Although the foregoing calculation shows how unprotected taps may be made from feeder conductors by satisfying the rules of 240.21(B)(3), the rules of 240.21 are all concerned with PROTECTION OF CONDUCTORS ONLY. Consideration must now be made of transformer protection, as follows:
1. Note 240.21(B)(3) makes no reference to transformer protection. [240.21(C) is followed by an FPN that references 450.3.] But 450.3 calls for protection of transformers, and there is no exception made for the conditions of part (B)(3) to 240.21.
2. It is clear from 450.3(B) and Table 450.3(B) that the transformer shown in Fig. 450-11 is not protected by a primary-side overcurrent device rated not more than 125 percent of primary current (54 A), because 1.25 × 54 A = 68 A, maximum.
3. But 450.3(B)(2) does offer a way to provide required protection. The 110-A protection at E is secondary protection rated not over 125 percent of rated secondary current (1.25 × 125 A secondary current = 156 A). With that secondary protection, a primary feeder overcurrent device rated not more than 250 percent of rated primary current will satisfy 450.3(B)(2). That would call for fuses in the feeder switch (or a CB) (at B in the diagram) rated not over
250 percent × 54 A primary current = 135 A
But, the fuses in the feeder switch are rated at 125 A—which are not in excess of 250 percent of transformer primary current and, therefore, satisfy Table 450.3(B).
In addition to the two basic methods for protecting transformers previously described, Note 3 to Table 450.3(B) also provides for protection with a built-in thermal overload protection, as shown in Fig. 450-12.
450.4. Autotransformers 600 V, Nominal, or Less. This section sets the rules for the permitted connections to and the required overcurrent protection for autotransformers. It is important to place these rules in the context of provisions in 210.9 and 215.11 that limit their use in some circumstances. In general, a branch circuit or a feeder may be supplied through an autotransformer only if the system supplied has a grounded conductor which is electrically connected to a grounded conductor of the system supplying the autotransformer, with the following exceptions:
1. An autotransformer can be used to extend or add a branch circuit for an equipment load without the connection to a similar grounded conductor when transforming from a nominal 208-V to a nominal 240-V supply or similarly from 240 to 208 V.
2. In industrial occupancies, where conditions of maintenance and supervision ensure that only qualified persons will service the installation, autotransformers can be used to supply nominal 600-V loads from nominal 480-V systems, and 480-V loads from nominal 600-V systems, without the connection to a similar grounded conductor.
Fig. 450-12. Built-in protection is another technique for transformers. [Note 3 to Table 450.3(B).]
This transformation is commonly employed to connect 480-V equipment in a factory with a 600-V distribution, or vice versa. A 480 to 120-V, two-winding transformer is connected with its windings in series so it functions as an autotransformer, as shown in Fig. 450-13. There is an easy way to determine how to size the two-winding transformer that will be connected in this way. If C = transformation capacity as an autotransformer; X = rated transformation capacity as a two-winding transformer; and R = the voltage ration of the two windings, then
example Suppose a 50-hp and a 3-hp motor, both rated 460 V, will be connected to a 575-V, 3-phase system. If two 480 V/120 V two-winding transformers will be connected in open delta to service the loads, what sized transformers should be purchased?
The full-load ratings of the motors must be evaluated using the NEC Table 430.250. The 50-hp motor draws 65 A and the 3-hp motor 4.8 A. Converting to kilovolt-amperes, 65 A × 460
/(1000) = 52 kVA and 4.8 A × 460/(1000) = 4 kVA.
Add 13 kVA (25 percent of the largest motor) to get 69 kVA total.
In the formula, R will be 480 V ÷ 120 V = 4, and C will be 69 kVA.
Then the capacity required will be 69 kVA ÷ 5 or 14 kVA.
Remember that the capacity of an open delta transformer bank is not the sum of the two transformers; instead, the capacity is that of the two transformers multiplied by half the square root of three. Two 10-kVA two-winding transformers would be a good choice in this case. The final capacity would be (10 kVA + 10 kVA)(½) = 17 kVA.
Fig. 450-13. A 480- to 120-V transformer connected as an autotransformer to run 460-V equipment on a 575-V system. (Sec. 450.4.)
Note that this 480-V transformer (120-V secondary) will be connected to a 600-V supply voltage. The NEC requires that field-connected autotransformers be identified for use at the elevated voltage because the dielectric withstand test is often done at a lower level for windings rated 250 V or less, and in this case the 120-V winding must be tested as though it were a 600-V winding. Transformer manufacturers are very familiar with this application and appropriate products are readily available.
The NEC requires autotransformers rated 600 V, nominal, or less be protected by an individual overcurrent device in series with each ungrounded conductor, rated or set at not more than 125 percent of the rated full-load input current of the autotransformer. If this calculation does not correspond to a standard rating and the rated input current is 9 A or more, the next higher standard device is permitted. No overcurrent device is permitted to be placed in series with the shunt winding.
Referring to Fig. 450-13, no overcurrent device should be placed between points “A” and “C.” If the overcurrent device opened and the load impedance were low, the input 575 V would return to the autotransformer at some voltage approaching 575 V across the series winding (between “B” and “C” in Fig. 450-13). Given the 1:4 winding ratio, that could result in a voltage on the 460 V side of the autotransformer exceeding 2000 V.
As an example of an overcurrent protective device sizing calculation, the two 10-kVA transformers used in the Sec. 5.117 analysis can supply 17.3 kVA of load transformation. In the formula, rearrange the terms to solve for C: C = X(R + 1), or in this case 17.3 kVA (4 + 1) = 87 kVA; 87,000 VA ÷ (575 V × ) = 87 A. This arrangement of autotransformers must be protected at not more than 125 percent of this number, or 108 A, with the next higher standard size protective device (110 A) permitted.
450.5. Grounding Autotransformers. An existing, ungrounded, 480-V system derived from a delta transformer hookup can be converted to grounded operation in two basic ways:
First, one of the three phase legs of the 480-V delta can be intentionally connected to a grounding electrode conductor that is then run to a suitable grounding electrode. Such grounding would give the two ungrounded phases (A and B) a voltage of 480 V to ground. The system would then operate as a grounded system, so that a ground fault (phase-to-conduit or other enclosure) on the secondary can cause fault-current flow that opens a circuit protective device to clear the faulted circuit.
But corner grounding of a delta system does not give the lowest possible phase-to-ground voltage. In fact, the voltage to ground of a corner-grounded delta system is the same as it is for an ungrounded delta system because voltage to ground for ungrounded circuits is defined as the greatest voltage between the given conductor and any other conductor of the circuit. Thus, the voltage to ground for an ungrounded delta system is the maximum voltage between any two conductors, on the assumption that an accidental ground on any one phase puts the other two phases at full line-to-line voltage aboveground.
In recognition of increasing emphasis on the safety of grounded systems over ungrounded systems, 450.5 covers the use of zig-zag grounding autotransformers to convert 3-phase, 3-wire, ungrounded delta systems to grounded wye systems. Such grounding of a 480-V delta system, therefore, lowers the voltage to ground from 480 V (when ungrounded) to 277 V (the phase-to-grounded-neutral voltage) when converted to a wye system (Fig. 450-14).
Fig. 450-14. Zig-zag transformer changes voltage to ground from 480 to 277 V. (Sec. 450.5.)
A zig-zag grounding autotransformer gets its name from the angular phase differences among the six windings that are divided among the three legs of the transformer’s laminated magnetic core assembly. The actual hookup of the six windings is an interconnection of two wye configurations, with specific polarities and locations for each winding. Just as a wye or delta transformer hookup has a graphic representation that looks like the letter Y or the Greek letter 
, so a zig-zag grounding autotransformer is represented as two wye hookups with pairs of windings in series but phase-displaced, as in Fig. 450-15.
With no ground fault on any leg of the 3-phase system, current flow in the transformer windings is balanced, because equal impedances are connected across each pair of phase legs. The net impedance of the transformer under balanced conditions is very high, so that only a low level of magnetizing current flows through the windings. But when a ground fault develops on one leg of the 3-phase system, the transformer windings become a very low impedance in the fault path, permitting a large value of fault current to flow and operate the circuit protective device—just as it would on a conventional grounded-neutral wye system, as shown in Fig. 450-16.
Fig. 450-15. Windings of zig-zag transformer provide for flow of fault or neutral current. (Sec. 450.5.)
Because the kilovoltampere rating of a grounding autotransformer is based on short-time fault current, selection of such transformers is much different from sizing a conventional 2-winding transformer for supplying a load. Careful consultation with a manufacturer’s sales engineer should precede any decisions about the use of these transformers.
Fig. 450-16. Zig-zag transformer converts ungrounded system to grounded operation. (Sec. 450.5.)
Such transformers shall have a continuous per-phase current rating and a continuous neutral current rating. Zig-zag connected transformers must never be connected on the load side of any system grounding connections, including those made in accordance with 250.24(B), 250.30(A)(1), or 250.32(B) Exception.
Part (A) of the section covers the use of these transformers when an actual 480Y/277-V distribution system is being created. Zig-zag grounding autotransformers are comprised of two three-phase wye connections in series. Figure 450-17 shows most of the requirements as well as the angular displacements that give the transformation its name. The inner connections are a straightforward wye, and close inspection of the three outer segments of the pinwheel shows that they are also oriented in a wye configuration. In fact, at least one major transformer manufacturer now sells zig-zag customers three 2-winding transformers (thus, six windings) along with instructions as to how to connect them to achieve this result. (See also the right side of Fig. 450-15.) The simultaneous disconnection rule is crucial because if a single-pole overcurrent device were to open, the grounding transformer will no longer function as intended, including a strong likelihood of dangerous transient overvoltages on its load side. A grounding autotransformer used to create a 3-phase, 4-wire distribution system must comply with the following four requirements:
Fig. 450-17. Zig-zag connections must meet numerous NEC requirements. (Sec. 450.5.)
1. The transformer must be directly connected to the ungrounded phase conductors and never switched or provided with overcurrent protection that is independent of the main switch and common-trip overcurrent protection for the 3-phase, 4-wire system. A transformer used to limit transitory overvoltages must be of suitable rating and must be connected in accordance with this rule.
2. An overcurrent sensing device must be provided that will cause the main switch or common-trip overcurrent protection previously discussed to open if the load on the autotransformer reaches or exceeds 125 percent of the continuous current per-phase or neutral rating. Delayed tripping for temporary overcurrents sensed at the autotransformer overcurrent device is permitted for the purpose of allowing proper operation of branch or feeder protective devices on the 4-wire system.
3. A fault-sensing system that causes the opening of a main switch or common trip overcurrent device for the 3-phase, 4-wire system must be provided to guard against single phasing or internal faults. An explanatory note advises that this can be accomplished by the use of two subtractive-connected donut-type current transformers installed to sense and signal when an unbalance occurs in the line current to the autotransformer of 50 percent or more of rated current.
4. The autotransformer must have a continuous neutral-current rating that is sufficient to handle the maximum possible neutral unbalanced load current of the 4-wire system.
Note that there is now an express prohibition in the parent language regarding installing a zig-zag on the load side of any system grounding connection. These transformer hookups are never a substitute for running a neutral to a load, regardless of how remote. If the supply system is grounded at any point, then a parallel neutral return path will exist and this will desensitize the protective controls for the transformer.
Part (B) covers grounding autotransformers that do not supply utilization loads, but instead merely provide a ground reference and a specified magnitude of ground-fault current for the operation of ground-responsive protective devices, typically so an ungrounded system can be grounded through an impedance. Refer to Fig. 450-17, but insert the grounding impedance as indicated and eliminate all load connections to the neutral. There are three requirements:
1. The autotransformer must have a continuous neutral current rating sufficient for the specified ground-fault current.
2. An overcurrent device having an interrupting rating suitable for the fault current available at its supply terminals, and that will open simultaneously all ungrounded conductors when it operates, must be connected in the grounding autotransformer branch circuit.
3. This overcurrent protection must be rated or set at a current not exceeding 125 percent of the autotransformer continuous per-phase current rating or 42 percent of the continuous-current rating of any series-connected devices in the autotransformer neutral connection. Delayed tripping for temporary overcurrents to permit the proper operation of ground-responsive tripping devices on the main system is permitted but must not exceed values that would be more than the short-time current rating of the grounding autotransformer or any series connected devices in the neutral connection.
The 42 percent rule reflects the fact that the current in each phase is 33 percent of the neutral return current, and 125 percent of this is about 42 percent. An exception follows for high-impedance grounded systems, for which the 125 percent rule is impracticable because the neutral current through the impedance is so low that 125 percent of this current does not come close to a conventional overcurrent device setting or rating.
For high-impedance grounded systems covered in 250.36, where the maximum ground-fault current is designed to be not more than 10 A, and where the grounding autotransformer and the grounding impedance are rated for continuous duty, an overcurrent device rated not more than 20 A that will simultaneously open all ungrounded conductors can be installed on the line side of the zig-zag.
450.6. Secondary Ties. In industrial plants having very heavy power loads it is usually economical to install a number of large transformers at various locations within each building, the transformers being supplied by primary feeders operating at voltages up to 13,800 V. One of the secondary systems that may be used in such cases is the network system.
The term network system as commonly used is applied to any secondary distribution system in which the secondaries of two or more transformers at different locations are connected together by secondary ties. Two network layouts of unit subs are shown in Fig. 450-18. In the spot network, two or three transformers in one location or “spot” are connected to a common secondary bus and divide the load. Upon primary or transformer fault, the secondary is isolated from the faulted section by automatic operation of the network protector, providing a high order of supply continuity in the event of faults. The general form of the network system is similar, except that widely separated individual substations are used with associated network protectors, and tie circuits run between the secondary bus sections. The system provides for interchange of power to accommodate unequal loading on the transformers. Limiters protect the ties. The purpose of the system is to equalize the loading of the transformers, to reduce voltage drop, and to ensure continuity of service. The use of this system introduces certain complications, and, to ensure successful operation, the system must be designed by an experienced electrical engineer.
The provisions of 450.3 govern the protection in the primary. Refer to Fig. 450-18. The network protector consists of a CB and a reverse-power relay. The protector is necessary because without this device, if a fault develops in the transformer, or, in some cases, in the primary feeder, power will be fed back to the fault from the other transformers through the secondary ties. The relay is set to trip the breaker on a reverse-power current not greater than the rated secondary current of the transformer. This breaker is not arranged to be tripped by an overload on the secondary of the transformer.
Fig. 450-18. These are the two basic types of network systems. (Sec. 450.6.)
450.6(A)(3) provides that:
1. Where two or more conductors are installed in parallel, an individual protective device is provided at each end of each conductor.
2. The protective device (fusible link or CB) does not provide overload protection, but provides short-circuit protection only.
In the case of a short circuit, the protective device must open the circuit before the conductor reaches a temperature that would injure its insulation. The principles involved are that the entire system is so designed that the tie conductors will never be continuously overloaded in normal operation—hence protection against overloads of less severity than short circuits is not necessary—and that the protective devices should not open the circuit and thus cause an interruption of service on load peaks of such short duration that the conductors do not become overheated.
A limiter is a special type of fuse having a very high interrupting capacity. Figure 450-19 is a cross-sectional view of one type of limiter. The cable lug, the fusible section, and the extension for connection to the bus are all made in one piece from a length of copper tubing, and the enclosing case is also copper. A typical device of this type is rated to interrupt a current of 50,000 A without perceptible noise and without the escape of flame or gases from the case.
Fig. 450-19. A limiter is a cable connection device containing a fusible element. (Sec. 450.6.)
Figure 450-20 is a single-line diagram of a simple 3-phase industrial-plant network system. The primary feeders may operate at any standard voltage up to 13,800 V, and the secondary voltage would commonly be 480 V. The rating of the transformers used in such a system would usually be within the range of 300 to 1000 kVA. The diagram shows two primary feeders, both of which are carried to each transformer, so that by means of a double-throw switch each transformer can be connected to either feeder. Each feeder would be large enough to carry the entire load. It is assumed that the feeders are protected in accordance with 450.3(A) so that no primary overcurrent devices are required at the transformers. The secondary ties consist of two conductors in multiple per phase and it will be noted that these conductors form a closed loop. Switches are provided so that any section of the loop, including the limiters protecting that section, can be isolated in case repairs or replacements should be necessary.
450.7. Parallel Operation. To operate satisfactorily in parallel, transformers should have the same percentage impedance and the same ratio of reactance to resistance. Information on these characteristics should be obtained from the manufacturer of the transformers.
450.8. Guarding. Figure 450-21 summarizes these rules. Refer to 110.27 on guarding of live parts. Safety to personnel is always important, particularly where a transformer is to operate with live parts. To protect against accidental contact with such components, isolate the unit or units in a room or place accessible only to qualified personnel and guard live parts, such as with a railing. When elevation is used for safeguarding live parts, consult 110.34(E) and 110.27.
As noted in part (C), switches and other equipment operating at up to 600 V and serving only circuits within a transformer enclosure may be installed within the enclosure if it is accessible to qualified persons only. This is intended to be part of the requirement that exposed, energized parts must be properly guarded. Part (D) requires that the operating voltage of exposed live parts be posted on the equipment or support structures.
Fig. 450-20. A typical industrial plant network distribution system. (Sec. 450.6.)
Fig. 450-21. Transformer installations must be effectively guarded. (Sec. 450.8.)
450.9. Ventilation. As noted in the last paragraph, ventilation grills or slots in the sides or back of transformer enclosures must have adequate clearance from walls and objects to ensure free and substantial airflow through the case. Further, the required clearance must be “clearly marked” on the transformer. Because of the tight quarters in today’s electrical rooms and the tendency to give as little space as possible to electrical equipment, this is a critically important installation requirement to ensure safety and trouble-free operation of enclosed transformers. The substantiation for this proposal stated the following:
Today’s widespread use of dry type power transformers indoors has resulted in the common practice of their being installed directly up against walls completely blocking the rear vents. As inspectors, we frequently wind up trying to find out if the transformer installation instructions are anywhere around so we can see what clearances the manufacturer has specified.
Clearly, this is not the best system. Furnaces, for example, commonly have required clearances marked on the nameplate. This proposal will let people know that clearances are required. Secondly, it will let us know what these clearances are.
(See Fig. 450-22, top.)
450.13. Accessibility. Accessibility is an important location feature of transformer installation. The NE Code generally requires a transformer (whether liquid-filled or dry-type) to be readily accessible to qualified personnel for inspection and maintenance (Fig. 450-22). That is, it must be capable of being reached quickly for operation, repair, or inspection without requiring use of a portable ladder to get at it, and it must not be necessary to climb over or remove obstacles to reach it. A transformer may be mounted on a platform or balcony, but there must generally be fixed stairs or a fixed ladder for access to the transformer (Fig. 450-23). However, there are some alternatives in subparts (A) and (B):
Part (A) permits dry-type transformers rated 600 V or less to be located “in the open on walls, columns, or structures”—without the need to be readily accessible. This allowance does not apply to a liquid-cooled transformer. A transformer suspended from the ceiling or hung on a wall—in which cases a ladder would be required to reach them because they are over 6½ ft (1.98 m) above the floor—would be okay, as shown in Fig. 450-24. There is no kilovoltampere limit on this provision, as opposed to the next paragraph. Part (B) permits dry-type transformers up to 600 V, 50 kVA, to be installed in fire-resistant hollow spaces of buildings not permanently closed in by structure, provided the transformer is designed to have adequate ventilation for such installation. (See Fig. 450-25.)
Note in part (A) that a transformer may be not readily accessible only if it is located in the open. The words “in the open” do not readily and surely relate to such words as “concealed” or “exposed.” But it is reasonable to conclude that “in the open” would be difficult to equate with “above a suspended ceiling.” The latter location is in a generally smaller, confined space in which the transformers must be readily accessible; part (A) must be taken as a condition in which a transformer does not have to be readily accessible—that is, that it may be mounted up high where a portable ladder would be needed to get at it. And, the wording of part (B) specifically recognizes installation of transformers rated 50 kVA or less to be installed in “hollow spaces” such as hung ceilings (Fig. 450-26).
Fig. 450-22. Basic rule calls for every transformer to have adequate ventilation and ready, easy, direct access for inspection or maintenance. (Secs. 450.9 and 450.13.)
Fig. 450-23. Transformers must be “readily accessible” without need for portable ladder to reach them. (Sec. 450.13.)
Fig. 450-24. Transformer mounted on wall (or suspended from ceiling) would be considered not readily accessible because a ladder would be needed to reach it. But, because it is “in the open,” such use conforms to 450.13(A). (Sec. 450.13.)
Fig. 450-25. Recessed mounting of dry-type transformers is permitted within ”fire-resistant hollow spaces,” as in hospitals, schools, and other commercial or institutional buildings. (Sec. 450.13.)
Fig. 450-26. Watch out for transformers above suspended ceilings. (Sec. 450.13.)
450.21. Dry-Type Transformers Installed Indoors. This rule differentiates between dry-type transformers based on kilovoltampere rating. All dry-type transformers rated at 112½ kVA or less, at up to 35 kV, must be installed so that a minimum clearance of 12 in. (305 mm) is provided between the transformer and any combustible material, or a fire barrier must be provided.
The last sentence in 450.21(A) recognizes the use of fire-resistant heat-insulating barriers instead of space separation for transformers rated not over 112½ kVA. But be aware that clearances required to ensure proper ventilation of the transformer must be provided to satisfy 450.9. That is, the minimum clearance called for by the manufacturer to ensure proper airflow for cooling must be provided between the insulating barrier and the transformer’s ventilation openings.
The exception permits those transformers rated 600 V or less that are completely enclosed to be installed closer than the 12-in. (300-mm) minimum, but consideration must always be given to the requirements of 450.9 for those enclosed transformers with ventilation openings, as indicated in the previous paragraph.
For units rated over 112½ kVA, part (B) of the rule basically calls for such transformers to be installed in vaults, with two exceptions for high-temperature insulation systems provided. Figure 450-27 shows the rules of this section. Related application recommendations are as follows:
Select a place that has the driest and cleanest air possible for installation of open-ventilated units. Avoid exposure to dripping or splashing water or other wet conditions. Outdoor application requires a suitable housing. Try to find locations where transformers will not be damaged by floodwater in case of a storm, a plugged drain, or a backed-up sewer.
Temperature in the installation area must be normal, or the transformer may have to be derated. Modern standard, ventilated, dry-type transformers are designed to provide rated kilovoltampere output at rated voltage when the maximum ambient temperature of the cooling air is 40°C and the average ambient temperature of the cooling air over any 24-h period does not exceed 30°C. At higher or lower ambients, transformer loading can be adjusted by the following relationships:
1. For each degree Celsius that average ambient temperature exceeds 30°C, the maximum load on the transformer must be reduced by 1 percent of rated kilovoltamperes.
2. For each degree Celsius that average ambient temperature is less than 30°C, the maximum load on the transformer may be increased by 0.67 percent of rated kilovoltamperes.
Depending on the type of insulation used, transformer insulation life will be cut approximately in half for every 10°C that the ambient temperature exceeds the normal rated value—or doubled for every 10°C below rated levels. Estimates assume continuous operation at full load. With modern insulations this rule is actually conservative for ambient temperature below normal operating temperatures and optimistic above it.
For proper cooling, dry-type transformers depend on circulation of clean air—free from dust, dirt, or corrosive elements. Filtered air is preferable and may be mandatory in some cases of extreme air pollution. In any case, it can reduce maintenance.
Fig. 450-27. Construction of dry-type transformer affects indoor installation rules. (Sec. 450.21.)
In restricted spaces—small basement mechanical rooms and the like—ventilation must be carefully checked to ensure proper transformer operating temperature. The usual requirement is for 100 cfm of air movement for each kilowatt of transformer loss. Areas of inlet and outlet vent openings should be at least 1 net sq ft per 100 kVA of rated transformer capacity.
Height of vault, location of openings, and transformer loading affect ventilation. One manufacturer calls for the areas of the inlet and outlet openings to be not less than 60 sq ft per 1000 kVA when the transformer is operating under full load and is located in a restricted space. And a distance of 1 ft should be provided on all sides of dry-type transformers as well as between adjacent units.
Freestanding, floor-mounted units with metal grilles at the bottom must be set up off the floor a sufficient distance to provide the intended ventilation draft up through their housings.
The installation location must not expose the transformer housing to damage by normal movement of persons, trucks, or equipment. Ventilation openings should not be exposed to vandalism or accidental or mischievous poking of rubbish, sticks, or rods into the windings. Adequate protection must be provided against possible entry of small birds or animals.
450.22. Dry-Type Transformers Installed Outdoors. A transformer that sustains an internal fault which causes arcing and/or fire presents the same hazard to adjacent combustible material whether it is installed indoors or outdoors. For that reason, a clearance of at least 12 in. (300 mm) is required between any dry-type transformer rated over 112½ kVA and combustible materials of buildings where installed outdoors.
In the second paragraph, the clearance of 12 in. (300 mm) from combustible building materials is not required for outdoor dry-type transformers that have a Class 155 insulation system and are completely enclosed, except for ventilation openings. The same consideration given for an 80°C-rise (Class 155) transformer is made outdoors as it is indoors, in Exception No. 2 in 450.21(B).
450.23. Less-Flammable Liquid-Insulated Transformers. 450.23 covers the liquid-filled transformers that have essentially replaced askarel-insulated transformers. Because oil-filled transformers used indoors require a transformer vault, the less-flammable (also called high-fire-point) insulated transformer offers an alternative to the oil-filled transformers, without the need for a vault. This Code section permits installation of these high-fire-point liquid-insulated transformers indoors or outdoors. Over 35 kV, such a transformer must be in a vault.
The rules of this section recognize that these various high-fire-point liquid-insulated dielectrics are less flammable than the mineral oil used in oil-filled transformers but not as fire-resistant as askarel. Because these askarel substitutes will burn to some degree, Code rules are aimed at minimizing any fire hazards:
1. Less-flammable liquid dielectrics used in transformers must, first of all, be listed—that is, tested and certified by a testing laboratory or organization and shown in a published listing as suitable for application. Less-flammable liquids for transformer insulation are defined as having “a fire point of not less than 300°C.”
2. Transformers containing the high-fire-point dielectrics may be used without a vault but only within buildings of noncombustible construction or limited combustibility (brick, concrete, etc.) and then only in rooms or areas that do not contain combustible materials. A Type I or Type II building is a building of noncombustible construction or limited combustibility, as described in the fine-print note, and there must be no combustible materials stored in the area where the transformer is installed.
3. The entire installation must satisfy all conditions of use, as described in the listing of the liquid.
4. A liquid-confinement area must be provided around such transformers that are not in a vault, because tests indicate these liquids are not completely nonpropagating—that is, if they are ignited, the flame will be propagated along the liquid. A propagating liquid must be confined to a given area to confine the flame of its burning (Fig. 450-28). The liquid-confinement area (a curb or dike around the transformer) must be of sufficient dimensions to contain the entire volume of liquid in the transformer.
Fig. 450-28. Transformers containing askarel-substitute liquids must satisfy specific installation requirements. (Sec. 450.23.)
A less-flammable liquid-insulated transformer installed in such a way that all of conditions 2 and 3 are not satisfied must be either
1. Provided with an automatic fire extinguishing system and a liquid-confinement area, or
2. Installed in a Code-specified transformer vault (part III of Art. 450), without need for a liquid-confinement area
Less-flammable liquid-insulated transformers rated over 35 kV and installed indoors must be enclosed in a Code-constructed transformer vault. All less-flammable liquid-insulated transformers installed outdoors may be attached to or adjacent to or on the roof of Type I or Type II buildings. Such installation at other than Type I or Type II buildings, where adjacent to combustible material, fire escapes, or door or window openings, must be guarded by fire barriers, space separation, and compliance with instructions for using the particular liquid.
Because these rules are general in nature and lend themselves to a variety of interpretations, application of these requirements may depend heavily on consultation with inspection authorities.
Although askarel-filled transformers up to 35 kV were used for many years for indoor applications because they do not require a transformer vault, there has been a sharp, abrupt discontinuance of their use over recent years. Growth in the ratings, characteristics, and availability of dry-type high-voltage transformers has accounted in major part for the reduction of askarel units. But another factor that has led to the rejection of askarel transformers in recent years is the environmental objections to the askarel liquid itself.
A major component of any askarel fluid is one or more polychlorinated biphenyls (PCBs), a family of chemical compounds designated as harmful environmental pollutants because they are nonbiodegradable and cannot be readily disposed of. Thus, although the askarels are excellent coolants where freedom from flammability is important, environmental objections to the sale, use, and disposal of PCBs have eliminated new applications of askarel transformers and stimulated a search for a nontoxic, environmentally acceptable substitute.
Proper handling and disposal of askarel is important for units still in use. A regulation of the EPA (Environmental Protection Agency), No. 311, required that all PCB spills of 1 lb (454 g) or more must be reported. Failure to report a spill is a criminal offense punishable by a $10,000 fine and/or 1-year imprisonment. Both the EPA and OSHA have objected to the use of askarels. A manufacturer of askarel has established a program for disposal of spent or contaminated PCB fluid using an incinerator that completely destroys the fluid by burning it at over 2000°F.
Non-PCB dielectric coolant fluids for use in small- and medium-sized power transformers as a safe alternate to askarels are available, and transformers using these new high-fire-point dielectric coolants have been widely used.
Extensive data from tests on available askarel substitutes show that they provide a high degree of safety. Such fluids do have NE Code and OSHA recognition. Responsibility for proper clearances with insurance underwriters, government regulating agencies, and local code authorities rests with the user or purchaser of the fluid in new or refilled transformers. Underwriters Laboratories does not test or list liquid-filled equipment. Both UL and Factory Mutual Research Laboratory have been involved in providing a classification service of flammability. The EPA has commented favorably on such high-fire-point fluids.
Few physical changes to transformers are necessary when using the new fluid dielectrics. However, load ratings on existing units may be reduced about 10 percent because of the difference in fluid viscosity and heat conductivity compared with askarel. The high-fire-point fluids cost about twice as much as askarel. Purchase price of a new transformer filled with the fluid (such as a typical 1000-kVA loadcenter unit) is about 10 to 15 percent more than an askarel-filled unit. But the economics vary for different fluids and must be carefully evaluated.
High-fire-point liquid-insulated transformers require no special maintenance procedures. The liquids exhibit good dielectric properties over a wide range of temperatures and voltage stress levels, and they have acceptable arc-quenching capabilities. They have a high degree of thermal stability and a high resistance to thermal oxidation that enables them to maintain their insulating and other functional properties for extended periods of time at high temperatures.
Since silicone liquids will ignite at 750°F (350°C), they are not classed as fire-resistant. However, if the heat source is removed or fluid temperature drops below 750°F, burning will stop. The silicone fluids are thus self-extinguishing.
450.24. Nonflammable Fluid-Insulated Transformers. This section permits indoor and outdoor use of transformers that utilize a noncombustible fluid dielectric, which is one that does not have a flash point or fire point and is not flammable in air (Fig. 450-29). As an alternative to askarel-insulated transformers, these transformers offer high BIL ratings and other features of operation similar to high-fire-point dielectric-insulated transformers—without concern for flammability. Such transformers do not, therefore, have the restrictions that are set down in 450.23 for the high-fire-point-liquid transformers.
Fig. 450-29. Comparison of the relative flammabilities of liquid dielectrics compared with ether. (Sec. 450.24.)
Nonflammable fluid-insulated transformers installed indoors must have a liquid-confinement area and a pressure-relief vent. In addition, such transformers must be equipped to absorb gases generated by arcing inside the tank, or the pressure-relief vent must be connected to a flue or duct to carry the gases to “an environmentally safe area.” Units rated over 35 kV must be installed in a vault when used indoors.
450.25. Askarel-Insulated Transformers Installed Indoors. Although askarel transformers are being phased out, the Code rule says such transformers installed indoors must conform to the following:
1. Units rated over 25 kVA must be equipped with a pressure-relief vent.
2. Where installed in a poorly ventilated place, they must be furnished with a means for absorbing any gases generated by arcing inside the case, or the pressure-relief vent must be connected to a chimney or flue which will carry such gases outside the building (Fig. 450-30).
3. Units rated over 35,000 V must be installed in a vault.
Fig. 450-30. Code rules still cover askarel transformers. (Sec. 450.25.)
450.26. Oil-Insulated Transformers Installed Indoors. The basic rule is illustrated in Fig. 450-31. Oil-insulated transformers installed indoors must be installed in a vault constructed according to Code specs, but the exceptions note general and specific conditions under which a vault is not necessary. The most commonly applied exceptions are as follows:
1. A hookup of one or more units rated not over 112½ kVA may be used in a vault constructed of reinforced concrete not less than 4 in. thick.
2. Units installed in detached buildings used only for providing electric service do not require a Code-constructed vault if no fire hazard is created and the interior is accessible only to qualified persons.
Fig. 450-31. Oil-filled transformers generally require installation in a vault. (Sec. 450.26.)
450.27. Oil-Insulated Transformers Installed Outdoors. Figure 450-32 shows how physical locations of building openings must be evaluated with respect to potential fire hazards from leaking transformer oil.
Fig. 450-32. Precautions must be taken for outdoor oil transformers. (Sec. 450.27.)
The outdoor locations need to be considered in light of the access requirement in 450.13. Some locations that comply with this section may be judged insufficiently accessible. The FPN following 450.27 points out that additional information can be found in the National Electrical Safety Code (not the NE Code). That code covers use of such transformers as shown in Fig. 450-33. But, because the FPN is only informational and presents no rule, the requirements of 450.13 must be observed.
450.28. Modification of Transformers. Askarel transformers that are drained and refilled with another liquid dielectric must be identified as such and must satisfy all rules of their retrofilled status. This rule is intended to maintain safety in all cases where askarel transformers are modified to eliminate PCB hazards. Marking must show the new condition of the unit and must not create code violations. For instance, an indoor askarel transformer that is drained and refilled with oil may require construction of a vault, which is required for oil-filled transformers as specified in 450.26.
450.41. Location. Ideally, a transformer vault should have direct ventilating openings (grilles or louvers through the walls) to outdoor space. Use of ducts or flues for ventilating is not necessarily a Code violation, but should be avoided wherever possible.
450.42. Walls, Roof, and Floor. Basic mandatory construction details are established for an NEC-type transformer vault, as required for oil-filled transformers and for all transformers operating at over 35,000 V. The purpose of a transformer vault is to isolate the transformers and other apparatus. It is important that the door as well as the remainder of the enclosure be of proper construction and that a substantial lock be provided. Details required for any vault are shown in Fig. 450-34 and include the following:
1. Walls and roofs of vaults shall be constructed of reinforced concrete, brick, load-bearing tile, concrete block, or other fire-resistive constructions with adequate strength and a fire resistance of 3 h according to ASTM Standard E119-99. “Stud and wall board construction” may not be used for walls, roof, or other surfaces of a transformer vault. Although the rule here does not flatly mandate concrete or masonry construction, that is essentially the objective of the wording. The substantiation for this rule said the following:
Fig. 450-33. High mounting of oil-filled transformers would require use of a portable ladder for access to the units. But 450.27 covers such units installed on poles or structures. (Sec. 450.27.)
Fig. 450-34. Transformer vault must assure containment of possible fire. (Sec. 450.42.)
The only guidance provided in Sec. 450.42 as to the type of material to be used in vault construction is in a fine-print note which states that ”six-in. (152-mm) thick reinforced concrete is a typical 3-hour construction.” This, of course, is only advisory. Sheet rock can be so installed as to have a 3-h fire rating, however, it should not be considered as being suitable for this type of installation. It would not have adequate structural strength in case of oil fire. It is not too difficult to break through such a wall, either intentionally or unintentionally. This is why the NEC specifically disallows it.
Your attention is also called to Sec. 230.6(3), which states that conductors installed in a transformer vault shall be considered outside of a building. The major thrust of 230.6 has been that the conductors are to be masonry-encased.
2. A vault must have a concrete floor not less than 4 in. (100 mm) thick when in contact with the earth. When the vault is constructed with space below it, the floor must have adequate structural strength and a minimum fire resistance of 3 h. Six-in. (150-mm)-thick reinforced concrete is a typical 3-h-rated construction.
3. Building walls and floors that meet the preceding requirements may serve for the floor, roof, and/or walls of the vault.
An exception to the basic regulations establishing the construction standards for transformer fireproof vaults notes that the transformer-vault fire rating may be reduced where the transformers are protected with automatic sprinkler, water spray, or carbon dioxide. The usual construction standards for transformer vaults (such as 6-in. [150-mm]-thick reinforced concrete) provide a minimum fire-resistance rating of 3 h. Where automatic sprinkler, water spray, or carbon dioxide is used, a construction rating of only 1 h will be permitted.
450.43. Doorways. Each doorway must be of 3-h fire rating as defined in the Standard for the Installation of Fire Doors and Windows (NFPA No. 80-1999). The Code-enforcing authority may also require such a door for doorways leading from the vault to the outdoors, in addition to any doorways into adjoining space in the building.
As required in part (C), vault doors must swing out and must be equipped with “panic bars” or other opening means that require only “simple pressure.”
This is intended to provide the greater safety of a push-open- rather than a rotating-knob-type of door release. As noted in the substantiation:
Conventional rotating door knob hardware is used on transformer vault doors due to lack of specific wording in the paragraph as presently written. The National Electrical Safety Code is believed to be very specific, or has been formally interpreted to be, requiring “panic type” door hardware. In an electrical flash or arc an electrical worker may lose the use of hands for twisting a conventional door knob.
In accordance with the concept of providing greater protection for personnel within the vault, other Code rules have also adopted similar wording. See 110.26(C)(3).
450.45. Ventilation Openings. This rule sets the size and arrangement of vent openings in a vault where such ventilation is required by ANSI C57.12.00-1993—“General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers,” as noted in 450.9. Figure 450-35 shows the openings as regulated by part (C) of this section. One or more openings may be used, but if a single vent opening is used, it must be in or near the roof of the vault—and not near the floor.
Fig. 450-35. Vault vent opening(s) may be in or near the roof—or near floor level also, if one is at or near roof level. (Sec. 450.45.)
Refer to Fig. 455-1, which shows details of the rules given in 455.2, 455.4, 455.6, 455.7, and 455.8.
455.2. Definitions. A phase converter is a static or a rotary device that converts single phase power to 3-phase power. The fine-print note calls attention to the fact that phase converters, which do not produce perfect 3-phase power, alter the starting torque and locked-rotor current of motors connected to them.
455.6. Conductors. Part (A)(1) addresses the general case and requires that conductors supplying a phase converter must have an ampacity at least equal to 125 percent of the “phase converter nameplate single-phase input full-load amperes.” Part (A)(2) covers instances where a specific load will be supplied, in which case the supply conductor ampacity must be at least 250 percent of the connected load. Currents are multiplied by the output to input voltage ratio as applicable if there is a voltage transformation involved. A fine-print note suggests 3 percent as a voltage drop allowance not to be exceeded in the single-phase portion of the circuit in order to promote proper starting and operation of the three-phase load.
Part (B) requires the manufactured phase conductor, which is the one originated by the converter and not connected to the line, to have a distinctive marking at all accessible locations and in a consistent manner throughout the system. The method of identification is not specified, but the color yellow appears to be the generally preferable choice. Section 455.9 prohibits connecting single-phase load to the manufactured phase, so it is essential to know which wire to avoid when making a single-phase load connection.
Fig. 455-1. Code rules on phase converters cover these considerations. (Art. 455.)
455.7. Overcurrent Protection. The rating of the overcurrent protective device protecting the phase converter supply conductors follows the same percentage rules (except stating the result as a maximum) as given for determining conductor ampacity, with an allowance for the next higher standard sized overcurrent device. The overcurrent protection cannot exceed 125 percent of the rated full-load input current.
There are no rules in this article for overcurrent protection on the load side of the converter. Presumably these conductors are a form of a tap and are subject to the rules in Secs. 240.4 and 240.21. Applying the 10-ft tap rule in 240.21(B) would clearly comply with code rules.
455.8. Disconnecting Means. A readily accessible disconnecting means that opens the single-phase ungrounded supply conductors simultaneously must be located in sight of the phase converter. The disconnecting means must be a circuit breaker or molded-case switch, or a switch rated in horsepower. Nonmotor loads may use a switch rated in amperes. For general purposes, the disconnecting means must have a rating equal to 115 percent of the “phase converter nameplate single-phase input full-load amperes.” If specific loads are supplied then the required rating depends on the type of disconnecting means used: Note that, in 455.20, the single-phase disconnecting means for a static phase converter supplying a single load may also serve as the disconnecting means for that load, provided it is in sight.
A circuit breaker or molded-case switch is to be based on at least 250 percent of the 3-phase full load amperes, adjusted for voltage as required. The procedure for these disconnects parallels that for sizing overcurrent protection.
A horsepower-rated switch is to be based on 200 percent of the equivalent horsepower of the 3-phase load. The horsepower-rated switch rule uses a summation of up to three numbers to obtain a lock-rotor ampere rating: the nonmotor loads; the 3-phase locked rotor current of the largest motor, as given in Table 430.251(B); and the full-load currents of all other motors operating at the same time. The sum of these numbers is then multiplied by the output to input voltage ratio if necessary, and then taken back into Table 430.151(B) for reconversion into horsepower.
455.21. Start-Up. A rotary converter must be started before the 3-phase load is energized. If a rotary phase converter is started under load, the output phase angles will not usually allow the 3-phase motor to start properly, risking burnout of components and fire. The next section requires that rotary converter installations be arranged so that if power to the converter fails, the loads cannot be reenergized (until the converter has successfully restarted, per Sec. 455.21). A fine-print note advises that magnetic contactors, etc., with manual or time-delay restarting will comply with this rule.
455.23. Capacitors. Capacitors installed for a motor load (and not as an integral part of the phase conversion system) must be installed on the line side of the motor running overload protection. This avoids the necessity to apply 460.9 to correct for improved power factor in sizing running overload protection if capacitors are on the load side of that protection. As implied in 455.2 (FPN), that power factor may be difficult to accurately predict.
460.1. Scope. The sections in this article apply chiefly to capacitors used for the power-factor correction of electric-power installations in industrial plants and for correcting the power factors of individual motors (Fig. 460-1). These provisions apply only to capacitors used for surge protection where such capacitors are not component parts of other apparatus.
In an industrial plant using induction motors, the power factor may be considerably less than 100 percent, particularly when all or part of the motors operate most of the time at much less than their full load. The lagging current can be counteracted and the power factor improved by installing capacitors across the line. By raising the power factor, for the same actual power delivered the current is decreased in the generator, transformers, and lines, up to the point where the capacitor is connected.
Figure 460-2 shows a capacitor assembly connection to the main power circuit of a small industrial plant, consisting of capacitors connected in a 3-phase hookup and rated at 90 kVA for a 460-V system. An externally operable switch mounted on the wall is used as the disconnecting means, and the discharge device required by 460.6 consists of two high-impedance coils inside the switch enclosure which consume only a small amount of power but, having a comparatively low DC resistance, permit the charge to drain off rapidly after the capacitor assembly has been disconnected from the line.
Fig. 460-1. A typical power-factor correction capacitor bank is this 300-kvar bank of twelve 25-kvar, 480-V capacitor units installed in a steel enclosure in an outdoor industrial substation. (Sec. 460.1.)
Fig. 460-2. Six internally delta-connected capacitors form a 3-phase capacitor bank. (Sec. 460.1.)
460.6. Discharge of Stored Energy. If no means were provided for draining off the charge stored in a capacitor after it is disconnected from the line, a severe shock might be received by a person servicing the equipment, or the equipment might be damaged by a short circuit. If a capacitor is permanently connected to the windings of a motor, as in Fig. 460-3, the stored charge will drain off rapidly through the windings when the circuit is opened. Reactors or resistors used as discharge devices either must be permanently connected across the terminals of the capacitor (such as within the capacitor housing) or a device must be provided that will automatically connect the discharge devices when the capacitor is disconnected from the source of supply. Most available types of capacitors have discharge resistors built into their cases. When capacitors are not equipped with discharge resistors, a discharge circuit must be provided.
Fig. 460-3. Capacitor voltage must be discharged when circuit is opened. (Sec. 460.6.)
Figure 460-3 shows a capacitor used to correct the power factor of a single motor. The capacitor may be connected to the motor circuit between the starter and the motor or may be connected between the disconnecting means and the starter, as indicated by the dotted lines in the diagram. If connected as shown by the dotted lines, an overcurrent device must be provided in these leads, as required by 460.8(B). The capacitor is shown as having discharge devices consisting of resistors.
Power capacitors, in most applications, are installed to raise the system power factor, which results in increased circuit or system current-carrying capacity, reduced power losses, and lower reactive power charges (most utility companies include a power-factor penalty clause in their industrial billing). Also, additional benefits derived as a result of a power capacitor installation are reduced voltage drop and increased voltage stability. Figure 460-4 presents basic data on calculating size of capacitors for power-factor correction. However, manufacturers provide tables and graphs to help select the capacitor for a given motor load.
In the past, the Code limited power-factor correction to unity (100 percent, or 1.0) when there is no load on the motor. That will result in a power factor of 95 percent or better when the motor is fully loaded. The old rule recognized the use of capacitors sized either for the value that will produce 100 percent power factor of the circuit when the motor is running at no load or for a value equal to 50 percent of the kilovoltampere rating of the motor input for motors up to 50 hp, 600 V (Fig. 460-5). Now the NEC makes no reference to this consideration.
Fig. 460-4. Capacitors reduce circuit current by supplying the magnetizing current to motors. (Sec. 460.6.)
A number of comments made in the discussions that supported and initially led to the adoption of what was formerly 460-7 are informative and important:
1. Setting the capacitor rating at 50 percent of the kilovoltampere rating of the motor input does not afford, in most cases, an appreciable gain in power factor over that achieved by setting the capacitor rating to limit the motor no-load power factor to unity. In two examples, 7.5-hp, 1200-rpm and 10-hp, 900-rpm ratings, the motor no-load kvar is more than 50 percent of the kilovoltampere rating of the motor input. In other examples, 40-hp, 1200-rpm and 200-hp, 1800-rpm ratings, the corrective kvar value to raise the motor no-load power factor to unity yields better power-factor improvement up through 75 percent load than attained by corrective kvar equal to 50 percent of the kilovoltampere rating of the motor input. For loads above 75 percent, the converse is true; but there is no notable overall difference in power-factor improvement between the two approaches.
Fig. 460-5. Maximum rating of capacitor kvar may be determined by either of two approaches. (Sec. 460.6.)
2. Most motor-associated capacitors are related to low-voltage, 5- to 50-hp, 1800- and 1200-rpm, across-the-line-start motors. In this range, the no-load rule for determination of maximum capacitor kvar restricts capacitors to less than 50 percent of horsepower.
Noticeable economies can be made by applying larger capacitors to such motors. This has been done for years, with excellent results and no field trouble, as has been attested by several engineers whose views have been made known to the Code panel.
The larger motors should certainly have capacitor kvars limited by stringent rules, but this should not interfere with the field-proved use of augmented capacitor sizing of the integral-horsepower motors most commonly used in industrial, commercial, and institutional plants.
The no-load power factor of a motor is a design constant of the motor and may be obtained from the manufacturer of the motor—or it may be measured or calculated. In Fig. 460-4, using the known no-load PF (power factor) of a motor, the kilowatts can be calculated from kW = PF × kVA1. Then kvar1 (the required rating of PF capacity to raise the no-load value of PF to 100 percent) equals the square root of (kVA1)2 – (kW)2, where kVA1 is calculated from circuit voltage and current measured with a clamp-on ammeter.
A handy rule-of-thumb method for determining the kilovar rating of a capacitor required to provide optimum power-factor correction for a given motor is:
1. With no load on the motor, measure the no-load kilovoltampere. That can be determined by using a clamp-on ammeter to measure the amount of current drawn by the motor under no-load condition and then using a voltmeter to get the phase-to-phase voltage of the motor circuit. Then, for a 3-phase motor, the kilovoltampere input to the motor is derived from the formula:
Input kVA = (phase-to-phase voltage × line current × 1.732) + 1000
2. Because the power factor of an unloaded motor is very low—say, about 10 percent—the kilovoltampere vector for the original PF condition as shown in Fig. 460-4 is lagging the kilowatt vector by an angle that is approaching 90°. That results from the working current being small (only the resistance of the windings) while the reactive (magnetizing) current is at its normal and very much larger value. In that condition, the reactive current causes the kilovar vector to be almost the same length as the kilovoltampere vector—so close in fact that it is generally safe to take the kilovoltampere input value as the required kilovar rating of capacitor needed to correct to 100 percent PF at no-load, which will result in a 95 to 98 percent PF at full load.
3. Then select a capacitor assembly that has a kilovar rating as close as possible to—but not in excess of—the calculated value of input kilovoltampere of the motor. This method may be used on rewound motors or on other motors where it is not possible to make a better determination of needed capacitor kilovar.
Capacitors of the type used for PF correction of motors are commonly rated in kilovoltamperes, or the rating may be in kilovars, meaning reactive kilovoltamperes, abbreviated kvar. The capacitors are usually designed for connection to a 3-phase system and constructed as a unit with three leads brought out.
Corrective measures for improving the power factor may be designed into motor branch circuits. Generally, the most effective location for installation of individual power-factor-correction capacitors is as close to the inductive load as possible. This provides maximum correction from the capacitor back to the source of power. At individual motor locations, power-factor-correcting capacitors offer improved voltage regulation. As shown in Fig. 460-6, power-factor capacitors installed at terminals of motors provide maximum relief from reactive currents, reducing the required current-carrying capacities of conductors from their point of application all the way back to the supply system. Figure 460-7 shows a typical example. Such application also eliminates extra switching devices, since each capacitor can be switched with the motor it serves.
Capacitors also may be installed as a group or bank at some central point, such as a switchboard, loadcenter, busway, or outdoor substation. Usually this method serves only to reduce the utility company penalty charges; however, in many instances, installation costs also will be lower.
When motors are small, numerous, and operated intermittently, it is often economically more desirable to install required capacitor kvar at the motor control center.
Capacitor installations may consist of an individual unit connected as close as possible to the inductive load (at the terminals of a motor, etc.) or of a bank of many units connected in multiple across a main feeder. Units are available in specific kvar and voltage ratings. Standard low-voltage capacitor units are rated from about 0.5 to 25 kvar at voltages from 216 to 600 V. For high-voltage applications, standard ratings are 15, 25, 50, and 100 kvar. Available in single-, 2-, or 3-phase configurations, power capacitors may be supplied either unfused or equipped with current-limiting or high-capacity fuses (single-phase units are furnished with one fuse; 3-phase capacitors usually have two fuses). On low-voltage units, fuses may be mounted on the capacitor bushings inside the terminal compartment.
Fig. 460-6. PF capacitors at individual motors offer maximum corrective advantages. (Sec. 460.6.)
Use of capacitor power-factor application is generally not acceptable for any motor application involving repetitive switching of the motor load, as in plugging, jogging, rapid reversals, reclosings, and so forth, because of the severe overvoltages and overtorques that are generated in such motor applications when capacitors of the permitted rating are connected on the load side of the motor starter. The objectionable effects can lead to premature failure of motor insulation.
460.8. Conductors. Part (A) of this section covers sizing of circuit conductors. The current corresponding to the kilovoltampere rating of a capacitor is computed in the same manner as for a motor or other load having the same rating in kilovoltamperes. If a capacitor assembly used at 460 V has a rating of 90 kVA, the current rating is 90,000 VA/(480 V × ) = 90,000 VA/(277 V × × ) = 90,000 VA/831 V = 108 A. The minimum required ampacity of the conductors would be 1.35 × 108 A, or 146 A.
The manufacturing standards for capacitors for power-factor correction call for a rating tolerance of “-0, + 15 percent,” meaning that the actual rating in kilovoltamperes is never below the nominal rating and may be as much as 15 percent higher. Thus, a capacitor having a nameplate rating of 100 kVA might actually draw a current corresponding to 115 kVA. The current drawn by a capacitor varies directly with the line voltage, so that, if the line voltage is higher than the rated voltage, the current will be correspondingly increased. Also, any variation of the line voltage from a pure sine wave form will cause a capacitor to draw an increased current. It is for these reasons that the conductors leading to a capacitor are required to have an ampacity not less than 135 percent of the rated current of the capacitor.
Fig. 460-7. Typical capacitor installation connected on the load side of a motor starter for a 500-hp motor consists of ceiling-suspended enclosed capacitors (arrow) that are rated at 80 kvar, connected through the pull box to the circuit conductors for the motor. Diagram shows how conductors were added to the equipment shown in the photo. (Sec. 460.6.)
example Given the kvar rating of capacitors to be installed for a motor, determining the correct capacitor conductor size is relatively simple. The rule here requires that the ampacity of the capacitor conductors be not less than one-third the ampacity of the motor circuit conductors and not less than 135 percent of the capacitor rated current. The capacitor nameplate will give rated kvar, voltage, and current. It is then a simple matter of multiplying rated current by 1.35 to obtain the ampacity value of the conductor to be installed and selecting the size of conductor required to carry that value of current, from Table 310.16. Then check that the ampacity is not less than one-third the ampacity of the motor circuit conductors.
For a motor rated 100 hp, 460 V, 121 A full-load current, a 25-kvar capacitor would correct power factor to between 0.95 and 0.98 at full load. The nameplate on the capacitor indicates that the capacitor is rated 460 V, 31 A. Then 31 × 1.35 = 42 A. From Code Table 310.16, a No. 6 TW or THW conductor rated to carry 55 A would do the job. (No. 8 THW rated at 45 A would most likely be considered not acceptable because UL generally calls for use of 60°C wires in circuits up to 100 A.) The motor circuit conductors are found to be 2/0 THW, with an ampacity of 175 A. Since 
× 175 = 58 A, the No. 6 THW, with an ampacity of 65 A, should be used.
If these conductors are connected to the load terminals of the motor controller, the overload protection heaters may have to be changed (or if the OL is adjustable, its setting may have to be reduced), because the capacitor will cause a reduction in line current and adjustment of relay setting is required by 460.9.
Although part (B) of the rule requires overcurrent protection (fuses or a CB) in each ungrounded conductor connecting a capacitor assembly to a circuit, the exception considers the motor-running overload relay in a starter to be adequate protection for the conductors when they are connected to the motor circuit on the load side of the starter. Where separate overcurrent protection is provided, as required for line-side connection, the device must simply be rated “as low as practicable.” When a capacitor is thrown on the line, it may momentarily draw an excess current. A rating or setting of 250 percent of the capacitor current rating will provide short-circuit protection. Being a fixed load, a capacitor does not need overload protection such as is necessary for a motor.
Most power capacitors are factory equipped with fuses which provide protection in case of an internal short circuit. These fuses are usually rated from 165 to 250 percent of the rated kilovar current to allow for maximum operating conditions and momentary current surges. When installed on the load side of a motor starter, as noted previously, capacitors do not require additional fusing. However, for bank installations, separate fuses are required.
Part (C) of the rule requires a disconnecting means for all the ungrounded conductors connecting a capacitor assembly to the circuit—but a disconnect is not needed when the capacitor is connected on the load side of a starter with overload protection. The disconnect must be rated at least equal to 1.35 times the rated current of the capacitor.
Note that part (C)(1) requires a multipole switching device for the disconnect. This rule was originally adopted because of the inherent danger of single-pole switching of low-voltage capacitors. Normal switching or closing on faults may cause arcs or splattering of molten metal.
Two accepted methods of wiring capacitors are illustrated in Fig. 460-8. Diagram A shows the method of connection at a central location, such as at a power center or on a busway feeder. In such an installation, the Code rule requires an overcurrent device in each ungrounded conductor, a separate disconnecting means, and a discharge resistor (usually furnished with capacitors). The current rating of both the capacitor disconnect switch and the conductors supplying the capacitor must be not less than 135 percent of the rated current of the capacitor. In B, the capacitor is connected directly to motor terminals. Installation on the load side of the motor starter eliminates the need for separate overcurrent protection and separate disconnecting means. However, motor-running overcurrent protection must take into account the lower running current of the motor, as required by 460.9.
Fig. 460-8. PF capacitor assembly may be connected on the line or load side of a starter. (Sec. 460.8.)
460.9. Rating or Setting of Motor Overload Device. When a power-factor capacitor is connected to a motor circuit at the motor—that is, on the load or motor side of the motor controller—the reactive current drawn by the motor is provided by the capacitor, and, as a result, the total current flowing in the motor circuit up to the capacitor is reduced to a value below the normal full-load current of the motor. With that hookup, the total motor full-load current flows only over the conductors from the capacitor connection to the motor, and the entire motor circuit up to that connection carries only the so-called working current or resistive current. That is shown in the top part of Fig. 460-9.
Under the conditions shown, it is obvious that setting the overload relay in the starter for 125 percent of the motor nameplate full-load current (as required by 430.32) would actually be an excessive setting for real protection of the motor, because considerably less than full-load current is flowing through the starter. The rule of this section clearly requires that the rating of a motor overload protective device connected on the line side of a power-factor correction capacitor must be based on 125 percent (or other percentage from 430.32) times the circuit current produced by the improved power factor—rather than the motor full-load current (see Fig. 460-9).
Fig. 460-9. Motor overload protection must be sized for the current at improved PF. (Sec. 460.9.)
The 25-kvar capacitor used on the 100-hp, 460-V motor in the example in 460.8 will reduce the motor line current by about 9 percent. Section 430.32(A) (also 430.34 and 460.9) requires that the running overload protection be sized not more than 125 percent of motor full-load current produced with the capacitor. If the OL protection heaters were originally sized at 125 percent of the motor full-load current (1.25 × 121), they would have been sized at 151 A. With the motor current reduced by 9 percent (0.09 × 121, or 11 A), the motor full-load current with the capacitor installed would be 121 - 11, or 110 A. Since 125 percent of 110 A is 137.5 A, the heaters must be changed to a size not larger than 137.5 A.
If the capacitor conductors could be connected on the line side of the heaters, the heaters would not have to be reduced in size, since the reduction of line current occurs only from the source back to the point of the capacitor connection. Conductor connections at this location are extremely difficult to make because of the lack of space and the large size of the connecting lugs. Controller load terminals are furnished with connectors that will accept an additional conductor, or they can be easily modified to permit a dependable connection.
460.10. Grounding. The metal case of a capacitor is suitably grounded by lock-nut and bushing connections of grounded metal nipples or raceways carrying the conductors connecting the capacitor into a motor circuit or feeder.
470.1. Scope. Except when installed in connection with switchboards or control panels that are so located that they are suitably guarded from physical damage and accidental contact with live parts, resistors should always be completely enclosed in properly ventilated metal boxes.
Large reactors are commonly connected in series with the main leads of large generators or the supply conductors from high-capacity network systems to assist in limiting the current delivered on short circuit. Small reactors are used with lightning arresters, the object here being to offer a high impedance to the passage of a high-frequency lightning discharge and so to aid in directing the discharge to ground. Another type of reactor, having an iron core and closely resembling a transformer, is used as a remote-control dimmer for stage lighting. Reactors, like resistors, are sources of heat and should therefore be mounted in the same manner as resistors.
480.1. Scope. Storage cells are of two general types: the so-called lead-acid type, in which the positive plates consist of lead grids having openings filled with a semisolid component, commonly lead peroxide, and the negative plates are covered with sponge lead, the plates being immersed in dilute sulfuric acid; and the alkali type, in which the active materials are nickel peroxide for the positive plate and iron oxide for the negative plate, and the electrolyte is chiefly potassium hydroxide (Fig. 480-1).
480.2. Definitions. “Stationary installations of storage batteries” provide an independent source of power for emergency lighting, switchgear control, engine-generator set starting, signal and communications systems, laboratory power, and similar applications. They are an essential component of UPS systems. This Code article does not cover batteries used to supply the motive power for electric vehicles.
Fig. 480-1. Article 480 applies only to “stationary installations of storage batteries”—whether they are used for supply to lighting, generator cranking, switchgear control, or in UPS (uninterruptible power supply) systems. (Sec. 480.1.)
The most commonly used battery is the lead-acid type—either lead-antimony or lead-calcium. Nickel-cadmium batteries offer a variety of special features that, in many instances, offset their higher initial cost. Other types include silver-zinc, silver-cadmium, and mercury batteries.
The lead-antimony battery is readily available at a moderate price, has a high efficiency (85 to 90 percent), is comparatively small, and has a relatively long life if operated and maintained properly under normal conditions. Voltage output is about 2 V per cell; ratings range to about 1000 A-h (based on an 8-h discharge rate).
Lead-calcium batteries offer features similar to the lead-antimony type, and they require less maintenance. They do not require an “equalizing” charge (application of an overvoltage for a period of time to ensure that all cells in a battery bank will produce the same voltage). For this reason, they are often selected for use in UPS systems.
This type of cell can usually be operated for a year or more without needing water, depending on the frequency and degree of discharge. Sealed or maintenance-free batteries of this type never need water. Voltage output is 2 V per cell, with ratings up to about 200 A-h (8-h rate).
Nickel-cadmium batteries are particularly useful for application in temperature extremes. They are reputed to have been successfully operated at temperatures from -40 to 163°F (-40 to 73°C). They have a very high short-time current capability and are well suited to such applications as engine starting and UPS operation. Initial cost is higher than lead-acid types; however, they offer long life (25 to 30 years), reliability, and small size per unit. Voltage is about 1.2 V per cell.
480.3. Wiring and Equipment Supplied from Batteries. As indicated in Fig. 480-2, whatever kinds of circuits and loads a battery bank serves, all rules of the NE Code covering operation at that voltage must be applied to the wiring and equipment.
Fig. 480-2. Applicable Code rules must be observed for load circuits fed by batteries. (Sec. 480.3.)
480.5. Disconnecting Means. A disconnecting means must now be provided for any ungrounded conductor supplied by a stationary battery bank operating over 30 V. It must be readily accessible and located within sight of the battery system. This rule correlates with the new 240.21(H) that specifies a comparable location for overcurrent protection for such conductors.
480.7. Insulation of Batteries of Over 250 V. Racks of adjacent batteries must be so placed as to have a minimum of 2 in. (50 mm) of air space between any pair of exposed, live battery terminals of opposite polarity.
480.9. Battery Locations. Although specific “battery rooms” or enclosures are no longer required for installation of any batteries, part (A) does require ventilation at battery locations. A specific battery room was previously required for open-tank or open-jar batteries, but such units are no longer made or in use.
The overcharging of a battery can result in the breaking down of the electrolyte into gases that, if permitted to accumulate in the room, may result in an explosive mixture. Overcharging indicates problems with the charging equipment requiring correction. Proper ventilation will resolve this explosive mixture, ensuring that the location is not a hazardous location subject to Art. 501.
Because the fumes given off by a storage battery are very corrosive, the type of wiring must be such that it will withstand the corrosive action, and special precautions are necessary as to the type of insulation used, as well as protection of all metalwork. It is stated by the respective manufacturers that conduit made of aluminum or red brass is well suited to withstand the corrosive effects of the fumes in battery rooms. If steel conduit is used, it is recommended that the conduit be zinc-coated and that it be kept well painted with asphaltum paint.
Batteries of the lead-acid type sometimes throw off a fine spray of the dilute acid which fills the air around the cells; hence, steel conduit or tubing should not be brought close to any cell.
There are no special requirements on the type of luminaires or other electrical equipment used in the battery room. Proper ventilation of the room will prevent explosions. See 300.6 and 410.10(B).
490.1. Scope. The requirements previously given in Art. 710 for equipment rated over 600 V nominal are now covered in Art. 490. As defined in 490.2, the term high-voltage applies to any equipment rated at more than 600 V nominal. The systems customarily utilized as premises wiring and therefore subject to the NEC are usually not over 69 kV and therefore the technically correct terminology is “medium voltage,” which is the usual term used in this book.
490.21. Circuit-Interrupting Devices. Medium-voltage power CBs provide load switching, short-circuit protection, electrical operation, adjustable time delays of trip characteristics for selectively coordinated protection schemes, quick reclosing after tripping, and various protective hookups such as differential relay protection of transformers. There are oil-type, oilless (or air-magnetic), and vacuum-break CBs. The air-magnetic CB is the common type for indoor applications in systems up to 15 kV and higher. Oil CBs are sometimes used for indoor and outdoor medium-voltage service equipment, where they provide economical disconnect and protection on the primary of a transformer.
Modern medium-voltage CB equipment meets all the needs of control and protection for electrical systems, from the simplest to the most complex and sophisticated. In particular, its use for selectively coordinated protection of services, feeders, and branch circuits is unique. In current ratings up to 3000 A, CB gear has the very high interrupting ratings required for today’s high-capacity systems. Available in metal-clad assemblies, all live parts are completely enclosed within grounded metal enclosures for maximum safety. For applications exposed to lightning strikes or other transient overcurrents, CB equipment offers quick reclosing after operation. Drawout construction of the CB units provides ease of maintenance and ready testing of breakers. CB gear offers unlimited arrangements of source and load circuits and is suited to a variety of AC or DC control power sources. Accessory devices are available for special functions.
Figure 490-1 covers the basic rules of this section on use of CBs. Figure 490-2 shows an oil CB, with the line-side isolating switch required by 230.204(A).
Part (B) covers power fuses, which are available in current-limiting and noncurrent-limiting types. (See Fig. 490-3 for an example of the uses to which power fuses may be put.) The current-limiting types offer reduction of thermal and magnetic stresses on fault by reducing the energy let-through. They are constructed with a silver-sand internal element, similar to 600-V current-limiting fuses. Such fuses generally have higher interrupting ratings at some voltages, but their continuous current-carrying ratings are limited.
Noncurrent-limiting types of power fuses are made in two types of operating characteristics: expulsion type and nonexpulsion type. The expulsion fuse gets its name from the fact that it expels hot gases when it operates. Such fuses should not be used indoors without a “snuffer” or other protector to contain the exhaust, because there is a hazard presented by the expelled gases. At the end of part (B)(1), the rule says that vented expulsion-type power fuses used indoors, underground, or in metal enclosures must be “identified for the use.” Vented power fuses are not safe for operation in confined space—unless specifically tested and identified for such use. Part (B)(5) of this section requires that fuses expelling flame in operation must be designed or arranged to prevent hazard to persons or property. The boric-acid fuse with a condenser or other protection against arcing and gas expulsion is a typical nonexpulsion, noncurrent-limiting fuse (Fig. 490-4).
Fig. 490-1. Detailed rules regulate use of medium-voltage circuit breakers. (Sec. 490.21.)
Parts (B)(6) and (B)(7) cover very important safeguards for the use of fuses and fuseholders:
Fig. 490-2. Oil circuit breaker for medium-voltage application has a disconnecting switch on its supply side to isolate the line terminals of the breaker. (Sec. 490.21.)
Fig. 490-3. Typical power fuses are used in load-interrupter switchgear, which is an alternative to circuit-breaker gear for control and protection in indoor medium-voltage systems. (Sec. 490.21.)
Fig. 490-4. Boric-acid fuse uses a device to protect against flame expulsion. (Sec. 490.21.)
In coordinating power fuses, care must be taken to account for ambient temperature adjustment factors, because time-current curves are based on an ambient of 25°C. Adjustment also must be made for preheating of fuses due to load current to ensure effective coordination of fuses with each other and/or with CBs. Manufacturer’s curves of adjustment factors for ambient temperature and fuse preloading are available.
Figure 490-5 shows the time-current characteristic for an R-rated, current-limiting (silver-sand) fuse designed for 2400- and 4800-V motor applications. Such fuses must be selected to coordinate with the motor controller overload protection, with the controller clearing overloads up to 10 times motor current and the fuse taking over for faster opening of higher currents up to the interrupting rating of the fuse. The amp rating of R-rated fuses is given in values such as 2R or 12R or 24R. If the number preceding the R is multiplied by 100, the value obtained is the ampere level at which the fuse will blow in 20 s. Thus, the rating designation is not continuous current but is based on the operating characteristics of the R-rated fuse. Continuous current rating of such fuses is given by the manufacturer at some value of ambient temperature.
Fused cutouts for medium-voltage circuits, as shown in Fig. 490-6, are available for outdoor applications only, as regulated by part (C) of this section in general, and (C)(1) in particular, which flatly prohibits this equipment from being used indoors. Pull-type fuse cutouts are used outdoors on pole-line crossarms where the ionized gas expelled during operation is unlikely to cause a flashover in a confined space, or damage someone’s hearing due to the loudness of the report. The use indoors in electric rooms where accessible only to qualified persons, as shown in Fig. 490-7, appears to be a violation of this rule. Such fused cutouts are acceptable for use as an isolating switch, as permitted by 490.22.
Fig. 490-5. R-rated fuses are used in motor starters for 2400- and 4800-V motors. (Sec. 490.21.)
Fig. 490-6. Expanded rules cover use of distribution cutouts. (Sec. 490.21.)
Fig. 490-7. Distribution cutouts are single-pole, fused, protective, and disconnect devices that are hook-stick-operable. Note voltage and current rating on case of each cutout (arrow), as required by part (C)(5) of 490.21. (Sec. 490.21.) The location appears to be indoors, and if so, would be controversial in light of the prohibition in 490.21(C)(1).
Part (D) of this section covers oil-filled cutouts. In addition to air CBs, oil CBs, and fused load-interrupter switches, another device frequently used for control of medium-voltage circuits is the oil-filled cutout. Compared to breakers and fused switchgear, oil-filled cutouts are inexpensive devices that provide economical switching and, where desired, overload and short-circuit protection for primary voltage circuits.
The oil-filled cutout is a completely enclosed, single-pole assembly with a fusible or nonfusible element immersed in the oil-filled tank that makes up the major part of the unit, and with two terminals on the outside of the housing. Figure 490-8 shows the basic construction of a typical cutout with a listing of available entrance fittings for the terminals to suit them to various cable and job requirements. The circuit is broken or closed safely and rapidly by the internal switching mechanism. The switch mechanism is made up of a rotating element that, in the closed position, bridges two internal contacts—each contact connecting to one of the outside terminals. The rotating element is completely insulated from the external case and from the external handle that operates the element. The rotating element may be simply a shorting blade when the cutout is used as an unfused switch. When the cutout is to be used as a fused switching unit, the rotating bridging element is fitted with a fuse. Operation of an oil-filled cutout is controlled at the top end of the shaft extending out through the top of the housing.
Fig. 490-8. Oil-filled cutout is a fused or unfused, single-pole disconnect device. (Sec. 490.21.)
As a single-pole switching device, the oil-filled cutout is not polarized—that is, either terminal may be a line or load terminal. This is a result of the symmetrical construction of the switching element and suits the device to use in circuit sectionalizing or as a tie device in layouts involving two or more primary supply circuits. Note that these Code rules on oil-filled cutouts are different from those in part (C) on distribution cutouts.
With an oil-filled cutout, the switching of load current or the breaking of fault current is confined within a sturdy metal housing. Operation is made safe and quiet by confining arcs and current rupture forces within the enclosure. This operating characteristic of the cutouts especially suits them to use where there are explosive gases or flammable dusts, where complete submersion is possible, where severe atmospheric conditions exist, or where exposure of live electrical parts might be hazardous.
Oil-filled (sometimes called oil-fuse) cutouts are made in three sizes based on continuous current—100, 200, and 300 A, up to 15 kV. In one line there are three basic types. Pole-type cutouts are equipped with rubber-covered leads from the terminals for use in open wiring. Pothead-type cutouts have a cable lead from one terminal for open wiring and a sleeve on the other for connecting a lead or rubber-covered cable from an underground circuit. Subway-type cutouts are for underground vaults and manholes, particularly where submersion might occur, and are equipped with a sleeve on each terminal for rubber-or lead-sheathed cable. Figure 490-9 lists the various types of terminal connections that are available on oil-filled cutouts.
Fig. 490-9. Terminals on oil-filled cutouts must be matched to application and cable type. (Sec. 490.21.)
For multiphase circuits, two or three single-phase cutout units can be group-mounted with a gang-operating mechanism for simultaneous operation. Figure 490-10 shows three-gang assemblies. For pole mounting, linkage and a long handle are available for operating cutouts from the ground. Or cutouts can be flange-mounted on a terminal box, as shown in Fig. 490-11, where the three-gang assembly was added to a medium-voltage switchgear on a modernization job.
Because oil-filled cutouts provide load-break capability and overcurrent protection, they may be used for industrial and commercial service equipment, for switching outdoor lighting of sports fields and shopping centers, for transformer loadcenters, for primary-voltage motor circuits, or for use in vaults and manholes of underground systems.
In 100- and 200-A ratings, oil-filled cutouts are available in combination with current-limiting power fuses in double-compartment indoor or outdoor enclosures. These fused oil interrupter switches provide moderate load-break and high fault-current interrupting capability in an economical package.
Part (E) of this section recognizes the use of so-called load-interrupter switches used in medium-voltage systems. In parts (1) to (6), a wealth of specific data provides guidance to design engineers, electrical installers, and electrical inspectors on the proper installation, operation, and maintenance of medium-voltage interrupter switches—with particular emphasis on safety to operators and maintenance personnel.
Switching for modern medium-voltage electrical systems can be provided by a number of different equipment installations. For any particular case, the best arrangement depends on several factors: the point of application—either for outside or inside distribution or as service equipment; the voltage; the type of distribution system—radial, loop, selective, network; conditions—accessibility, type of actual layout of the equipment; job atmosphere; use; future system expansion; and economic considerations.
Fig. 490-10. Oil-filled cutouts can be assembled as a 3-pole device for 3-phase circuits. (Sec. 490.21.)
Types of switches used in medium-voltage applications include:
1. Enclosed air-break load-interrupter switchgear with or without power fuses
2. Oil-filled cutouts (fused or unfused)
3. Oil-immersed-type disconnect switches
Modern load-interrupter switchgear in metal safety enclosures finds wide application in medium-voltage distribution systems, in combination with modern power fuses (Fig. 490-12). 230.208 of the NE Code covers use of air load-interrupter switches, with fuses, for disconnect and overcurrent protection of medium-voltage service-entrance conductors, and refers the reader to part II of Art. 490. Part (E) of 490.21 also covers use of fused air load-interrupter switches for medium-voltage feeder in distribution systems, as well as at service equipment.
Metal-enclosed fused load interrupters offer a fully effective alternative to use of power CBs, with substantial economies, in 5- and 15-kV distribution systems for commercial, institutional, and industrial buildings. Typical applications for such switchgear parallel to those of power CBs include the following:
Fig. 490-11. Gang-operated 3-pole assembly of oil-filled cutouts provided addition of a new medium-voltage circuit on a modernization project, but location of the units was questioned because part (D)(7) imposes a 5-ft (1.5 m) maximum mounting height. (Sec. 490.21.)
1. In switching centers—Switchgear is set up for control and protection of individual primary feeders to transformer loadcenters.
2. In substation primaries—Load-interrupter switchgear is used for transformer switching and protection in the primary sides of substations.
3. In substation secondaries—Here the switchgear is used as a switching center closely coupled to a medium-voltage transformer secondary.
4. In service entrances—This is a single-unit application of a switchgear bay for service-entrance disconnect and protection in a primary supply line.
Fused load-interrupter switchgear, typically rated up to 1200 A, can match the ratings and required performance capabilities of power CBs for a large percentage of applications in which either might be used.
Fuse-interrupter switches for medium-voltage circuits are available with manual or power operation—including types with spring-powered, over-center mechanisms for manual operation or motor-driven, stored-energy operators. Available in indoor and outdoor housings, assemblies can be equipped with a variety of accessory devices, including key-interlocks for coordinating switch operation with remote devices such as transformer secondary breakers.
Fig. 490-12. Load-interrupter switchgear is generally used with fuses to provide protection as well as load-break switching for medium-voltage circuits. The fuses must be rated to provide complete protection for the load interrupter on closing, carrying, or interrupting current—up to the assigned maximum short-circuit rating. (Sec. 490.21.)
Vacuum switchgear, with their contacts operating in a vacuum “bottle” that is enclosed in a compact cylindrical assembly, has gained wide acceptance as load interrupters for medium-voltage switching and sectionalizing. Available in 200- and 600-A ratings for use at 15.5, 27, and 38 kV, this switching equipment is suited to full-load interruption and is rated for 15,000- or 20,000-A short-circuit current under momentary and make-and-latch operations. BIL ratings are 95, 125, or 150 kV.
Vacuum switch assemblies, with a variety of accessories, including stored-energy operators and electric motor operators for remote control, are suited to all indoor and outdoor switching operations—including submersible operation for underground systems. The units offer fireproof and explosionproof operation, with virtually maintenance-free life for their rated 5000 load interruptions. Units are available in standard 2-, 3-, and 4-way configurations, along with automatic transfer options. Accessory CTs and relays can be used with stored-energy operators to apply vacuum switches for fault-interrupting duty.
490.22. Isolating Means. Air-break or oil-immersed switches of any type may be used to provide the isolating functions described in this section. Distribution cutouts or oil-filled cutouts are also used as isolating switches.
Oil-immersed disconnect switches are used for load control and for sectionalizing of primary-voltage underground-distribution systems for large commercial and industrial layouts (Fig. 490-13). Designed for high-power handling—such as 400 A up to 34 kV—this type of switch can be located at transformer loadcenter primaries or at other strategic points in medium-voltage circuits to provide a wide variety of sectionalizing arrangements to provide alternate feeds for essential load circuits.
Fig. 490-13. Oil switches are commonly used for isolating equipment and circuits for sectionalizing and for transfer from preferred to emergency supply. (Sec. 490.22.)
Oil-immersed disconnect switches are available for as many as five switch positions and ground positions to ground the feeder or test-ground positions for grounding or testing. Ground positions are used in such switches to connect circuits to ground while they are being worked on to assure safety to personnel.
Oil switches for load-break applications up to 15 kV are available for either manual or electrically powered switching for all types of circuits. When electrical operation is used, the switch functions as a medium-voltage magnetic contactor.
Switches intended only for isolating duty must be interlocked with other devices to prevent opening of the isolating switch under load, or the isolating switch must be provided with an obvious sign warning against opening the switch under load.
490.24. Minimum Space Separation. For field fabricated installations, these are the distances that have to be met in terms of clearances between uninsulated live parts and from such parts to ground. If equipment has been formally manufactured and evaluated in accordance with recognized standards, then the dimensions in this table no longer apply.
490.30. General. Where the previous sections presented regulations on the individual switching and protective devices, this section covers enclosure and interconnection of such unit devices into overall assemblies. Basically, the rules of this section are aimed at the manufacturers and assemblers of such equipment.
Use of all medium-voltage switching and control equipment must be carefully checked against information given with certification of the equipment by a test laboratory—such as data given by UL in their Green Book. Typical data are as follows:
Unit substations listed by UL have the secondary neutral bonded to the enclosure and have provision on the neutral for connection of a grounding conductor. A terminal is also provided on the enclosure near the line terminals for use with an equipment grounding conductor run from the enclosure of primary equipment feeding the unit sub to the enclosure of the unit sub. Connection of such an equipment grounding conductor provides proper bonding together of equipment enclosures where the primary feed to the unit sub is directly buried underground or is run in nonmetallic conduit without a metal conduit connection in the primary feed (Fig. 490-14).
The rule of 490.44 is particularly aimed at the designers and installers of equipment, rather than at manufacturers. Part (B) emphasizes that careful layout and application of switching components of all types is important. Figure 490-15 shows the kind of condition that can be extremely hazardous in medium-voltage layouts where there is the chance of a secondary to primary feedback—such as the intentional one shown to provide emergency power to essential circuits in Building 2. Under emergency conditions, the main fused interrupter is opened and the secondary CB for the generator is closed, feeding power to the 480-V switchboard in Building 1 and then feeding through two transformers to supply power to the 480-V circuits in Building 2. This hookup makes the load side of the main interrupter switch live, presenting the hazard of electrocution to any personnel who might go into the switch thinking that it is dead because it is open. A second switch can eliminate this difficulty, if applied with interlocks.
Fig. 490-14. NEC rules on equipment construction are supported by UL data. (Sec. 490.24.)
Fig. 490-15. Feedback in medium-voltage hookups can be hazardous. (Sec. 490.24.)
490.35. Accessibility of Energized Parts. Equipment operating at 600 V and below must be excluded from medium-voltage enclosures with exposed live parts [part (A)] unless access under hazardous conditions is eliminated due to interlocks or other procedures. However, space heaters, and also medium-voltage instrument or control transformers [part (C)] are permitted in such spaces without any access limitations beyond the locked doors required in part (A).
490.41. Location of Devices. Operating handles for control and instrument transfer switches must be readily accessible, and not over 2.0 m (78 in.) high. Part (B), however, waives this requirement for “infrequently operated devices” and that is proving to be somewhat controversial. On one level, just about all medium-voltage switching mechanisms are infrequently operated, so do they all get a waiver? One way to approach this is in terms of infrequent operation among the various medium-voltage operators, that is, equipment qualifies under (B) if it is very infrequently operated, as opposed to merely infrequently so. Another question in circulation is whether the criterion should even be infrequent operation, as opposed to difficult operation, as is done right now in (A) and its exception.
490.46. Circuit Breaker Locking. This is the medium-voltage version of the locking-mechanism-to-remain-in-place rule, addressing drawout gear, etc.
490.47. Metal-Enclosed and Metal-Clad Service Equipment. This rule requires medium-voltage service equipment manufacturers to provide a grounding busbar that extends into the compartment where the service conductors will terminate. The busbar will provide a termination point for cable shielding connections to earth, as well as an appropriate point to attach safety grounds for the protection of personnel who may need to work on the equipment.
