered in the last sentence of 430.6(A)(1), “motors built for low speeds (less than 1200 rpm) or high torques may have higher full-load currents, . . . in which case the nameplate current ratings shall be used.”
Fig. 430-60. For multimotor machines, the disconnect may be “adjacent” to controller. (Sec. 430.102.)
Part (B) basically requires a disconnect means (switch or CB) to be within sight and not more than 50 ft away from “the motor location and the driven machinery location.” But the exception to that basic requirement says that a disconnect does not have to be within sight from the motor and its load if the required disconnect ahead of the motor controller is capable of being locked in the open position. This exception has been severely limited in recent code cycles. The disconnect in sight of the controller must be “individually capable of being locked in the open position” which eliminates the possibility of a locked panel door qualifying. In addition, as in most instances throughout Chap. 4, the locking provisions must remain with the circuit breaker or switch even with the lock removed.
The above limitations apply in many comparable situations, but the following limitations are unique to the application of this exception. It only applies to (1) industrial occupancies with written safety procedures that ensure that only qualified persons will service the motor or (2) other installations if the additional disconnect would introduce additional hazards or would be impracticable.
For example, it would be plainly impracticable to place a disconnect 50 ft down a well shaft to be “in sight” (not over 50 ft distant) from a submersible pump motor 100 ft down the same shaft. Variable frequency drives should not have the motor disconnected unless the drive itself is disconnected, and multi-motor equipment may cause hazards unless a coordinated stop is arranged. Large motors (over 100 hp) only need isolation switches anyway, so their disconnects can be remote, and additional disconnects in hazardous (classified) locations only exacerbate the explosion hazards. These issues are covered quite well in a note that follows the exception. Never again assume that a locking disconnect in the motor control center solves the local disconnect requirement.
According to the basic rule of part (B), a manually operable switch, which will provide disconnection of the motor from its power supply conductors, must be placed within sight from the motor location. And this switch may not be a switch in the control circuit of a magnetic starter. (The NE Code at one time permitted a switch in the coil circuit of the starter installed within sight of the motor. Such a condition is not acceptable to the present Code.)
These requirements are shown in Fig. 430-61. Specific layouts of the two conditions are shown in Fig. 430-62. (Note: The portions of these drawings covering the exception only apply when and if the restrictive conditions just discussed have been met.) The intent of the exception to part (B) is to permit maintenance workers to lock the disconnecting means ahead of the controller in the open position and keep the key in their possession so that the circuit cannot be energized while they are working on it.
Fig. 430-61. Disconnect means must be within sight from the motor and its driven load, unless out-of-sight disconnect can be locked open. [Sec. 430.102(B).] This exception is now severely limited (see text).
Fig. 430-62. Here’s an example of the rules, showing physical layout. [Sec. 430.102(B).] As previously noted, this exception is now severely limited. As illustrated here, it would probably only apply in an industrial occupancy under written safety procedures and with qualified supervision and staff.
The pushbutton station in Fig. 430-62, “Exception,” operates only the holding coil in the magnetic starter. The magnetic starter controls the current to the motor; for example, the control wires to a pushbutton station could become shorted after the motor is in operation, and pushing the STOP button would not release the holding coil in the magnetic starter and the motor would continue to run. This is the reason that a disconnecting means is required to be installed within sight from the motor and its load or a lock-open switch installed ahead of the controller. In this case, operating the disconnecting means will open the supply to the controller and shut off the motor.
430.103. Operation. This rule actually defines the meaning of disconnecting means.
In order that necessary periodic inspection and servicing of motors and their controllers may be done with safety, the Code requires that a switch, CB, or other device shall be provided for this purpose. Because the disconnecting means must disconnect the controller as well as the motor, it must be a separate device and cannot be a part of the controller, although it could be mounted on the same panel or enclosed in the same box with the controller. The disconnect must be installed ahead of the controller. And note that the disconnect need open only the ungrounded conductors of a motor circuit.
In case the motor controller fails to open the circuit if the motor is stalled, or under other conditions of heavy overload, the disconnecting means can be used to open the circuit. It is therefore required that a switch used as the disconnecting means shall be capable of interrupting very heavy current.
A 2008 NEC amendment added the requirement that the disconnecting means be designed so it cannot close automatically. This addresses time-clock switches that can be placed in an open position, and even have the door locked shut, but unless the trippers are physically removed the clock will, in time, reclose the circuit.
430.105. Grounded Conductors. Although 430.103 requires a disconnect means only for the ungrounded conductors of a motor circuit, if a motor circuit includes a grounded conductor, one pole of the disconnect may switch the grounded conductor, provided all poles of the disconnect operate together—as in a multipole switch or CB. For instance, a 120-V, 2-wire circuit with one of its conductors grounded only requires a single-pole disconnect switch, but a 2-pole switch could be used, with one pole simultaneously switching the grounded leg.
430.107. Readily Accessible. Although a motor circuit may be provided with more than one disconnect means in series ahead of the controller—such as one at the panel where the motor circuit originates and one at the controller location—only one of the disconnects is required to be “readily accessible,” as follows:
Readily accessible: Capable of being reached quickly for operation, renewal, or inspection, without requiring those to whom ready access is requisite to climb over or remove obstacles or to resort to portable ladders, chairs, etc. (See “Accessible.”)
The disconnecting means must be reached without climbing over anything, without removing crates or equipment or other obstacles, and without requiring the use of portable ladders.
Note carefully: A disconnect that has to be “readily accessible” must be so only for “those to whom ready access is requisite”—which clearly and intentionally allows for making equipment not readily accessible to other than authorized persons, such as by providing a lock on the door, with the key possessed by or available to those who require ready access.
Because the definition of readily accessible contains a last phrase that says “See ‘Accessible’,” logic dictates that the installation must also satisfy the definition of accessible. And the wording of the definition clearly establishes that there is no Code violation in putting the disconnect means in a room or area under lock and key to make it accessible only to authorized persons.
The definition reads:
Accessible: (As applied to Equipment.) Admitting close approach because not guarded by locked doors, elevation, or other effective means. (See “Readily Accessible.”)
Again note carefully: That definition does not say that a door to an electrical room is prohibited from being locked. In fact, the wording of the definition, by referring to “locked doors,” actually presumes the existence and, therefore, the acceptability of locked doors in electrical systems. The only requirement implied by the wording is that locked doors, where used, must not guard against access—that is, disposition of the key to the lock must be such that those requiring access to the room are not positively excluded. The rule is satisfied if the key is available to provide access to authorized persons.
In reference to the definition of accessible, the critical word is guarded. The definition is not intended to mean that equipment cannot be behind locked doors or that equipment cannot be mounted up high where it can be reached with a portable ladder. To make equipment “not accessible,” a door lock or high mounting must be such that it positively guards against access. Equipment behind a locked door for which a key is not possessed by or available to persons who require access to the equipment is not “accessible.” A common example of that latter condition occurs in multitenant buildings where a disconnect for the tenant of one occupancy unit is located behind the locked door of another tenant’s occupancy unit from which the first tenant is effectively and legally excluded. And even that application is Code-acceptable if the disconnect is not required by the NEC to be “readily accessible.”
Equipment may be fully “accessible” even though installed behind a locked door or at an elevated height. Equipment that is high-mounted but can be reached with a ladder that is fixed in place or a portable ladder is “accessible” (although the equipment would not be “readily accessible” if a portable ladder had to be used to reach it). Similarly, equipment behind a locked door is “accessible” to anyone who possesses a key to the lock or to a person who is authorized to obtain and use the key to open the locked door. In such cases, conditions do not guard against access.
Refer to the definitions of accessible and readily accessible in Art. 100 of this book.
430.109. Type. In a motor branch circuit, every switch or CB in the circuit, from where the circuit is tapped from the feeder to the motor itself, must satisfy the requirements on type and rating of disconnect means. A CB switching device with no automatic trip operation, a so-called molded-case switch, may be used as a motor disconnect instead of a conventional CB or a horsepower-rated switch. Such a device either must be rated for the horsepower of the motor it is used with or must have an amp rating at least equal to 115 percent of that of the motor with which it is used. Figure 430-63 covers the basic rules on types of disconnect means. An instantaneous-trip circuit breaker, as part of a listed combination motor controller, has the status of a circuit breaker in this classification and is an acceptable disconnecting means. The following items also qualify, with the limitations as described.
Self-protected combination controller—This combination device controls the motor’s performance, and qualifies as a disconnecting means. The device must be listed to qualify. It is also on the list of short-circuit and ground-fault protective devices, as covered in 430.52(C)(6).
Fig. 430-63. One of these disconnects must be used for a motor branch circuit. (Sec. 430.109.) There are a few others; see text.
Manual motor controller additionally marked SUITABLE AS MOTOR DISCONNECT—These devices carry principal listings as controllers, intended to turn a motor on and off manually. Even though it acts manually, and says OFF and ON, it does not qualify as a disconnecting means without meeting additional qualifications. Motor controllers, being designed as the manual equivalents of automatic controllers generally wired on the load side of a conventional disconnect, don’t have as robust internal spacings as full-fledged disconnect switches. The NEC allows these devices, if so listed, to be used as formal disconnects in two circumstances. The first, covering small motors, allows them to be used as disconnects for motors of 2 hp or less, just as snap switches (described later in this list). The second, covering larger motors, allows them to be used as disconnects provided they are on the load side of the final branch-circuit short-circuit and ground-fault protective device. In either case, their horsepower rating must not be less than the motor.
General-use switch—A switch intended for use in general distribution and branch circuits. It is rated in amperes, and is capable of interrupting its rated current at its rated voltage. Its ampere rating must be not less than twice the full-load current rating of the motor. It generally cannot be used for a motor larger than 2 hp, unless it additionally qualifies as a motor-circuit switch, as described earlier. It also qualifies if the installation (2 hp up to 100 hp) involves an autotransformer-type controller and the motor is driving a generator with overload protection, provided the controller has (1) no-voltage release (shuts off when power is discontinued, with manual restart only), (2) running overload protection limited to 125 percent of motor full-load current, and (3) the capability to interrupt locked-rotor current. In addition, the branch circuit must include separate fuses or an inverse-time circuit breaker not over 150 percent of the motor full-load current.
General-use snap switc h—A form of general-use switch constructed so that it can be installed in flush device boxes or on outlet box covers, or otherwise used in conjunction with wiring systems recognized by the NEC. They are for ac motors only. To qualify, the switch must be rated ac-only (general-use ac-dc snap switches are not acceptable) and the motor full-load current must not exceed 80 percent of the ampere rating of the switch.
Isolating switch—A switch intended for isolating an electric circuit from the source of power. It has no interrupting rating, and it is intended to be operated only after the circuit has been opened by some other means. This is permitted only for dc motors over 40 hp and for ac motors over 100 hp. These switches are available in larger ratings, as covered at the end of this list.
Plug and receptacle—A cord- and plug-connected motor need not have an additional disconnecting means if the plug and receptacle have a suitable horsepower rating. Refer to the UL guide card information under the heading “Receptacles for Attachment Plugs and Plugs” (Guide Card Designator RTRT) for the complete list, and manufacturer’s literature. The separate horsepower rating is not required for appliances, room air conditioners, or portable motors rated hp or less.
System isolation equipment—New in the 2005 NEC (and defined in 430.2), this concept uses a contactor on the load side of a motor circuit switch, circuit breaker, or molded case switch as a disconnecting means. Because contactors can be closed inadvertently, these devices must be listed and include means to redundantly monitor the contactor position. This equipment is designed for extremely large industrial applications, typically involving multiple personnel entry points to a piece of equipment and therefore making remote lockout provisions in a control circuit desirable.
Section 430.109(E) sets the maximum horsepower rating required for motor-circuit switches at 100 hp. Higher-rated switches are now available and will provide additional safety. The first sentence of this section makes a basic requirement that the disconnecting means for a motor and its controller be a motor-circuit switch rated in horsepower. For motors rated up to 500 hp, this is readily complied with, inasmuch as the UL lists motor-circuit switches up to 500 hp and the manufacturers mark switches to conform. But for motors rated over 100 hp, the Code does not require that the disconnect have a horsepower rating. It makes an exception to the basic rule and permits the use of an ampere-rated switch or isolation switch, provided the switch has a carrying capacity of at least 115 percent of the nameplate current rating of the motor [430.110(A)]. And UL notes that horsepower-rated switches over 100 hp must not be used as motor controllers. And part (E) notes that isolation switches for motors over 100 hp must be plainly marked “Do not operate under load,” if the switch is not rated for safely interrupting the locked-rotor current of the motor. Figure 430-64 shows an example of disconnect switch application for a motor rated over 100 hp.
Fig. 430-64. Above 100 hp, a switch does not have to be horsepower-rated. (Sec. 430.109.)
example Provide a disconnect for a 125-hp, 3-phase, 460-V motor. Use a nonfusible switch, inasmuch as short-circuit protection is provided at the supply end of the branch circuit.
The full-load running current of the motor is 156 A, from NEC Table 430.250. A suitable disconnect must have a continuous carrying capacity of 156 × 1.15, or 179 A, as required by 430.110(A).
This calls for a 200-A, 3-pole switch rated for 480 V. The switch may be a general-use switch, a current- and horsepower-marked motor-circuit switch, or an isolation switch. A 200-A, 3-pole, 480-V motor-circuit switch would be marked with a rating of 50 hp, but the horsepower rating is of no concern in this application because the switch does not have to be horsepower-rated for motors larger than 100 hp.
If the 50-hp switch were of the heavy-duty type, it would have an interrupting rating of 10 × 65 A (the full-load current of a 460-V, 50-hp motor), or 650 A. But the locked-rotor current of the 125-hp motor might run over 900 A. In such a case, the switch is required by part (E) to be marked “Do not operate under load.”
If a fusible switch had to be provided for the example motor to provide both disconnect and short-circuit protection, the size of the switch would be determined by the size and type of fuses used. Using a fuse rating of 250 percent of motor current (which does not exceed the 300 percent maximum in Table 430.152) for standard fuses, the application would call for 400-A fuses in a 400-A switch. This switch would certainly qualify as the motor disconnect. However, if time-delay fuses are used, a 200-A switch would be large enough to take the time-delay fuses and could be used as the disconnect (because it is rated at 115 percent of motor current).
In the foregoing, the 400-A switch might have an interrupting rating high enough to handle the locked-rotor current of the motor. Or the 200-A switch might be of the CB-mechanism type or some other heavy current construction that has an interrupting rating up to 12 times the rated load current of the switch itself. In either of these cases, there would be no need for marking “Do not operate under load.”
Up to 100 hp, a switch which satisfies the Code on rating for use as a motor controller may also provide the required disconnect means—the two functions being performed by the one switch—provided it opens all ungrounded conductors to the motor, is protected by an overcurrent device (which may be the branch-circuit protection or may be fuses in the switch itself), and is a manually operated air-break switch or an oil switch not rated over 600 V or 100 A—as permitted by 430.111.
430.110. Ampere Rating and Interrupting Capacity. An ampere-rated switch or a CB must be rated at least equal to 115 percent of a motor’s full-load current if the switch or CB is the disconnect means for the motor.
When two or more motors are served by a single disconnect means, as permitted by 430.112, or where one or more motors plus a nonmotor load (such as electric heater load) make use of a single common disconnect, part (C) must be used in sizing the disconnect. Refer to the discussion at 430.87 for a worked-out example of the front half of this process, that of coming up with the equivalent horsepower rating. Now the second half of the calculation must be done, that is, determining the required ampere rating of the disconnect. This is the sum of the current ratings of the components, all multiplied by 1.15. In this case 62 A × 1.15 = 71 A. The combined disconnect must have a horsepower rating of 50 hp and the current rating must not be less than 71 A at the stated voltage of 460 V. If the disconnect is not rated in horsepower, such as a molded case switch, this ampere rating is sufficient.
430.111. Switch or Circuit Breaker as Both Controller and Disconnecting Means. As described under 430.84, a manual switch—capable of starting and stopping a given motor, capable of interrupting the stalled-rotor current of the motor, and having the same horsepower rating as the motor—may serve the functions of controller and disconnecting means in many motor circuits, if the switch opens all ungrounded conductors to the motor. That is also true of a manual motor starter. A single manually operated CB may also serve as controller and disconnect (Figs. 430-65 and 430-66). However, in the case of an autotransformer type of controller, the controller itself, even if manual, may not also serve as the disconnecting means. Such controllers must be provided with a separate means for disconnecting controller and motor.
Fig. 430-65. A manual switch or CB may serve as both controller and disconnect means. (Sec. 430.111.)
Although this Code section permits a single horsepower-rated switch to be used as both the controller and the disconnect means of a motor circuit, UL rules note that “enclosed switches rated higher than 100 hp are restricted to use as motor disconnecting means and are not for use as motor controllers.”
The acceptability of a single switch for both the controller and the disconnecting means is based on the single switch satisfying the Code requirements for a controller and for a disconnect. It finds application where general-use switches or horsepower-rated switches are used, as permitted by the Code, in conjunction with time-delay fuses which are rated low enough to provide both running overload protection and branch-circuit (short-circuit) protection. In such cases, a single fused switch may serve a total of four functions: (1) controller, (2) disconnect, (3) branch-circuit protection, and (4) running overload protection. And it is possible for a single CB to also serve these four functions.
For sealed refrigeration compressors, 440.12 gives the procedure for determining the disconnect rating, based on nameplate rated-load current or branch-circuit selection current, whichever is greater, and locked-rotor current of the motor-compressor.
Fig. 430-66. Use of a single controller disconnect is limited. (Sec. 430.111.)
430.112. Motors Served by Single Disconnecting Means. In general, each individual motor must be provided with a separate disconnecting means. However, a single disconnect sometimes may serve a group of motors under the conditions specified, which are the same as in 430.87. Such a disconnect must have a rating sufficient to handle a single load equal to the sum of the horsepower ratings or current ratings.
Exception A
In 610.32 it is required that the main collector wires of a traveling crane shall be controlled by a switch located within sight of the wires and readily operable from the floor or ground. This switch would serve as the disconnecting means for the motors on the crane. When repair or maintenance work is to be done on the electrical equipment of the crane, it is safer to cut off the current from all this equipment by opening one switch, rather than to use a separate switch for each motor. Also, in the case of a machine tool driven by two or more motors, a single disconnecting means for the group of motors is more serviceable than an individual switch for each motor, because repair and maintenance work can be done with greater safety when the entire electrical equipment is “dead.”
Exception B
Such groups may consist of motors having full-load currents not exceeding 6 A each, with circuit fuses not exceeding 20 A at 125 V or less, or 15 A at 600 V or less. Because the expense of providing an individual disconnecting means for each motor is not always warranted for motors of such small size, and also because the entire group of small motors could probably be shut down for servicing without causing inconvenience, a single disconnecting means for the entire group is permitted.
Exception C
“Within sight” should be interpreted as meaning so located that there will always be an unobstructed view of the disconnecting switch from the motor, and Sec. 430.102 limits the distance in this case between the disconnecting means and any motor to a maximum of 50 ft.
These conditions are the same as those under which the use of a single controller is permitted for a group of motors. (See 430.87.) The use of a single disconnecting means for two or more motors is quite common, but in the majority of cases the most practicable arrangement is to provide an individual controller for each motor.
If a switch is used as the disconnecting means, it must be of the type and rating required by 430.109 for a single motor having a horsepower rating equal to the sum of the horsepower ratings of all the motors it controls. Thus, for six 5-hp motors the disconnecting means should be a motor-circuit switch rated at not less than 30 hp. If the total of the horsepower ratings is over 2 hp, a horsepower-rated switch must be used.
430.113. Energy from More than One Source. The basic rule of this section, which is similar to that of 430.74, requires a disconnecting means to be provided from each source of electrical energy input to equipment with more than one circuit supplying power to it, such as the hookup shown in Fig. 430-67, where two switches or a single 5-pole switch could be used. And each source is permitted to have a separate disconnecting means. This Code rule is aimed at the need for adequate disconnects for safety in complex industrial layouts. But an exception to the Code rule states that where a motor receives electrical energy from more than one source (such as a synchronous motor receiving both alternating current and direct current energy input), the disconnecting means for the main power supply to the motor shall not be required to be immediately adjacent to the motor—provided that the controller disconnecting means, which is the disconnect ahead of the motor starter in the main power circuit, is capable of being locked in the open position. If, for instance, the motor control disconnect can be locked in the open position, it may be remote; but the disconnect for the other energy input circuit would have to be adjacent to the machine itself, as indicated in Fig. 430-68.
430.122. Conductors—Minimum Size and Ampacity. The majority of motors going into industrial and commercial occupancies are going in with adjustable speed drives for both customization of output and energy efficiency. The incoming branch circuit or feeder to power conversion equipment included as part of an adjustable-speed drive system must be based on the rated input to the power conversion equipment, and is to be taken at 125 percent of that value. If the drive has a bypass mechanism that allows the motor to run directly across the line, the conductors must meet the larger of two calculations, one being 125 percent of the input rating to the drive, and the other being 125 percent of the 430.6 determined rating (usually, therefore, the Table 430.250 rating), as applicable.
Fig. 430-67. A disconnect must be used for each power input to motorized equipment. (Sec. 430.113.)
Fig. 430-68. An exception is made for disconnects for multiple power sources. (Sec. 430.113.)
430.126. Motor Overtemperature Protection. Adjustable speed drive systems must be capable of detecting overheated motors that are in that condition not because of excessive load resulting in additional current draw, but for other reasons. Traditional methods that monitor current will protect the circuit conductors from overload, but not the motor if it is drawing its nameplate current but operating below its rated speed because of factors involved with the application, because a low speed may not produce enough ventilation or coolant circulation, etc. A thermal protector (see 430.32) may be required in addition to the drive programming. This section is followed by an extensive fine-print note calling attention to the problem and offering suggestions as to causes and remedies.
430.224. Size of Conductors. For motors rated over 600 V, the circuit conductors to the motor are selected to have a current rating equal to or greater than the trip setting of the running overload protective device for the motor.
430.225. Motor Circuit Overcurrent Protection. Overload protection must protect the motor and other circuit components against overload currents up to and including locked-rotor current of the motor. A CB or fuses must be used for protection against ground faults or short circuits in the motor circuit.
430.242. Stationary Motors. Usually, stationary motors are supplied by wiring in a metal raceway or metal-clad cable. The motor frames of such motors must be grounded, the raceway or cable armor being attached to the frame and serving as the grounding conductor. (See 250.118.)
Any motor in a wet location constitutes a serious hazard to persons and should be grounded unless it is so located or guarded that it is out of reach. All water pump motors, including those in the submersible-type pump, must be grounded, regardless of location, to comply with 250.112(L).
430.245. Method of Grounding. Good practice requires in nearly all cases that the wiring to motors which are not portable shall, at the motor, be installed in rigid or flexible metal conduit, electrical metallic tubing, or metal-clad cable and that such motors should be equipped with terminal housings. The method of connecting the conduit to the motor where some flexibility is necessary is shown in Fig. 430-69. The motor circuit is installed in rigid conduit and a short length of liquidtight flexible metal conduit is provided between the end of the rigid conduit and the terminal housing on the motor. But because the size of flex is over metric designator 35 (trade size 1¼), a separate equipment grounding conductor (or bonding jumper) must be used within or outside the flex as noted in 350.60 and 250.118. Refer to 430.12(E), which requires provision of a suitable termination for an equipment grounding conductor at every motor terminal housing, as shown in Fig. 430-70.
This section permits the use of fixed motors without terminal housings. If a motor has no terminal housing, the branch-circuit conductors must be brought to a junction box not over 6 ft (1.8 m) from the motor. Between the junction box and the motor, the specified provisions apply.
According to 300.16, the conduit, tubing, or metal-clad cable must terminate close to the motor in a fitting having a separable bushed hole for each wire. The method of making the connection to the motor is not specified; presumably, it is the intention that the wire brought out from the terminal fitting shall be connected to binding posts on the motor or spliced to the motor leads. The conduit, tubing, or cable must be rigidly secured to the frame of the motor.
Table 430-251(B). This the locked-rotor table that is necessary to determine equivalent horsepower ratings and other activities. It does not apply to Design A motors, which are not covered in this table and not limited to its provision; the manufacturer would need to be consulted in such cases.
Fig. 430-69. Liquidtight flex provides flexible connection from rigid conduit supply to motor terminals but does require a separate equipment grounding conductor run within the flex with the circuit conductors or a separate external bonding jumper from the rigid metal conduit to the metal terminal box for each of the two runs. (Sec. 430.245.)
ARTICLE 440. AIR-CONDITIONING AND REFRIGERATING EQUIPMENT
440.2. Definitions. The first definition for “Branch-Circuit Selection Current” is extremely important because of the differences between it and the last definition, covering “rated-load current.” The latter corresponds to full-load current in many other places. The former includes the effects of the degree of sustained overload that the unit is capable of under defined test conditions. It always at least equals, and usually exceeds the rated-load current. The definition is critical because, where given, it becomes the number upon which the other size or ratings of elements of the circuit are determined, including conductors, over-current protective devices, and disconnects.
The third definition, “Leakage Current Detection and Interruption,” describes a device capable of detecting and interrupting current flow “from the cord conductor,” such as where a ground fault has occurred, and “between” the “cord conductors,” such as where the device is unintentionally submerged. This definition ties into the rule of 440.65, which mandates such protective devices for all single-phase (generally, 120- or 220-V) cord-and-plug-connected room air conditioners. Although this is a manufacturer’s concern, designers and installers must ensure that any such cord-and-plug-connected room air conditioners are factory equipped with a leakage current detector and interrupter (LCDI) or an arc-fault circuit interrupter (AFCI).
Fig. 430-70. Motor terminal housings must include some lug or terminal for connecting an equipment grounding conductor that may be run inside the raceway with the circuit wires or may be run as a bonding jumper around a length of flex or liquidtight flex, as commonly used for vibration-free motor connections. The terminal box here must have internal provision for connecting the equipment grounding conductor, required for this short length of liquidtight flex, that is larger than metric designator 35 (trade size 1¼) in size. The static grounding connection shown here (arrow) on the box does not satisfy 350.60 and 250.102(E) as a bonding jumper for the flex, and it does not satisfy 250.134 as an equipment ground for an AC motor. (Sec. 430.245.)
440.3. Other Articles. Article 440 is patterned after Art. 430, and many of its rules, such as on disconnecting means, controllers, conductor sizes, and group installations, are identical or quite similar to those in Art. 430. This article contains provisions for such motor-driven equipment and for branch circuits and controllers for the equipment, taking into account the special considerations involved with sealed (hermetic-type) motor-compressors, in which the motor operates under the cooling effect of the refrigeration.
It must be noted that the rules of Art. 440 are in addition to or are amendments to the rules given in Art. 430 for motors in general. The basic rules of Art. 430 also apply to A/C (air-conditioning) and refrigerating equipment unless exceptions are indicated in Art. 440.
Article 440 further clarifies the application of NE Code rules to air-conditioning equipment and refrigeration equipment as follows:
1. A/C and refrigerating equipment which does not incorporate a sealed (hermetic-type) motor-compressor must satisfy the rules of Art. 422 (Appliances), Art. 424 (Space Heating Equipment), or Art. 430 (Conventional Motors)—whichever apply. For instance, where refrigeration compressors are driven by conventional motors, the motors and controls are subject to Art. 430, not Art. 440. Furnaces with air-conditioning evaporator coils installed must satisfy Art. 424. Other equipment in which the motor is not a sealed compressor and which must be covered by Arts. 422, 424, or 430 includes fan-coil units, remote forced-air-cooled condensers, remote commercial refrigerators, and similar equipment.
2. Room air conditioners are covered in part VII of Art. 440 (440.60 through 440.64), but must also comply with the rules of Art. 422.
3. Household refrigerators and freezers, drinking-water coolers, and beverage dispensers are considered by the Code to be appliances, and their application must comply with Art. 422 and must also satisfy Art. 440, because such devices contain sealed motor-compressors.
Air-conditioning equipment (other than small room units and large custom installations) is manufactured in the form of packaged units having all necessary components mounted in one or more enclosures designed for floor mounting, for recessing into walls, for mounting in attics or ceiling plenums, for locating outdoors, and so forth. Figure 440-1 shows the difference between room A/C units (such as window units) and the larger so-called packaged units or central air conditioners. Room units consist of a complete refrigeration system in a unit enclosure intended for mounting in windows or in the wall of the building, with ratings up to 250 V, single phase. Unitary assemblies may be console type for individual room use, rated up to 250 V, single phase, or central cooling units rated up to 600 V for commercial or domestic applications. This type may consist of one or more factory-made sections. If it is made up of two or more sections, each section is designed for field interconnection with one or more matched sections to make the complete assembly. Dual-section systems consist of separate packaged sections installed remote from each other and interconnected by refrigerant tubing, with the compressor either within the outdoor section or within the indoor section.
Electrical wiring in and to units varies with the manufacturer, and the extent to which the electrical contractor need be concerned with fuse and CB calculations depends on the manner in which the units’ motors are fed and the type of distribution system to which they are to be connected. A packaged unit is treated as a group of motors. This is different from the approach used with a plug-in room air conditioner, which is treated as an individual single-motor load of amp rating as marked on the nameplate.
440.4. Marking on Hermetic Refrigerant Motor-Compressors and Equipment. Important in the application of hermetic refrigerant motor-compressors are the terms rated-load current and branch-circuit selection current. As previously noted, definitions of these terms are given in 440.2. When the equipment is marked with the branch-circuit selection current, this greatly simplifies the sizing of motor branch-circuit conductors, disconnecting means, controllers, and over-current devices for circuit conductors and motors.
Fig. 440-1. Code rules differentiate between unit room conditioners and central systems. (Sec. 440.3.)
For some A/C equipment that is not required to have a branch-circuit selection current, the value of rated-load current will appear on the equipment nameplate; and that same value of current will also appear in the nameplate space reserved for branch-circuit selection current. In such cases, the branch-circuit selection current is to be equal to the rated-load current.
440.5. Marking on Controllers. Note that a controller may be marked with ”full-load and locked-rotor current (or horsepower) rating.” That possibility of two methods of marking requires careful application of the rules in 440.41 on selecting the correct rating of controller for motor-compressors.
440.6. Ampacity and Rating. Selection of the rating of branch-circuit conductors, controller, disconnect means, short-circuit (and ground-fault) protection, and running overload protection is not made the same for hermetic motor-compressors as for general-purpose motors. In sizing those components, the rated-load current marked on the equipment and/or the compressor must be used in the calculations covered in other rules of this article. That value of current must always be used, instead of full-load currents from Code Tables 430.248 to 430.250, which are used for sizing circuit elements for non-hermetic motors. And if a branch-circuit selection current is marked on equipment, that value must be used instead of rated-load current.
440.12. Rating and Interrupting Capacity. Note that the rules here are qualifications that apply to the rules of 430.109 and 430.110 on disconnects for general-purpose motors.
A disconnecting means for a hermetic motor, as covered in part (A)(2), must be a motor-circuit switch rated in horsepower or a CB—as required by 430.109.
If a CB is used, it must have an amp rating not less than 115 percent of the nameplate rated-load current or the branch-circuit selection current—whichever is greater.
But, if a horsepower-rated switch is to be selected, the process is slightly involved for hermetic motors marked with locked-rotor current and rated-load current or rated-load current plus branch-circuit selection current—but not marked with horsepower. In such a case, determination of the equivalent horsepower rating of the hermetic motor must be made using the locked-rotor current and either the rated-load current or the branch-circuit selection current—whichever is greater—based on Code Tables 430.248, 430.249, or 430.250 for rated-load current or branch-circuit selection current and Table 430.251(B) for locked-rotor current, as follows:
For example, a 3-phase, 460-V hermetic motor rated at 11-A branch-circuit selection and 60-A locked-rotor is to be supplied with a disconnect switch rated in horsepower. The first step in determining the equivalent horsepower rating of that motor is to refer to Code Table 430.250. This table lists 7½ hp as the required size for a 460-V, 11-A motor. To ensure adequate interrupting capacity, Code Table 430.251(B) is used. For a 60-A locked-rotor current, this table also shows 7½ hp as the equivalent horsepower rating for any locked-rotor current over 45 to 66 A for a 400-V motor. Use of both tables in this manner thus establishes a 7½ hp disconnect as adequate for the given motor in both respects. Had the two ratings as obtained from the two tables been different, the higher rating would have been chosen.
Figure 440-2 shows an example of disconnect sizing for a horsepower-rated switch when a hermetic motor is used, in accordance with 430.53(C) and (D), along with fan motors on a single circuit, as covered in part (B) and in 440.34. Fan motors are usually wired to start slightly ahead of the compressor motor through use of interlock contacts or a time-delay relay. In some units, however, all motors start simultaneously, and that is covered by part (B) of this section in sizing the horsepower-rated disconnect switch. Where this is the case, the starting load will be treated like a single motor to the disconnect switch, and the sum of the locked-rotor currents of all motors should be used with Code Table 430.251(B) to determine the horsepower rating of the disconnect. The disconnect normally must handle the sum of the rated-load or branch-circuit selection currents; hence the rating as checked against Code Table 430.250 will be on the basis of the sum of the higher of those currents for all the motors. Code Table 430.250, using the full-load total of 38 A (6.0 A + 26 A + 6.0 A) in this example, indicates a 15-hp disconnect. Code Table 430.251(B), assuming simultaneous starting of all three motors and using the total locked-rotor current of 230 A (40 A + 150 A + 40 A), also shows 15 hp as the required size.
If motors do not start simultaneously, the compressor locked-rotor current (150 A) used with Code Table 430.251(B) gives a 10-hp rating. However, the higher of the two horsepower ratings must be used; hence, the running currents impose the more severe requirements and dictate use of a 15-hp switch. See data under 440.22.
Fig. 440-2. Disconnect for multiple motors is sized from rated-load or branch-circuit selection currents and locked-rotor currents. (Sec. 440.12.)
As required by part (D) of this section, all disconnects in a branch circuit to a refrigerant motor-compressor must have the required amp or horsepower rating and interrupting rating. This provides for motor-compressor circuits the same conditions that 430.108 requires for other motor branch circuits.
440.14. Location. 440.13 recognizes use of a cord-plug and receptacle as the disconnect for such cord-connected equipment as a room or window air conditioner. But this section (440.14) applies to fixed-wired equipment—such as central systems or units with fixed circuit connection. For conditioners with fixed wiring connection to their supply circuits, the rule poses a problem. If the branch-circuit breaker or switch which is to provide disconnect means is located in a panel that is out of sight (or more than 50 ft [15.0 m] away) from the unit conditioner, another breaker or switch must be provided at the equipment. If the panel breaker or switch does not satisfy the rule here, a separate disconnect means would have to be added in sight from the conditioner, as shown in Fig. 440-3. This is also true if the service switch is installed as shown in Fig. 440-4.
As stated in the second sentence, the required disconnect means for air-conditioning or refrigeration equipment may be installed on or within the equipment enclosure; however, it must not be mounted in a way that obstructs the removal of service panels or the visibility of nameplates. If suitably located on the unit, the location is recognized as an equivalent of the basic rule that the disconnect must be readily accessible and within sight (visible and not over 50 ft [15.0 m] away) from the A/C or refrigeration equipment. Such equipment is being manufactured now with the disconnect incorporated as part of the assembly.
Fig. 440-3. For any fixed-wire A/C equipment, disconnect must be “within sight.” (Sec. 440.14.)
Fig. 440-4. “Within sight” disconnect must also be “readily accessible” at the equipment. (Sec. 440.14.)
440.21. General. Part III of this article covers details of branch-circuit makeup for A/C and refrigeration equipment; 440.3(A) says that the provisions of Art. 430 apply to A/C and refrigeration equipment for any considerations that are not covered in Art. 440. Thus, because Art. 440 does not cover feeder sizing and feeder overcurrent protection for A/C and refrigeration equipment, it is necessary to use applicable sections from Art. 430. 430.24 covers sizing of feeder conductors for standard motor loads and for A/C and refrigeration loads. 430.62 and 430.63 cover rating of overcurrent protection for feeders to both standard motors and A/C and refrigeration equipment. That fact is noted in the language at the end of the first paragraph of 430.62(A).
440.22. Application and Selection. Part (A) of this section is illustrated in Fig. 440-5, where a separate circuit is run to the compressor and to each fan motor of a packaged assembly, containing a compressor with 26-A rated-load current and fan motors rated at 6.0-A full load each. The compressor protection is sized at 1.75 × 26 A (175 percent of rated-load current), or 45.5 A—calling for 45-A fuses, maximum. Note carefully that the wording of this rule makes it a “not to exceed” or “round down” rule. Hermetic compressor windings sit in their own refrigerant, and in effect operate overloaded, which is only possible because of the refrigerant. This makes them more sensitive to problems and the overcurrent protection boundaries are more tightly drawn accordingly. In this case, if and only if the basic rule doesn’t work, then a modest increase to 225 percent is allowable, and that too is a not to exceed number. However, it is never necessary to decrease the protection below 15 A.
Sizing of branch-circuit protection for a single branch circuit to the same three motors is permitted by 430.53(C) as well as by 440.22(B) and is shown in Fig. 440-6. That layout is a specific example of the general rules covered in 440.22(B)(1), which ties the rules of 430.53(C) and (D) into the rules of 440.22(B), as shown in Fig. 440-7. Such application is based on certain factors, as covered in the UL Electrical Appliance and Utilization Directory, listed under “Air Conditioners, Central Cooling,” as follows:
Fig. 440-5. A separate circuit may be run to each motor of A/C assembly. (Sec. 440.22.)
Fig. 440-6. Single multimotor branch circuit must conform to several rules. (Sec. 440.22.)
The proper method of electrical installation (number of branch circuits, disconnects, etc.) is shown on the wiring diagram and/or marking required to be attached to the air conditioner.
In air conditioners employing two or more motors or a motor(s) and other loads operating from a single supply circuit, the motor running overcurrent protective devices (including thermal protectors for motors) and other factory-installed motor circuit components and wiring are investigated on the basis of compliance with the motor-branch-circuit short circuit and ground fault protection requirements of 430.53(C) of the National Electrical Code. Such multimotor and combination load equipment is to be connected only to a circuit protected by fuses or a circuit breaker with a rating which does not exceed the value marked on the data plate. This marked protective device rating is the maximum for which the equipment has been investigated and found acceptable. Where the marking specifies fuses, or “HACR Type” circuit breakers, the circuit is intended to be protected by the type of protective device specified.
The electrical contractor and inspector charged with wiring and approving such an installation can be sure that Code requirements have been met, provided that the branch-circuit protection as specified on the unit is not exceeded and the wiring and equipment is as indicated on the wiring diagram. Provision is made in such a unit for direct connection to the branch-circuit conductors; motors are wired internally by the manufacturer.
Fig. 440-7. Fan circuits are sometimes fused in multimotor assemblies. (Sec. 440.22.)
Units are sometimes encountered in which the manufacturer has wired separate fuse cutouts for the fan motors inside the enclosure to avoid meeting the requirements of 430.53(C) for group fusing, as shown in Fig. 440-8. The cutouts are normally fed from the line terminals of the compressor starter.
Starter and disconnect sizes are the same as in Fig. 440-2, but starters and their overcurrent protection no longer need be approved for group fusing, and wiring inside the unit need not conform to 430.53(C). Fan motors may now be wired with 14 AWG wire and protected with 15-A fuses with no 1/3 ratio calculation involved. The supply circuit, feeding the same motors, will again be 6 AWG.
Since the fan motors are not subject to group fusing requirements, they will not restrict the maximum value of the main fuses. However, these fuses provide the only short-circuit protection for the compressor starter and conductors. Unless the compressor starter is approved for group fusing at a higher fuse rating, the fuses must not exceed 175 percent of the compressor full-load rating, or 45.5 A, calling for 45-A fuses. The wire from the panel now gets sized more as a conventional motor feeder needing to accommodate the 45-A main compressor protective device plus the other full-load current supplying the fans, or 8.8 A, for a total of 54 A. The 6 AWG wire on a 60-A fuse is appropriate for this hookup.
Fig. 440-8. Each motor may have individual short-circuit protection. (Sec. 440.22.)
Figure 440-9 shows an arrangement which includes, in addition to the branch-circuit panel, a feeder panel for distribution to other units. The breakers in the branch-circuit panel serve as branch-circuit protection as well as the disconnecting means, and their ratings are computed from 430.52 and 440.22. Code Tables 430.250 and 430.251 would not be involved, since breakers are not rated in horsepower. Ratings of CBs in the branch-circuit panel are computed at 175 percent of motor current for the hermetic motor and at 250 percent of full-load current for the fan motors, to satisfy Table 430.52. Breakers in subfeeder panel are rated using 430.62 and the conductors by 440.33. Because 440.22(A) does not distinguish between types of short-circuit and ground-fault protective devices in setting the upper limits on short-circuit protection, most of these numbers agree with those developed in Fig. 440-8 for fuses.
Part (C) points out that data on a manufacturer’s heater table take precedence over the maximum ratings set by 440.22(A) or (B). This is the correlating language in this article to 430.52(C)(2) on the same subject.
440.32. Single Motor-Compressor. Branch-circuit conductors supplying a motor in a packaged unit are not sized in the same manner as other motor loads (430.22). Instead of using the full-load current from Code Tables 430.148 to 430.150, the marked rated-load current or the marked branch-circuit selection current must be used in determining minimum required conductor ampacity. Note that branch-circuit selection current must be used where it is given.
Fig. 440-9. Circuit breakers may be used for multimotor A/C assemblies. (Sec. 440.22.)
Examples are shown in the typical circuits shown in Figs. 440-2 and 440-6.
For a wye-start, delta-run compressor, the conductors between the controller and the unit are sized, just exactly as in the case of the same wires in 430.22(C), based on 58 percent of the full-load current of the compressor. When you apply the mandatory 125 percent factor to that base number, as covered in the first paragraph, the result is a 72 percent ampacity requirement. This number does not need to be increased further. Take care not to be confused by the difference in approach between this rule and the same one in 430.22(C).
440.33. Motor-Compressor(s) With or Without Additional Motor Loads. Where more than one motor is connected to the same feeder or branch circuit, calculation of conductor sizes must provide ampere capacity at least equal to the sum of the nameplate rated-load currents or branch-circuit selection currents (using the higher of those values in all cases) plus 25 percent of the current (either rated-load or branch-circuit selection current for a hermetic motor or NEC table current for standard motor) of the largest motor of the group. Examples are shown in Figs. 440-2 and 440-7.
In Fig. 440-7, the question arises as to whether the No. 6 conductors feeding the unit may be decreased to No. 8 inside the unit to feed the compressor motor in the absence of fuses for this motor at the point of reduction. The status of the main feed to the unit—whether it should be considered a feeder or a branch circuit—is in doubt, since it is a branch circuit as far as the compressor motor is concerned and a feeder in that it also supplied the two fused fan circuits.
Considered solely as a branch circuit to the compressor, these conductors normally would be No. 8 to handle the 26-A compressor motor full-load current, protected at not more than 175 percent or 50-A fuses. Therefore, since 45-A fuses (or less) will actually be used for the main feed, they constitute proper protection for No. 8 conductors and their use should be permitted. The existence of No. 6 conductors over part of the circuit adds to its capacity and safety rather than detracting from it.
It is particularly important to keep in mind when selecting conductor sizes that the nameplate current ratings of air-conditioning motors are not constant maximum values during operation. Ratings are established and tested under standard conditions of temperature and humidity. Operation under weather conditions more severe than those at which the ratings are established will result in a greater running current, which can approach the maximum value permitted by the overcurrent device. Operating voltage less than the limits specified on the motor nameplate also contributes to higher full-load current values, even under standard conditions. Conductor capacity should be sufficient to handle these higher currents. Motor feeders are sized according to 440.33. Since overload protection may permit motors to run continuously overloaded (up to 140 percent full load), feeders must be sized to handle such overload. By basing calculations on the largest motor of the group, the extra capacity thus provided will normally be enough to handle any unforeseen overload on the smaller motors involved with enough diversity existing in any normal group of motors to make consistent overloads on all motors at one time unlikely. However, a group of air-conditioning compressor motors all of the same size on a single feeder have a common function—reducing the ambient temperature. Except for slight possible variations, weather conditions affect each conditioner to the same degree and at the same time. Therefore, if one unit is operating at an overload, it is likely that the rest are also.
440.35. Multimotor and Combination-Load Equipment. This rule ties into the data required by UL to be marked on such equipment. Refer to the UL data quoted in 440.22.
440.41. Rating. The basic rule calls for a compressor controller to have a full-load current rating and a locked-rotor current rating not less than the compressor nameplate rated-load current or branch-circuit selection current (whichever is greater) and locked-rotor current. But, as noted for the disconnect under 440.12, for sealed (hermetic-type) refrigeration compressor motors, selection of the size of controller is slightly more involved than it is for standard applications. Because of their low-temperature operating conditions, hermetic motors can handle heavier loads than general-purpose motors of equivalent size and rotor-stator construction. And because the capabilities of such motors cannot be accurately defined in terms of horsepower, they are rated in terms of full-load current and locked-rotor current for polyphase motors and larger single-phase motors. Accordingly, selection of controller size is different from the case of a general-purpose motor where horsepower ratings must be matched, because controllers marked in horsepower only must be carefully related to hermetic motors that are not marked in horsepower.
For controllers rated in horsepower, selection of the size required for a particular hermetic motor can be made after the nameplate rated-load current, or branch-circuit selection current, whichever is greater, and locked-rotor current of the motor have been converted to an equivalent horsepower rating. To get this equivalent horsepower rating, which is the required size of controller, the tables in Art. 430 must be used. First, the nameplate full-load current at the operating voltage of the motor is located in Code Tables 430.248, 430.249, or 430.250 and the horsepower rating which corresponds to it is noted. Then the nameplate locked-rotor current of the motor is found in Code Table 430.251(B), and again the corresponding horsepower is noted. In all tables, if the exact value of current is not listed, the next higher value should be used to obtain an equivalent horsepower, by reading horizontally to the horsepower column at the left side of those tables. If the two horsepower ratings obtained in this way are not the same, the larger value is taken as the required size of controller.
A typical example follows:
Given: A 230-V, 3-phase, squirrel-cage induction motor in a compressor has a nameplate rated-load current of 25.8 A and a nameplate locked-rotor current of 90 A.
Procedure: From Code Table 430.250, 28 A is the next higher current to the nameplate current of 25.8 under the column for 230-V motors and the corresponding horsepower rating for such a motor is 10 hp.
From Code Table 430.251(B), Art. 430, a locked-rotor current rating of 90 A for a 230-V, 3-phase motor requires a controller rated at 5 hp. The two values of horsepower obtained are not the same, so the higher rating is selected as the acceptable unit for the conditions. A 10-hp motor controller must be used.
Some controllers may be rated not in horsepower but in full-load current and locked-rotor current. For use with a hermetic motor, such a controller must simply have current ratings equal to or greater than the nameplate rated-load current and locked-rotor current of the motor.
440.52. Application and Selection. The basic rule of part (A)(1) calls for a running overload relay set to trip at not more than 140 percent of the rated-load current of a motor-compressor. As shown in the other subpart (3), if a fuse or inverse time CB is used to provide overload protection, it must be rated not over 125 percent of the compressor rated-load current. Note that those are absolute maximum values of overload protection and no permission is given to go to “the next higher standard rating” of protection where 1.4 or 1.25 times motor current does not yield an amp value that exactly corresponds to a standard rating of a relay or of a fuse or CB.
Running overload protective devices for a motor are necessary to protect the motor, its associated controls, and the branch-circuit conductors against heat damage due to excessive motor currents. High currents may be caused by the motor being overloaded for a considerable period of time, by consistently low or unbalanced line voltage, by single-phasing of a polyphase motor, or by the motor stalling or failing to start.
Damage may occur more quickly to a hermetic motor which stalls or fails to start than to a conventional open-type motor. Due to the presence of the cool refrigerant atmosphere under normal conditions, a hermetic motor is permitted to operate at a rated current which is closer to the locked-rotor current than is the same rated current of an open-type motor of the same nominal horsepower rating. The curves of Fig. 440-10 show the typical relation between locked-rotor and full-loaded currents of small open-type and hermetic motors. Because a hermetic motor operates within the refrigerant atmosphere, it is constantly cooled by that atmosphere. As a result, a given size of motor may be operated at a higher current than it could be if it were used as an open, general-purpose motor without the refrigerant cycle to remove heat from the windings. In effect, a hermetic motor is operated overloaded because the cooling cycle prevents overheating. For instance, a 5-hp open motor can be loaded as if it were a 7½-hp motor when it is cooled by the refrigerant. The full-load operating current of such a motor is higher than the normal current drawn by a 5-hp load and is, therefore, closer to the value of locked-rotor current, which is the same no matter how the motor is used.
Fig. 440-10. Hermetic motors operate at full-load currents closer to locked-rotor currents. (Sec. 440.52.)
When the rotor of a hermetic motor is slowed down because of overload or is at a standstill, there is not sufficient circulation of the refrigerant to carry away the heat; and heat builds up in the windings. Special quick-acting thermal and hydraulic-magnetic devices have been developed to reduce the time required to disconnect the hermetic motor from the line before damage occurs when an overload condition develops.
Room air conditioners and packaged unit compressors are normally required to incorporate running overload protection which will restrict the heat rise to definite maximum safe temperatures in case of locked-rotor conditions. Room conditioners normally use inherent protectors built into the compressor housing which respond to the temperature of the housing. Larger units also often use inherent protection in addition to quick-acting overload heaters installed in the motor starter, which respond only to current. These protective methods are covered in paragraphs (2) and (4) of 440.52(A).
The electrical installer will normally be concerned with the running overcurrent protection of a hermetic motor only when it becomes necessary to replace the existing devices supplied with the equipment. For this purpose, compressor manufacturers’ warranties explicitly specify catalog numbers of replacements which are to be used to ensure proper operation of the equipment.
440.60. General. These rules on room air-conditioning units recognize that such units are basically appliances, are low-capacity electrical loads, and may be supplied either by an individual branch circuit to a unit conditioner or by connection to a branch circuit that also supplies lighting and/or other appliances. For all Code discussion purposes, an air-conditioning unit of the window, console, or through-the-wall type is classified as a fixed appliance—which is described in Art. 100 as “fastened or otherwise secured at a specific location.” Such an appliance may be cord-connected or it may be fixed-wired (so-called permanently connected).
210.23 of in Art. 210 on “Branch Circuits” must also be applied in cases where a unit room air conditioner is connected to a branch circuit supplying lighting or other appliance load.
When a unit air conditioner is connected to a circuit supplying lighting and/or one or more appliances that are not motor loads, the rules of Art. 210 must be observed:
1. For plug connection of the A/C unit, 210.7(A) says that receptacles installed on 15- and 20-A branch circuits must be of the grounding type and must have their grounding terminals effectively connected to a grounding conductor or grounded raceway or metal cable armor.
2. On 15- and 20-A branch circuits, the total rating of a unit air conditioner (“utilization equipment fastened in place”) must not exceed 50 percent of the branch-circuit rating when lighting units or portable appliances are also supplied [210.23(A)]. It was on the basis of that rule that the 7½-A air conditioner was developed. Being 50 percent of a 15-A branch circuit, such units are acceptable for connection to a receptacle on a 15- or 20-A circuit that supplies lighting and receptacle outlets.
3. A branch circuit larger than 20 A may not be used to supply a unit conditioner plus a lighting load. Circuits rated 25, 30, 40, or 50 A may be used to supply fixed lighting or appliances—but not both types of loads.
440.61. Grounding. Air-conditioner units that are connected by permanent wiring must be grounded in accordance with the basic rules of 250.110 covering equipment that is “fastened in place or connected by permanent wiring.” 250.114 covers grounding of cord-and-plug-connected air conditioners by means of an equipment grounding conductor run within the supply cord for each such unit.
The nameplate marking of a room air conditioner shall be used in determining the branch-circuit requirements, and each unit shall be considered as a single motor unless the nameplate is otherwise marked. If the nameplate is marked to indicate two or more motors, 430.53 and 440.22(B)(1) must be satisfied, covering the use of several motors on one branch circuit.
440.62. Branch-Circuit Requirements. Even though a room air conditioner contains more than one motor (usually the hermetic compressor motor and the fan motor), this rule notes that for a cord-and-plug-connected air conditioner the entire unit assembly may be treated as a single-motor load under the conditions given.
Examples of the rule of part (B) are shown in Fig. 440-11. The total marked rating of any cord-and-plug-connected air-conditioning unit must not exceed 80 percent of the rating of a branch circuit which does not supply lighting units or other appliances, for units rated up to 40 A, 250 V, single phase.
Fig. 440-11. Room air conditioners must not load an individual branch circuit over 80 percent of rating. (Sec. 440.62.)
As noted under 440.60, 210.2 seems to say that only Art. 440 and not Art. 210 applies when the circuit supplies only a motor-operated load. But, since Arts. 430 and 440 do not rate branch circuits—either on the basis of the size of the short-circuit protective device or the size of the conductors—a question arises about the meaning of the phrase “80 percent of the rating of a branch circuit.” Does that mean 80 percent of the rating of the fuse or CB? The answer is: It means 80 percent of the rating of the protective device, which rating is not more than the amp rating of the circuit wire. The circuit as described here is taken to be a circuit with “rating” as given in Art. 210 and covered by part (4) of 440.62(A).
As part (C) of this section notes, the total marked rating of air-conditioning equipment must not exceed 50 percent of the rating of a branch circuit which also supplies lighting or other appliances. (See Fig. 440-12.) From the rule, we can see that the Code permits air-conditioning units to be plugged into existing circuits which supply lighting loads or other appliances. By the provisions of this section, such a conditioner must not draw more than 7½-A full load (nameplate rating) when connected to a 15-A circuit and not more than 10 A when connected to a 20-A circuit. This rule effectively makes the 50% limitation for fixed loads on general purpose branch circuits in 210.23(A) applicable to a load that is cord-and-plug-connected.
Fig. 440-12. Room air conditioner must not exceed 50 percent of circuit rating if other loads are supplied. (Sec. 440.62.)
A problem exists in connecting two or more conditioners to the same circuit. Compressor and fan motors and their controls, when installed in the same enclosure and fed by one circuit, are approved by UL for group installation when tested as a unit appliance. However, an air conditioner’s component parts carry no general group-fusing approval which would permit the several separate conditioners to operate on the same circuit in accordance with 430.53(C). To connect more than one conditioner to the same branch circuit, the provisions of either 430.53(A) or 430.53(B) must be fulfilled, treating each cord-connected conditioner as a single-motor load. Note that 440.62(A) does classify a room air conditioner as a single motor load (that permission is conditional, but the terms are met in most cases).
According to 430.53(A), which applies only to motors rated not over 6 A, two 115-V, 6-A conditioners could be used on a 15-A circuit; three 5-A conditioners could be used on a 20-A circuit which supplies no other load; and two 220-V, 6-A units could be operated on a 15-A circuit, as shown in Fig. 440-13. But it could be argued that the maximum load in any such application may be calculated at 125 percent times the current of the largest air conditioner plus the sum of load currents of the additional air conditioners, with that total current being permitted right up to the rating of the circuit.
Fig. 440-13. Rules limit use of two or more room conditioners on single circuit. (Sec. 440.62.)
However, most conditioners sold today exceed 6-A full-load current. As a result, the application of two or more units as permitted by 430.53(A) is limited. But 430.53(B) does offer considerable opportunity for using more than one air conditioner on a single circuit. Figure 440-14 shows two examples of such application, which can be used if the branch-circuit protective device will not open under the most severe normal conditions which might be encountered. Although that usage is a complex connection among several Code rules and requires clearance with inspection authorities, it can provide very substantial economies.
Fig. 440-14. These hookups have been accepted as conforming to rules of Arts. 440 and 430. (Sec. 440.62.)
Many local codes avoid the complications of connecting conditioners to existing circuits and connecting more than one conditioner to the same circuit by requiring a separate branch circuit for each conditioner. Multiple installations involving many room conditioners such as are frequently encountered in hotels, offices, and so forth, require careful planning to meet Code requirements and yet minimize expensive branch-circuit lengths.
Watch Out!
The NE Code refers to motor-operated appliances and/or to room air conditioners in Arts. 210, 422, and 440. Great care must be exercised in correlating the various Code rules in these different articles to ensure effective compliance with the letter and spirit of Code meaning. There is much crossover in terminology and references, making it difficult to tell whether a room air conditioner should be treated as an appliance circuit load or a motor load. However, a step-by-step approach to the problem which keeps in mind the intent of these provisions can resolve confusing points. Since the manufacturer is required to supply the motor-running overcurrent protection, no problems should arise concerning these devices. For larger units connected permanently to the distribution system, these can be treated directly as hermetic motor loads, using the provisions of Art. 440.
It may be assumed that a window or through-the-wall unit will operate satisfactorily on a standard fuse of the same rating as its attachment plug cap if there is no marking to the contrary on the unit. In any event, a time-delay fuse of the same or smaller rating could be substituted. If CBs are used for branch-circuit protection, a 15-A breaker will normally hold the starting current if a standard 15-A fuse will, since such breakers have inherent time delay. If the unit is marked to require a 15-A time-delay fuse and a 15-A breaker will not hold the starting current, few inspectors will object to the use of a 20-A breaker, since Art. 430 permits such a procedure for motor loads.
Normally, starting problems are not severe with these units, since the low inertia of present-day motor-compressor combinations permits them to reach full speed within a few cycles. Such a rapid drop in starting current is usually well within the time permitted by the trip or rupture characteristics of the breaker or fuse.
Similarly, the question of wire size may be resolved by application of Arts. 210, 422, or 440. Rarely do room conditioners even as large as 2 tons take more than 13-A running current; hence No. 12 copper or No. 10 aluminum conductors are more than sufficient. In addition, many local codes prohibit use of conductors smaller than No. 12. In localities where No. 14 wire may be used, provisions of 440.62(B), restricting the loading to 80 percent of the circuit rating, must determine the wire size, where “rating” is interpreted as referring to the conductor carrying capacity. If Art. 440 is used to determine the wire size, the 125 percent requirement of 440.32 gives the same result.
Figure 440-15 shows one feeder of an installation involving many room conditioners which practically eliminates branch-circuit wiring and will serve to illustrate the complications of circuit calculations for a multiple-unit installation. Total running current of each unit is 12 A as shown; hence, No. 14 copper wire could be used for branch-circuit conductors, protected by a 15-A fuse—either standard or time-delay. However, if the appropriate conductors of a 4-wire, 3-phase feeder were routed to the location of each conditioner and a combination fuseholder and receptacle was installed as shown, the only existing branch-circuit conductors would be the jumpers between the feeder, the fuseholder, and the receptacles. These jumpers, then, could be No. 14 wire. Assuming that the fuse-receptacle unit is mounted directly on or in close proximity with the junction box in which the tap to the feeder is made, the No. 14 wire is justified from the fuse to the feeder since it is not over 10 ft (3.0 m) long and is sufficient for the load supplied [240.21(B)(1)]. Since both motors usually start simultaneously, the total unit current is used to compute feeder conductor size and protection: 125 percent times 12 plus 24 is 39 A, permitting No. 8 conductors. However, this is practically the limit of the circuit’s capacity; there is no provision for overload, and voltage drop is very likely to be a factor at the end of the feeder. Therefore No. 6 conductors should be used.
Fig. 440-15. This type of circuiting was used for air conditioners in a hotel modernization project. (Sec. 440.62.)
Feeder protection is calculated on the basis of 300 percent times 12 plus 24, or 60-A fuses. Substitution of time-delay fuses for this 39-A feeder load would likely permit 45-A fuses.
Important: The rules of 440.62 apply only to cord-and-plug-connected room air conditioners. A unit room air conditioner that has a fixed (not cord-and-plug) connection to its supply must be treated as a group of several individual motors and protected in accordance with 430.53 and 440.22(B), covering several motors on one branch.
440.63. Disconnecting Means. A disconnect is required for every unit air conditioner. An attachment plug and receptacle or a separable connector may serve as the disconnecting means (Fig. 440-16).
Fig. 440-16. Plug-and-receptacle serves as required disconnect means. (Sec. 440.63.)
If a fixed connection is made to an A/C unit from the branch-circuit wiring system (i.e., not a plug-in connection to a receptacle), consideration must be given to a means of disconnect, as required in 422.33 and 422.34 for appliances:
For unit air conditioners in any type of occupancy, the branch-circuit switch or CB may, where readily accessible to the user of the appliance, serve as the disconnecting means. Figure 440-17 shows this, but the switch or CB is permitted to be out of sight by the exception to 422.34 when the A/C unit has an internal OFF switch—which all units do have. And 422.34 requires the disconnect means for a motor-driven appliance to be within sight from the air-conditioner unit.
Fig. 440-17. Branch circuit CB or switch may serve as disconnect. (Sec. 440.63.)
Because air conditioners have unit switches within them, the disconnect provisions of 422.34 may be applied. The internal unit switch with a marked OFF position that opens all ungrounded conductors may serve as the disconnect and is considered within sight as required by 422.34 in any case where there is another disconnect means as follows:
In multifamily (more than two) dwellings, the other disconnect means must be within the apartment where the conditioner is installed or on the same floor as the apartment.
In two-family dwellings, the other disconnect may be outside the apartment in which the appliance is installed. It may be the service disconnect.
In single-family dwellings, the service disconnect may serve as the other disconnect means—whether the branch circuit to the conditioner is fed from plug fuses or from a breaker or switch (Fig. 440-18).
Fig. 440-18. Service disconnect may be the “other disconnect” for A/C unit in private house. (Sec. 440.63.)
440.65. Leakage Current Detection and Interruption and Arc Fault Circuit Interrupter (AFCI). This rule requires a factory installed LCDI or AFCI protective device in the cord and within 300 mm (12 in.) of the plug cap for all single-phase, cord-and-plug-connected room A/Cs.
ARTICLE 445. GENERATORS
445.12. Overcurrent Protection. Alternating-current generators can be so designed that on excessive overload the voltage falls off sufficiently to limit the current and power output to values that will not damage the generator during a short period of time. Whether automatic overcurrent protection of a generator should be omitted in any particular case is a question that can best be answered by the manufacturer of the generator. It is a common practice to operate an exciter without overcurrent protection, rather than risk the shutdown of the main generator due to accidental opening of the exciter fuse or CB. In the case of constant-voltage generators, the NEC leaves this to the manufacturer, providing various options including “inherent design” as well as conventional over-current devices.
Figure 445-1 shows the connections of a 2-wire DC generator with a single-pole protective device. If the machine is operated in multiple with one or more other generators, and so has an equalizer lead connected to the positive terminal, the current may divide at the positive terminal, part passing through the series field and positive lead and part passing through the equalizer lead. The entire current generated passes through the negative lead; therefore the fuse or CB, or at least the operating coil of a CB, must be placed in the negative lead. The protective device should not open the shunt-field circuit, because if this circuit were opened with the field at full strength, a very high voltage would be induced which might break down the insulation of the field winding.
Fig. 445-1. With this connection, a single-pole CB can protect a 2-wire DC generator. (Sec. 445.12.)
Paragraph (C) is intended to apply particularly to generators used in electrolytic work. Where such a generator forms part of a motor-generator set, no fuse or CB is necessary in the generator leads if the motor-running protective device will open when the generator delivers 150 percent of its rated full-load current.
In paragraph (D), use of a balancer set to obtain a 3-wire system from a 2-wire main generator is covered, as shown in Fig. 445-2. Each of the two generators used as a balancer set carries approximately one-half the unbalanced load; hence these two machines are always much smaller than the main generator. In case of an excessive unbalance of the load, the balancer set might be overloaded while there is no overload on the main generator. This condition may be guarded against by installing a double-pole CB, with one pole connected in each lead of the main generator and with the operating coil properly designed to be connected in the neutral of the 3-wire system. In Fig. 445-2, the CB is arranged so as to be operated by either one of the coils A in the leads from the main generator or by coil B in the neutral lead from the balancer set.
Fig. 445-2. A balancer set supplies the unbalanced neutral current of a 3-wire system, with each generator carrying 25 of the 50-A unbalance. (Sec. 445.12.)
445.13. Ampacity of Conductors. The first three sentences clarify sizing of circuit conductors connecting a generator to the control and protective device(s) it serves:
1. The ampacity of the conductors between the generator terminals and the first panelboard, switchboard, or other comparable equipment that contains overcurrent protective devices must not be less than 115 percent of the generator nameplate current rating. Note that there is no requirement that the conductors arrive at the terminals of a single overcurrent device of a particular rating, or that the sum of the ratings of the overcurrent devices at the terminating location not exceed some quantity. The tap rules in 240.21(G) recognize whatever protection is provided under 445.12, and allow the generator output conductors indefinite length without additional protection as long as they meet the 115 percent ampacity criterion.
2. The neutral of the generator feeder may have its size reduced from the minimum capacity required for the phase legs. As with any feeder or service circuit, the neutral has to have only enough ampacity for the unbalanced load it will handle—as covered by 220.61.
3. When a generator neutral is not grounded at its terminals, as is permitted by 250.20, the neutral conductor from the generator must be sized not only for its unbalanced load, as required in 220.61, but also for carrying ground-fault current. For a generator feeder neutral to be adequately sized as an equipment grounding conductor, to effectively carry enough current to operate overcurrent devices in a grounded system, 250.24 requires that the neutral be not smaller than required by 250.66 and Table 250.66, or, where the phase conductors are larger than 1100-kcmil copper, or 1750-kcmil aluminum, the neutral must be not less than 12½ percent of the cross-sectional area of the largest phase leg of the generator feeder. (See Fig. 445-3.)
445.14. Protection of Live Parts. As a general rule, no generator should be “accessible to unqualified persons.” If necessary to place a generator operating at over 50 V to ground in a location where it is so exposed, the commutator or collector rings, brushes, and any exposed terminals should be provided with guards which will prevent any accidental contact with these live parts.
Fig. 445-3. Neutral must have adequate capacity for generator that is not a “separately derived” system source (i.e., does not have its neutral bonded and grounded). (Sec. 445.13.)
445.18. Disconnecting Means Required for Generators. Lockable disconnect(s) are required for generators unless the generator’s driving machine can be readily shut down and it is not running in parallel with another source of voltage, such as another generator. The “(s)” is a recent NEC change, and reflects the useful idea that an excellent design option is to establish multiple disconnects right at the generator so multiple systems can originate at the generator (such as emergency, legally required standby, and optional standby) without issues of system separation at the occupancy served.
ARTICLE 450. TRANSFORMERS AND TRANSFORMER VAULTS (INCLUDING SECONDARY TIES)
450.1. Scope. The exceptions indicate those transformer applications that are not subject to the rules of Art. 450. The exceptions shown in Fig. 450-1 are as follows:
Exception No. 2 excludes any dry-type transformer that is a component part of manufactured equipment, provided that the transformer complies with the requirements for such equipment. Those requirements include UL standards on the construction of the particular equipment. This exclusion applies, for instance, to control transformers within a motor starter or within a motor control center. However, although such transformers do not have to be protected in accordance with 450.3(B), such control transformer circuits must have their control conductors protected as described under 430.72(B). But a separate control transformer—one that is external to other equipment and is not an integral part of any other piece of equipment—must conform to the protection rules of 450.3 and other rules in Art. 450.
Fig. 450-1. These transformer applications are exempt from the rules of Art. 450. (Sec. 450.1.)
Exception No. 6 points out that ballasts for electric-discharge lighting (although they are transformers—either autotransformers or separate-winding, magnetically coupled types) are treated as lighting accessories rather than transformers.
Exception No. 8 notes that liquid-filled or dry-type transformers used for research, development, or testing are exempt from the requirements of Art. 450, provided that effective arrangements are made to safeguard any persons from contacting energized terminals or conductors. Again, in the interest of the unusual conditions that frequently prevail in industrial occupancy, this rule recognizes that transformers used for research, development, or testing are commonly under the sole control of entirely competent individuals and exempts such special applications from the normal rules that apply to general-purpose transformers used for distribution within buildings and for energy supply to utilization equipment, controls, signals, communications, and the like. (See Fig. 450-2.)
Fig. 450-2. Transformers that are set up in a laboratory to derive power for purposes of testing other equipment or powering an experiment are exempt from the rules of Art. 450, provided care is taken to protect personnel from any hazards due to exposed energized parts. (Sec. 450.1.)
UL listing The Electrical Construction Materials Directory of the UL lists “Transformers—Power and General Purpose, Dry Type.” To satisfy NE Code and OSHA regulations, as well as local code rules on acceptability of equipment, any transformers of the types and sizes covered by UL listing must be so listed. Use of an unlisted transformer of a type and size covered by UL listing would certainly be considered a violation of the spirit of NE Code 110.3. UL listing covers air-cooled types rated up to 500 kVA for single-phase transformers and up to 1500 kVA for 3-phase units (all up to 600-V rating).
450.2. Definitions. A transformer is an individual transformer, single or polyphase, identified by a single nameplate, unless otherwise indicated in this article. Three single-phase transformers connected for a 3-phase transformation must be taken as three transformers, not one. This definition helps to clarify the contents of some of the rules of Art. 450.
450.3. Overcurrent Protection. This section covers overcurrent protection in great detail, and other Code rules (240.21, 240.40, and 408.16 in particular) usually get involved in transformer applications. Although there is no rule on disconnects, use of required overcurrent protection results in the presence of a fused switch or CB that may serve as disconnecting means.
It should be understood that the overcurrent protection required by this section is for transformers only. Such overcurrent protection will not necessarily protect the primary or secondary conductors or equipment connected on the secondary side of the transformer. Using overcurrent protection to the maximum values permitted by these rules would require much larger conductors than the full-load current rating of the transformer (other than permitted in the 25-ft [7.5-m] tap rule in 240.21). Accordingly, to avoid using oversized conductors, overcurrent devices should be selected at about 110 to 125 percent of the transformer full-load current rating. And when using such smaller overcurrent protection, devices should be of the time-delay type (on the primary side) to compensate for inrush currents which reach 8 to 10 times the full-load primary current of the transformer for about 0.1 s when energized initially.
In approaching a transformer installation it is best to use a one-line diagram, such as shown in the accompanying sketches. Then by applying the tap rules in 240.21, proper protection of the conductors and equipment, which are part of the system, will be achieved. See comments following 240.21.
240.4(F) is the only Code rule that considers properly sized primary overcurrent devices to protect the secondary conductors without secondary protection and no limit to the length of secondary conductors.
On 3- and 4-wire transformer secondaries, it is possible that an unbalanced load may greatly exceed the secondary conductor ampacity, which was selected assuming balanced conditions. Because of this, the NE Code does not permit the protection of secondary conductors by overcurrent devices operating through a transformer from the primary of a transformer having a 3-wire or 4-wire secondary. For other than 2-wire to 2-wire or 3-wire to 3-wire delta-delta transformers, protection of secondary conductors has to be provided completely separately from any primary-side protection. In designing transformer circuits, the rules of 450.3 can be coordinated with 240.21(C), which provides special rules for tap conductors used with transformers. This general procedure allows for simultaneous protection of both the transformer and the conductors connected on both sides of it. Refer to the extensive discussion in 240.21 in Chap. 2 of this book. Those rules need not be covered again here.
Part (A) of this section mandates compliance with Table 450.3(A). That table sets rules for overcurrent protection of any transformer (dry-type or liquid-filled) rated over 600 V, and, as covered in Note 4 to Table 450.3(A), electronically actuated fuses that are adjustable to a “specific current” may also be rated at 300 percent. Protection may be provided either by a protective device of specified rating on the transformer primary or by a combination of protective devices of specified ratings on both the primary and secondary. Figure 450-3 shows the basic rules of such overcurrent protection. The fact that E-rated fuses used for high-voltage circuits are given melting times at 200 percent of their continuous-current rating explains why this Code rule used to set 150 percent of primary current as the maximum fuse rating but permits CBs up to 300 percent. In effect, the 150 percent for fuses times 2 becomes 300 percent—the maximum value allowed for a CB. Now such fuses may be rated up to 250 percent (instead of 150 percent) for transformers with 6 percent or less impedance.
The basic requirement for “Any Location” in Table 450.3(A) says that any high-voltage transformer must have both primary and secondary protection based on Table 450.3(A) for maximum ratings of the primary and secondary fuses or circuit breakers.
The rules under “supervised locations only” give two alternative ways of protecting high-voltage transformers where “conditions of maintenance and supervision assure that only qualified persons will monitor and service the transformer.” The two alternatives are as follows:
1. Primary protection only may be used, with fuses set at not over 250 percent of the primary current or circuit breakers set at not over 300 percent of primary current. And if that calculation results in a fuse or CB rating that does not correspond to a standard rating or setting, the next higher standard rating or setting may be used.
2. Primary and secondary protection based on Table 450.3(A) is the alternative.
These rules resulted from concerted industry action to provide better transformer protection. As stated in the substantiation for 450.3(A), “It is felt that this approach will aid in reducing the number of transformer failures due to overload, as well as maintaining the flexibility of design and operation by industry and the more-complex commercial establishments.”
Part (B) of this section covers all transformers—oil-filled, high-fire-point liquid-insulated, and dry-type—rated up to 600 V. The step-by-step approach to such protection is as follows:
1. For any transformer rated 600 V or less (i.e., the rating of neither the primary nor the secondary winding is over 600 V), the basic overcurrent protection may be provided just on the primary side [Table 450.3(B) (top row)] or may be a combination of protection on both the primary and secondary sides [Table 450.3(B) (bottom row)].
If a transformer is to be protected by means of a CB or set of fuses only on the primary side of the transformer, the basic arrangement is as shown in Fig. 450-4.
In that layout, a CB or a set of fuses rated not over 125 percent of the transformer rated primary full-load current provides all the overcurrent protection required by the NE Code for the transformer. This overcurrent protection is in the feeder circuit to the transformer and is logically placed at the supply end of the feeder, so the same overcurrent device may also provide the overcurrent protection required for the primary feeder conductors. There is no limit on the distance between primary protection and the transformer. When the correct maximum rating for transformer protection is selected and installed at any point on the supply side of the transformer (either near or far from the transformer), then feeder circuit conductors must be sized so that the CB or fuses selected will provide the proper protection as required for the conductors. The ampacity of the feeder conductors must be at least equal to the amp rating of the CB or fuses unless 240.4(B) is satisfied. That is, when the rating of the overcurrent protection selected is not more than 125 percent of rated primary current, the primary feeder conductor may have an ampacity such that the overcurrent device is the next higher standard rating.
Fig. 450-3. High-voltage transformers (rated over 600 V, dry or fluid-filled) with any impedance must be protected in one of these ways. However, the value of OC protection for transformers with known internal impedances as indicated in Table 450.3(A) may be based on the particular impedance. [Table 450.3(A).]
Fig. 450-4. This is the basic rule on primary-side protection for transformers with primary current over 9 A. (Table 450.3.)
The rules set down for protection of a 600-V transformer by a CB or set of fuses in its primary circuit are given in Fig. 450-5 for transformers with rated primary current of 9 A or more. Note 1 to Table 450.3(B) says that “the next higher standard” rating of protection may be used, if needed. Figure 450-6 shows the absolute maximum values of protection for smaller transformers. When using the 1.67 or 3 times factor, if the resultant current value is not exactly equal to a standard rating of fuse or CB, then the next lower standard rated fuse or CB must be used.
When the rules of Table 450.3(B) are observed, the transformer itself is properly protected and the primary feeder conductors, if sized to correspond, may be provided with the protection required by 240.4. But all considerations on the secondary side of the transformer then have to be separately and independently evaluated. When a transformer is provided with primary-side overcurrent protection, a whole range of design and installation possibilities are available for secondary arrangement that satisfies the Code. The basic approach is to provide required overcurrent protection for the secondary circuit conductors right at the transformer—such as by a fused switch or CB attached to the transformer enclosure, as shown in Fig. 450-7. Or 10- or 25-ft (3.0- or 7.5-m) taps may be made, as covered in 240.21.
2. There is another acceptable way to protect a 600-V transformer, described in Table 450.3(B). In this method, the transformer primary may be fed from a circuit which has overcurrent protection (and circuit conductors) rated up to 250 percent (next higher standard size permitted) of rated primary current (instead of 125 percent, as previously)—but, in such cases, there must be a protective device on the secondary side of the transformer, and that device must be rated or set at not more than 125 percent of the transformer’s rated secondary current (Fig. 450-8). This secondary protective device must be located right at the transformer secondary terminals or not more than the length of a 10- or 25-ft (3.0- or 7.5-m) tap away from the transformer, and the rules of 240.21 on tap conductors must be fully satisfied.
Fig. 450-5. Protection sizing for larger transformers is 125 percent of primary current. (Sec. 450.3.)
The secondary protective device covered by Table 450.3(B) may readily be incorporated as part of other required provisions on the secondary side of the transformer, such as protection for a secondary feeder from the transformer to a panel or switchboard or motor control center fed from the switchboard. And a single secondary protective device rated not over 125 percent of secondary current may serve as a required panelboard main as well as the required transformer secondary protection, as shown at the bottom of Fig. 450-9.
The use of a transformer circuit with primary protection rated up to 250 percent of rated primary current offers an opportunity to avoid situations where a particular set of primary fuses or CB rated at only 125 percent would cause nuisance tripping or opening of the circuit on transformer inrush current. But the use of a 250 percent rated primary protection has a more common and widely applicable advantage in making it possible to feed two or more transformers from the same primary feeder. The number of transformers that might be used in any case would depend on the amount of continuous load on all the transformers. But in all such cases, the primary protection must be rated not more than 250 percent of any one transformer, if they are all the same size, or 250 percent of the smallest transformer, if they are of different sizes. And for each transformer fed, there must be a set of fuses or CB on the secondary side rated at not more than 125 percent of rated secondary current, as shown in Fig. 450-10.
Fig. 450-6. Higher-percent protection is permitted for smaller transformers. (Table 450.3.)
Fig. 450-7. Protection of secondary circuit must be independent of primary-side transformer protection. (Sec. 450.3.)
Fig. 450-8. Secondary protection permits higher-rated primary protective device. (Table 450.3.)
Fig. 450-9. With 250 percent primary protection, secondary protection may be located like this. (Table 450.3.)
Figure 450-11 shows an example of application of 250 percent primary protection to a feeder supplying three transformers (such as at the bottom of Fig. 450-10). The example shows how the rules of Table 450.3(B) must be carefully related to 240.21 and other Code rules:
Fig. 450-10. With primary 250 percent protection, primary circuit may vary. (Sec. 450.3.)
Part (B)(3) of 240.21 of the NE Code is a rule that covers use of a 25-ft (7.5-m) unprotected tap from feeder conductors, with a transformer inserted in the 25-ft (7.5-m) tap. This rule does not eliminate the need for secondary protection—it makes a special condition for placement of the secondary protective device. It is a restatement of part (B)(3) as applied to a tap containing a transformer and applies to both single-phase and 3-phase transformer feeder taps.
Figure 450-11 shows a feeder supplying three 45-kVA transformers, each transformer being fed as part of a 25-ft (7.5-m) feeder tap that conforms to part (B)(3) of 240.21.
Fig. 450-11. Code rules must be tied together. (Sec. 450.3.)
Although each transformer has a rated primary current of 54 A at full load, the demand load on each transformer primary was calculated to be 41 A, based on secondary loading. No. 1 THW copper feeder conductors were considered adequate for the total noncontinuous demand load of 3 × 41 A, or 123 A. A step-by-step analysis of this system follows. Refer to circled letters in the figure:
A. The primary circuit conductors are No. 6 TW rated at 55 A, which gives them “an ampacity at least that of the overcurrent protection from which they are tapped . . . ,” because these conductors are tapped from the feeder conductors protected at 125 A. No. 6 TW is okay for the 41-A primary current.
B. The 125-A fuses in the feeder switch properly protect the No. 1 THW feeder conductors, which are rated at 130 A.
C. The conductors supplied by the transformer secondary must have “an ampacity that, when multiplied by the ratio of the secondary-to-primary voltage, is at least the ampacity of the conductors or overcurrent protection from which the primary conductors are tapped . . .” The ratio of secondary-to-primary voltage of the transformer is
Note that phase-to-phase voltage must be used to determine this ratio.
Then, for the secondary conductors, 240.21(B)(3)b says that
Then, minimum conductor ampacity equals
The No. 1 TW secondary conductors, rated at 110 A, are above the 96-A minimum and are, therefore, satisfactory.
D. The total length of the unprotected 25-ft (7.5-m) tap—that is, the primary conductor length plus the secondary conductor length (x + y) for any circuit leg—must not be greater than 25 ft (7.5 m).
E. The secondary tap conductors from the transformer must terminate in a single CB or set of fuses that will limit the load on those conductors to their rated ampacity from Table 310.16. Note that there is no exception given to that requirement and the “next higher standard device rating” may not be used if the conductor ampacity does not correspond to the rating of a standard device.
The overcurrent protection required at E, at the load end of the 25-ft (7.5-m) tap conductors, must not be rated more than the ampacity of the No. 1 TW conductors.
Max. rating of fuses or CB at E = 110 A
But a 100-A main would satisfy the 96-A secondary load. (Note: The overcurrent protective device required at E could be the main protective device required for a panel fed from the transformer.)