3. Based on the size of the service-entrance conductors (5 × 500 kcmil = 2500 kcmil per phase leg), the minimum size of grounding electrode conductor to the water pipe is 3/0 AWG copper or 250-kcmil aluminum or copper-clad aluminum.

4. The connection to the ground rod at B satisfies the rule of 250.53 (D)(2), requiring a water-pipe electrode to be supplemented by another electrode.

5. But, the minimum size of grounding electrode conductor B required between the neutral bus and the rod, pipe, or plate electrode is 6 AWG copper or 4 AWG aluminum, as covered by part (A) of 250.66 . And the Code does not require the conductor to a rod, pipe, or plate electrode to be larger than 6 AWG, regardless of the size of the service phases. As discussed under Sec. 250.53(C) and shown in Fig. 250-37, the conductor at B in Fig. 250-57 can be considered to be a bonding jumper, as covered by the sizing rule in 250.64 (F) which also says in effect that the conductor to the ground rod need not be larger than 6 AWG copper or 4 AWG aluminum due to the reference to 250.66 (A).

Parts (B) and (C) of the rule make clear that a grounding electrode conductor does not have to be larger than a conductor-type electrode to which it connects. 250.52 (A)(3) recognizes a “concrete-encased” electrode—which must be at least 20 ft (6.0 m) of one or more ½-in. (13-mm)-diameter steel reinforcing bars or rods in the concrete or at least 20 ft (6.0 m) of bare 4 AWG copper conductor (or a larger conductor), concrete-encased in the footing or foundation of a building or structure. Section 250.52(A)(4) recognizes a “ground-ring” electrode made up of at least 20 ft (6.0 m) of 2 AWG bare copper conductor (or larger), buried directly in the earth at a depth of at least 2½ ft (750 mm). Those electrodes described under 250.52(A)(1) through (4) are not “rod, pipe, or plate electrodes.” As electrodes from 250.52(A)(1) through (4), which must be bonded where present, such electrodes would normally be subject to the basic rule of 250.66, which calls for connection to any such electrode by a grounding electrode conductor sized from Table 250.66—requiring up to No. 3/0 copper for use on high-capacity services. But, that is not required, as explained in these paragraphs of the two parts.

Parts (B) and (C) recognize that there is no reason to use a grounding electrode conductor that is larger than a conductor-electrode to which it connects. The grounding electrode conductor need not be larger than 4 AWG copper for a 4 AWG concrete-encased electrode and need not be larger than 2 AWG copper if it connects to a ground-ring electrode—as in part (C) of 250.66. Where Table 250.66 would permit a grounding electrode conductor smaller than 4 AWG or 2 AWG (based on size of service conductors), the smaller conductor may be used—but the electrode itself must not be smaller than 4 AWG or 2 AWG. See Fig. 250-36.

The first note under Table 250.66 correlates to 230.40, Exception No. 2, and 250.64(D), as follows:

When two to six service disconnects in separate enclosures are used at a service, with a separate set of SE conductors run to each disconnect, the size of a single common grounding electrode conductor must be based on the largest sum of the cross sections of the same phase leg of each of the several sets of SE conductors. When using multiple service disconnects in separate enclosures, with a set of SE conductors run to each from the drop or lateral (230.40, Exception No. 2) and using a single, common grounding electrode conductor, either run continuous and unspliced from one disconnect to another and then to the grounding electrode, as in the upper right drawing of Fig. 250-49 (which will be impracticable in almost all cases due to termination restrictions in the enclosures), or with taps from each disconnect to a common grounding electrode conductor run to the electrode—as in 250.64(D)(1), this note is used to determine the size of the common grounding electrode conductor from Table 250.66. The “equivalent area” of the size of SE conductors is the largest sum of the cross-sectional areas of one ungrounded leg of each of the several sets of SE conductors.

250.68. Grounding Electrode Conductor Connection and Bonding Jumper Connections to Grounding Electrodes. The rule requires that the connection of a grounding electrode conductor to the grounding electrode “shall be accessible” (Fig. 250-58). [250.104(A) also requires that any clamp for a bonding jumper to interior metal water piping must be accessible.] Inspectors want to be able to see and/or be able to get at any connection to a grounding electrode. But because there are electrodes permitted in 250.52 that would require underground or concrete-encased connections, an Exception was added to the basic rule to permit inaccessible connections in such cases (Fig. 250-59). Electrode connections that are not encased or buried—such as where they are made to exposed parts of electrodes that are encased, driven, or buried—must be accessible. This section now places the burden on the installer to make such connections accessible wherever possible.

The second exception to 250.68(A) covers accessibility when connections are made to steel framing members that will be subsequently encased in fireproofing compounds, rendering such connections inaccessible. In such cases, there are often two connection issues in the same location. The first issue is the connection of the grounding electrode connection to a lug, which must be an exothermic weld or an irreversible crimp in order to be acceptable without remaining accessible. The second issue is how the lug is attached to the metal framing. In this case any mechanical connection is acceptable, even if reversible.

The second part of this section requires an effective grounding path, and to make this to be the case on a metal piping system electrode, use the proper clamp shown in Fig. 250-58. If the connection is to be remote from the point of entry for the water pipe into the building, as shown in Fig. 250.59, make sure the interior home run for the piping qualifies in terms of access to the piping and the qualifications of those supervising the installation. In addition, add bonding jumpers around any insulated joints or around equipment likely to be removed for repair, as covered in 250.53(D)(1). In a typical case of grounding for a local transformer within a building, 250.30(A)(7)(1) and 250.52(A)(1) note that grounding of the secondary neutral, where there is no building steel available, must be made to the water pipe within 1.5 m (5 ft) from its point of entry in the building unless the piping qualifies for the exception. And 250.68(B)

Fig. 250-58. Whenever possible, connections to grounding electrodes must be “accessible.” Note that the water-pipe clamp, if used on copper water tubing as opposed to heavy-wall red brass or galvanized steel pipe, must be marked for this service, as required by the UL Grounding and Bonding Standard, #467. [Secs. 250.68. and 110.3(B).]

requires that bonding jumpers be used to ensure continuity of the ground path back to the underground pipe for that portion permitted to serve as a grounding electrode by 250.52(A)(1) wherever the piping may contain insulating sections or is liable to become disconnected. Bonding jumpers around unions, valves, water meters, and other points where a water piping system electrode might be opened must have enough slack to permit removal of the part. Hazard is created when bonding jumpers are so short that they have to be removed to remove the equipment they jumper. Dangerous conditions have been reported about this matter. Bonding jumpers must be long enough to ensure grounding integrity along piping systems under any conditions of maintenance or repair.

Fig. 250-59. Although required, bonding of metal piping can pose problems. And even though the connection to the pipe may be made without regard to the pipe’s point of entry in some occupancies, that permission only applies to water pipes that are supervisable. Unless this is an “industrial” or “commercial” installation with qualified maintenance people and the water pipe is completely exposed and visible for the entire distance from its point of entry to the grounding electrode conductor connection, such connection from the grounded conductor must be made no further than 5 ft (1.52 m) from the pipe’s entry point. (Sec. 250.52(A)(1) Exception.)

250.70. Method of Grounding Conductor Connection to Electrodes. Because 250.53(G) requires buried or protected connections of grounding electrode conductors to ground rods, the third sentence of this section requires that a buried ground clamp be of such material and construction that it has been designed, tested, and marked for use directly in the earth. And any clamp that is used with two or more conductors must be designed, tested, and marked for the number and types of conductors that may be used with it. This is shown in the bottom drawing of Fig. 250-60. Connections depending on solder cannot be used due to the likelihood that the solder will melt during a sustained fault. Figure 250-58 gives some listing information about water-pipe clamps.

250.86. Other Conductor Enclosures and Raceways. The basic rule requires all enclosures and raceways to be grounded. But, Exception No. 1 permits the installation of short runs as extensions from existing open wiring, knob-and-tube work, or nonmetallic-sheathed cable without grounding where there is little likelihood of an accidental connection to ground or of a person touching both the conduit, raceway, or armor and any grounded metal or other grounded surface at the same time. Additionally, the Exception permits “short sections” of enclosures and raceways to be used as sleeving or to otherwise support cables. The exact length of such ungrounded metallic enclosures and raceways is not given. That determination will ultimately be made by the local electrical inspector.

The last exception covers instances where metal conduit sweeps are inserted into nonmetallic conduit runs to prevent the pulling rope, when under high tension, from sawing through the inner radius of the bend and destroying the integrity of the conduit system. If the sweep is buried low enough so its upper end is still 450 mm (18 in.) below grade level, or if it is entirely encased including the ends in 50 mm (2 in.) of concrete, (i.e., the concrete has to encase the steel/PVC raceway system so it overlaps the ends of the steel sweep by at least 50 mm [2 in.]), bonding is not required.

Fig. 250-60. Encased and buried electrode connections are permitted by exception to the basic rule. The bonding bushing at the bottom of the sweep almost certainly does not have a direct burial listing and would require modifications and inspection approval to be used in this way. A better option in this case would be a “U-bolt” style ground clamp around the conduit at the end of the sweep. They are available in constructions that are listed for direct burial. Another option would be to use PVC conduit and skip the bonding issue altogether. (Sec. 250.68.)

Part V. Bonding

250.90. General. One of the most interesting and controversial phases of electrical work involves the grounding and bonding of secondary-voltage service-entrance equipment. Modern practice in such work varies according to local interpretations of Code requirements and specifications of design engineers. In all cases, however, the basic intent is to provide an installation which is essentially in compliance with National Electrical Code rules on the subject, using practical methods for achieving objectives.

In order to ensure electrical continuity of the grounding circuit, bonding (special precautions to ensure a permanent, low-resistance connection) is required at all conduit connections in the service equipment and where any nonconductive coating exists which might impair such continuity. This includes bonding at connections between service raceways, service cable armor, all service-equipment enclosures containing service-entrance conductors, including meter fittings, boxes, and the like.

The need for effective grounding and bonding of service equipment arises from the electrical characteristics of utility-supply circuits. In the common arrangement, service conductors are run to a building and the service overcurrent protection is placed near the point of entry of the conductors into the building, at the load end of the conductors. With such a layout, the service conductors are not properly protected against ground faults or shorts occurring on the supply side of the service overcurrent protection. Generally, the only protection for the service conductors is on the primary side of the utility’s distribution transformer. By providing “bonded” connections (connecting with special care to reliable conductivity), any short circuit in the service-drop or service-entrance conductors is given the greatest chance of burning itself clear—because there is no effective overcurrent protection ahead of those conductors to provide opening of the circuit on such heavy fault currents. And for any contact between an energized service conductor and a grounded service raceway, fittings, or enclosures, bonding provides discharge of the fault current to the system grounding electrode—and again burning the fault clear. This condition of services is shown in Fig. 250-61.

250.92. Services. Because of the requirement set forth in 250.92, all enclosures for service conductors must be grounded to prevent a potential above ground on the enclosures as a result of fault—which would be a very definite hazard—and to facilitate operation of overcurrent devices anywhere on the supply side of the service conductors. However, because of the distant location of the protection and the normal impedance of supply cables, it is important that any fault to an enclosure of a hot service conductor of a grounded electrical system find a firm, continuous, low-impedance path to ground to ensure sufficient current flow to operate the primary protective device or to burn the fault clear quickly. This means that all enclosures containing the service conductors—service raceway, cable armor, boxes, fittings, cabinets—must be effectively bonded together; that is, they must have low impedance through themselves and must be securely connected to each other to ensure a continuous path of sufficient conductivity to the conductor which makes the connection to ground (Fig. 250-62).

Fig. 250-61. Service bonding must ensure burn-clear on shorts and grounds in service conductors. (Sec. 250.90.)

Fig. 250-62. Bonding ensures low-impedance path through all service conductor enclosures. (Sec. 250.92.)

The spirit of the Code and good engineering practice have long recognized that the conductivity of any equipment ground path should be at least equivalent to 25 percent of the conductivity of any phase conductor with which the ground path will act as a circuit conductor on a ground fault. Or, to put it another way, making the relationship without reference to insulation or temperature rise, the impedance of the ground path must not be greater than four times the impedance of any phase conductor with which it is associated.

In ungrounded electrical systems, the same careful attention should be paid to the matter of bonding together the noncurrent-carrying metal parts of all enclosures containing service conductors. Such a low-impedance ground path will quickly and surely corner ground any hot conductor which might accidentally become common with the enclosure system, and if a second fault occurs from a different phase, the low-impedance path will pass enough current so the fault will burn free.

Specific NE Code requirements on grounding and bonding are as follows:

1. 250.80 requires that service raceways, metal sheath of service cables, metering enclosures, and cabinets for service disconnect and protection be connected to either the grounding electrode conductor or the grounded circuit conductor. An exception to this rule is made in the case of certain lead-sheathed cable services as covered in 250.84. And, the Code requires that flexible metal conduit or liquidtight flexible metal conduit used in a run of service raceway must be bonded around (Fig. 250-63). 230.43 states that rule on flex and lists the only types of raceway that may enclose service-entrance conductors.

2. 250.80 also requires that service raceways and service cable sheaths or armoring—when of metal—be grounded.

3. 250.92 sets forth the service equipment which must be bonded—that is, the equipment for which the continuity of the grounding path must be specifically ensured by using specific connecting devices or techniques. As indicated in Fig. 250-64, this equipment includes (1) service raceway, cable trays, cable sheath, and cable armor; and (2) all service-equipment enclosures containing service-entrance conductors, including meter fittings, boxes, etc., interposed in the service raceway or armor.

Fig. 250-63. Flex may be used as a service raceway, with a jumper. (Sec. 250.92.)

Fig. 250-64. “Bonding” consists of using prescribed fittings and/or methods for connecting components enclosing SE conductors. (Sec. 250.92.)

Note that this section used to have requirement that reiterated requirements in 250.64(E) on the required continuity of ferrous metal enclosures for grounding electrode conductors, as shown in Fig. 250-65. Since it is adequately covered there it has been deleted here, but the requirements still apply.

250.92(B) covers “Method of Bonding at the Service.” 250.92(A) is very specific in listing the many types of equipment that require bonding connections, but the actual “how to” is often hazy. For virtually every individual situation where a bonding connection must be made, there is a variety of products available in the market which present the installer with a choice of different methods.

This section sets forth the specific means which may be used to connect service-conductor enclosures together to satisfy the bonding requirements of 250.92(A). These means include:

1. Bonding equipment to the grounded service conductor by means of suitable lugs, pressure connectors, clamps, or other approved means—except that soldered connections must not be used. 250.142 permits grounding of meter housings and service equipment to the grounded service conductor on the supply side of the service disconnecting means.

2. Threaded couplings in rigid metal conduit or IMC (intermediate metal conduit) runs and threaded bosses on enclosures to which rigid metal conduit or IMC connects.

3. Threadless couplings and connectors made up tight for rigid metal conduit, IMC, or electrical metallic tubing, or for metal-clad cables.

4. Other devices (not standard locknuts and bushings) listed for the purpose, such as bonding locknuts, wedges, and bushings.

Fig. 250-65. Grounding-conductor enclosure must be “bonded” at both ends of ferrous metal enclosures. (Sec. 250.92.)

In general, bonding jumpers must be used around concentric or eccentric knockouts which are punched or otherwise formed in such a manner that would impair the electrical current flow through the reduced cross section of metal that bridges between the enclosure wall and punched ring of the KO (knockout). And the bonding jumpers must be sized from 250.102(C).

Based on those briefly worded Code requirements, modern practice follows more or less standard methods.

Where a rigid conduit is the service raceway, threaded or threadless couplings are used to couple sections of a conduit together. A conduit connection to a meter socket may be made by connecting a threaded conduit end to a threaded hub or boss on the socket housing, where the housing is so constructed; by a locknut and bonding bushing; by a locknut outside with a bonding wedge or bonding locknut and a standard metal or completely insulating bushing inside; or sometimes by a locknut and standard bushing where the socket enclosure is bonded to the grounded service conductor. Conduit connections to KOs in sheet metal enclosures can be made with a bonding locknut (Fig. 250-66), a bonding wedge, or a bonding bushing where no KO rings remain around the opening through which the conduit enters and where the box is listed for such use, even where the KOs have not all been removed. But, generally, where a KO ring does remain around the conduit entry hole, a bonding bushing or wedge with a

Fig. 250-66. Bonding locknut is a recognized method for bonding a service conduit nipple to a meter socket, when the KO is clean (no rings left in enclosure wall) or is cut on the job. With plastic bushing permitted, this is the most economical of the several methods for making a bonded conduit termination. (Sec. 250.94.)

jumper wire must be used to assure a path of continuity from the conduit to the enclosure. Figure 250-67 summarizes the various acceptable techniques. It should be noted that the use of the common locknut and bushing type of connection is not allowed. Neither is the use of double locknuts—one inside, one outside—and a bushing, although that is permitted on the load side of the service equipment. The special methods set forth in 250.92(B) are designed to prevent poor connections or loosening of connections due to vibration. This minimizes the possibility of arcing and consequent damage which might result when a service conductor faults to the grounded equipment.

Fig. 250-67. Methods for “bonding” wiring methods to sheet metal enclosures. (Sec. 250.94.)

Similar provisions are used to ensure continuity of the ground path when EMT is the service raceway or when armored cable is used. EMT is coupled or connected by threadless devices—compression-type, indenter-type, or set-screw type, using raintight type outdoors. Although a threadless box connector is suitable to provide a bonded connection of the connector to the metal raceway (rigid metal, IMC, EMT), it is also necessary to provide a “bonded” connection between the connector and the metal enclosure. A threadless box connector on the end of EMT used as a service raceway provides satisfactory bonding of the EMT to the connector, but the last sentence of this rule says that a standard locknut or a standard bushing connected to the threaded end of the connector does not provide the required bonding of the connector to the metal service equipment to which the connector is connected. On the end of the connector, a bonding locknut or bonding bushings with or without jumpers must be used if the knockout is clean (all rings punched out or clean knockout punched on the job). If concentric or eccentric rings are left, a grounding locknut with a jumper, a grounding bushing with a jumper, or a grounding wedge with a jumper must be used to provide bonded connection around the perforated knockout. And fittings used with service cable armor must assure the same degree of continuity of the ground path.

The use of bonding bushings, bonding wedges, and bonding locknuts is recognized without reference to types of raceways or types of connectors used with the raceways or cable armor. As a result, common sense and experience have molded modern field practice in making raceway and armored service cable connection to service cabinets. The top of Fig. 250-68 shows how a bonding wedge is used on existing connections at services or for raceway connections on the load side of the service—such as required by 501.30(B) for Class I hazardous locations. A bonding bushing with provision for connecting a bonding jumper, is the common method for new service installations where

Fig. 250-68. Bonding bushings and similar fittings must be used in their intended manners. (Sec. 250.94.)

some of the concentric or eccentric “doughnuts” (knockout rings) are left in the wall of the enclosure, therefore requiring a bonding jumper. Great care must be taken to ensure that each and every type of bushing, locknut, or other fitting is used in the way for which it is intended to best perform the bonding function.

Figure 250-69 shows detailed application of the preceding rules to typical meter-socket installations. Meter-enclosure bonding techniques are shown in Fig. 250-70. Bonding details for current-transformer installations are shown in Fig. 250-71. Those illustrations are intended to portray typical field practice aimed at satisfying the various Code rules.

250.94. Bonding for Other Systems. This section calls for a ready, effective “inter-system” bonding and grounding of different systems, such as communications (telephone), lightning rod systems, and community antenna television (CATV)

Fig. 250-69. Typical meter socket applications. (Sec. 250.94.)

Fig. 250-70. Typical meter-enclosure installations (120/208- or 120/240-V services). (Sec. 250.94.)

systems at the service equipment for all buildings, not just dwellings, and at both services and at building disconnects. The rule requires that there be an “intersystem bonding termination . . . accessible for connection and inspection . . . [with the] capacity for connection of not less than three intersystem bonding conductors.” This makes provision for bonding metal enclosures of, say, telephone equipment to metal enclosures of electrical system components to reduce voltage differences between such metal enclosures as a result of lightning

Fig. 250-71. Bonding at CT cabinets. (Sec. 250.94.)

or power contacts. This rule was placed in the NE Code because 800.100(D) and 800.100(B)(1) requires bonding interconnection between a building’s power grounding electrode system and the “protector ground” (grounding electrode conductor) of telephone and other communications systems, and because making that bonded interconnection has become more difficult. Both 810.21(J) and 810.21(F)(1) and 820.100(D) and 820.100(B)(1) require such grounding interconnections, as does 830.100(D) and 830.100(B)(1), which covers “Network-Powered Broadband Communications Systems.”

The proposal for this Code addition in its original form for the 1981 NEC included the following commentary:

In the past, the bond between communications and power systems was usually achieved by connecting the communications protector grounds to an interior water pipe. Where the power was grounded to a ground rod, the bond was connected to the power grounding-electrode conductor or to metallic service conduit, which were usually accessible. With growing use of plastic water pipe, the tendency for service equipment to be installed in finished areas where the grounding electrode conductor is often concealed, and the use of plastic entrance conduit, communications installers no longer have an easily identifiable point for connecting bonds or grounds.

Where lightning or external power fault currents flow in protective grounding systems, there can be dangerous potential differences between the equipment of those systems. Even with the required common or bonded electrodes, lightning currents flowing in noncommon portions of the grounding system result in significant potential differences as a result of inductive voltage drop in the noncommon conductor. If a current flows through a noncommon grounding conductor 10 ft (3.05 m) long, there can be an inductive voltage drop as high as 4000 volts. If that noncommon conductor is either the power grounding-electrode conductor or the communications-protector grounding conductor, the voltage will appear between communication-equipment and power-equipment enclosures. The best technical solution to minimizing that voltage is with a short bond between the service equipment and the communications-protector ground terminal. The conductor to the grounding electrode is then common, and the voltage drop in it does not result in a potential difference between systems.

An externally accessible point for intersystem bonding should be provided at the electrical service if accessible metallic service-entrance conduit is not present or if the grounding-electrode conductor is not accessible. This point could be in the form of a connector, tapped hole, external stud, a combination connector-SE cable clamp, or some other approved means located at the meter base or service equipment enclosure.

Prior to the 2008 NEC intersystem bonding terminations were done using exposed portions of service equipment, or grounding electrode conductors, and if they were not accessible for external connections, as in finished basements, then a short length of 6 AWG was to be left or other approved means. Where this has been done in existing construction, it can remain by virtue of an exception that, in effect, was the prior rule.

A definite shock hazard can arise if a common grounding electrode conductor is not used to ground both the bonded service neutral and the communications protector. The problem can be solved by simply bonding the ground terminal of the protector to a grounded enclosure of the service equipment (the service panelboard enclosure or the meter socket) and not using the separate telephone grounding electrode conductor. The basic concept has been in the NEC for a very long time. The changes in 2008 mandate specific design components and performance criteria, including the requirement that the terminating device not obstruct the opening of an enclosure.

250.96. Bonding Other Enclosures. Metal raceways, cable sheaths, equipment frames and enclosures, and all other metal noncurrent-carrying parts must be carefully interconnected with Code-recognized fittings and methods to ensure a low-impedance equipment grounding path for fault current—whether or not an equipment grounding conductor (a ground wire) is run within the raceway and connected enclosures. The interconnected system of metal raceways and enclosures must itself form a Code-conforming equipment grounding path—even if a “supplementary equipment grounding conductor” is used within the metal-enclosure grounding system (Fig. 250-72).

Fig. 250-72. Interconnected metal enclosures (boxes, raceways, cabinets, housings, etc.) must form a continuous equipment grounding path, even if a separate equipment grounding wire is run within the metal enclosure system, except as shown in bottom part of the figure, to eliminate “noise” on the grounding circuit. (Sec. 250.96.)

The rule of 250.96(B) recognizes that to help reduce electromagnetic noise or interference on a grounding circuit, an insulating “spacer or fitting” may be used to interrupt the electrical continuity of a metallic raceway system used to enclose the branch-circuit conductors at the point of connection to the metal enclosure of a single piece of equipment.

This rule permits interrupting the current path between a metal equipment enclosure and the metal conduit that supplies the enclosure—but only if the metal conduit is grounded at its supply end and an equipment grounding conductor is run through the conduit into the metal enclosure and is connected to an equipment grounding terminal of the enclosure, to provide safety grounding of the metal enclosure. Provisions for an equipment ground reference separate from the metallic raceway system is covered by 250.146(D) for electronic equipment that is cord- and plug-connected. This rule covers a separate equipment ground reference for hard-wired sensitive electronic equipment.

Fig. 250-73. For circuits over 250 V to ground, a bonding jumper may be needed at conduit termination. (Sec. 250.97.)

250.97. Bonding for Over 250 V. Single locknut-and-bushing terminations are permitted for 120/240- and 120/208-V systems. Any 480/277-V grounded system, 480-V ungrounded system, or higher must generally use double locknut-and-bushing terminals on clean knockouts of sheet metal enclosures (no concentric rings in wall) for rigid metal conduit and IMC (Fig. 250-73).

Where good electrical continuity is desired on installations of rigid metal conduit or IMC, two locknuts should always be provided on clean knockouts (no rings left) of sheet metal enclosures so that the metal of the box can be solidly clamped between the locknuts, one being on the outside and one on the inside. The reason for not relying on the bushing in place of the inside locknut is that both the conduit and the box may be secured in place and if the conduit is placed so that it extends into the box to a greater distance than the thickness of the bushing, the bushing will not make contact with the inside surface of the box. But that possible weakness in the single-locknut termination does not exclude it from use on systems up to 250 V to ground.

The Exception to the main rule here has the effect of requiring that a bonding jumper must be used around any “oversized, concentric, or eccentric knockouts” in enclosures for circuits over 250 V to ground that are run in a metal raceway or cable unless the enclosure or fittings have been investigated and listed for use without a bonding jumper. Many such enclosures and fittings are so listed and are readily available, in particular, most conventional steel outlet boxes with concentric knockouts. Clearly, the use of that equipment will serve to reduce labor costs. But, generally for such circuits, a bonding jumper must be used at any conduit or cable termination in other than a clean, unimpaired opening in an enclosure (Fig. 250-73). For example, the majority of larger enclosures with concentric knockouts, including most panelboards and wireways, are not listed as providing the required continuity and a bonding jumper will be required.

In any case where all the punched rings (the “doughnuts”) are not removed and the box is not listed for use without a bonding jumper, or, where all the rings are removed but a reducing washer is used to accept a smaller size of conduit, a bonding jumper must be installed from a suitable ground terminal in the enclosure to a lug on the bushing or locknut of the termination of any conduit or cable containing conductors operating at over 250 V to ground. Such circuits include 480/277-V circuits (grounded or ungrounded); 480-, 550-, 600/347-, and 600-V circuits; and higher-voltage circuits. A bonding jumper is not needed for terminations of conduit that carry such circuits through KOs that are punched on the job to accept the corresponding size of conduit. But, double locknuts (one inside and one outside the enclosure) must be used on threaded conduit ends, or suitable threadless connectors or other fittings must be used on rigid or flexible conduit, EMT, or cable.

250.98. Bonding Loosely Jointed Metal Raceways. Provision must be made for possible expansion and contraction in concrete slabs due to temperature changes by installing expansion joints in long runs of raceways run through slabs. See 300.7(B). Because such expansion joints are loosely jointed to permit back-and-forth movement to handle changes in gap between butting slabs, bonding jumpers must be used for equipment grounding continuity (Fig. 250-74). Expansion fittings may be selected as vibration dampers and deflection mediums as well as to provide for movement between building sections or for expansion and contraction due to temperature changes in long conduit runs. The fitting diagrammed in Fig. 250-74 provides for movement from the normal in all directions plus 30° deflection, is available up to 4-in. (101.5 mm) in diameter, and may be installed in concrete. Expansion fittings are available from some manufacturers that have special internal designs and which have enabled them to be listed as providing the required continuity without the use of supplemental bonding jumpers.

Fig. 250-74. Conduit expansion fitting includes bonding jumper for ground continuity. (Sec. 250.98.)

250.100. Bonding in Hazardous (Classified) Locations. All raceway terminations in hazardous locations must be made by one of the techniques shown in Fig. 250-67 for service raceways. And as required by 501.30(A) such bonding techniques must be used in “all intervening raceways, fittings, boxes, enclosures, etc., between hazardous areas and the point of grounding for service equipment.” Refer to 501.30(A), 502.30(A), and 503.30(A) (Fig. 250-75).

Fig. 250-75. Bonded raceway terminations must be used at sheet metal KOs in hazardous areas. (Sec. 250.100.)

250.102. Equipment Bonding Jumpers. The jumper shown in Fig. 250-76 running from one conduit bushing to the other and then to the equipment ground bus is defined by the NE Code as an “equipment bonding jumper.” Equipment bonding jumpers must be made of copper or other corrosion-resistant material, and they can be in the form of wires, busbars, screws, or any other suitable conductor. They must be connected using one of the methods in 250.8 (or 250.70 if connecting to a grounding electrode). The rest of this section covers sizing calculations on the supply side of the service (Part C), sizing calculations on the load side of the service (Part D), and how to install these jumpers in the event they run with their associated conductors (Part E) instead of simply making bonding connections at terminations, as shown in Fig. 250-76.

(Part C) applies to the supply side of services, and actually, on paralleled installations, incorporates many different calculations in one paragraph. The first involves one jumper to all the raceways of the circuit (Fig. 250-76). Since, in the process of making its connection to the bus, it sees all the raceways, it is sized on the basis of the entire service, for which the largest phase conductor (in this as in most cases all are equal) is 3 × 500 kcmil = 1500 kcmil. Since this is larger than the coverage of Table 250.66, its size must be calculated on the basis of (12.5 percent) of the phase area just determined. 1,500,000 cm ÷ 8 = 187,500 cm. Consulting Table 8 in Chap. 9 of the NEC, the next larger wire size is 4/0 copper, and that is the size required for this wire. This procedure is identical to the one in 250.28(D)(1) for a main bonding jumper.

Fig. 250-76. Sizing main bonding jumper and other jumpers at service equipment. (Sec. 250.102.)

In the sketch of Fig. 250-76, if each of the three 4-in. (101.5-mm) conduits has a separate bonding jumper connecting each one individually to the equipment ground bus, then last sentence of part (C) may be applied to an individual bonding jumper for each separate conduit (Fig. 250-77). The size of a separate bonding jumper for each conduit in a parallel service must be not less than the size of the grounding electrode conductor for a service of the size of the phase conductor used in each conduit. Referring to Table 250.66, a 500-kcmil copper service calls for at least a No. 1/0 grounding electrode conductor. Therefore, the bonding jumper run from the bushing lug on each conduit to the ground bus must be at least a No. 1/0 copper (or 3/0 aluminum). Since these are bonding jumpers and not grounding electrode conductors, on a very large service or if multiple parallel sets of conductors ran in single conduits, the area of the largest phase conductor might exceed the reach of Table 250.66, and in such an instance the (12.5 percent) procedure would apply to determine the area of the bonding conductor.

The third sentence of part (C) requires separate bonding jumpers when the service is made up of multiple conduits and the equipment bonding jumper is run within each raceway (such as plastic pipe) for grounding service enclosures. According to the third sentence of part (C), when service-entrance conductors are paralleled in two or more raceways, an equipment bonding jumper that is routed within the raceways must also be run in parallel, one in each raceway, as at the right in Fig. 250-77. This clarifies application of nonmetallic service raceway where parallel conduits are used for parallel service-entrance conductors. As worded, the rule applies to both nonmetallic and metallic conduits where the bonding jumper is run within the raceways rather than from lugs on bonding bushings on the conduit ends. But for metallic conduits stubbed up under service equipment, if the conduit ends are to be bonded to the service equipment enclosure by jumpers from lugs on the conduit bushings, either a single large common bonding jumper may be used—from one lug, to another lug, to another, and so on, and then to the ground bus—or an individual bonding jumper (of smaller size from Table 250.66, based on the size of conductors in each conduit) may be run from each bushing lug to the ground bus.

Fig. 250-77. An individual bonding jumper may be used for each conduit (left) and must be used as shown at right. [Sec. 250.102(C).]

Do not confuse an equipment bonding jumper run in a raceway with a grounded circuit conductor run in the same raceway. For grounded systems, that will be the usual application, and the only equipment bonding will be to the metallic service raceways in the event a metallic wiring method is in use. Remember that the grounded circuit conductor is permitted to, and usually does provide, the supply-side bonding in upstream enclosures such as metering cabinets, as covered in 250.142(A)(1). This rule generally applies to ungrounded or impedance-grounded services where there are no grounded circuit conductors running through the raceways, and the necessity is to maintain bonding of conductive surfaces upstream of the service disconnect.

One other point in this regard. These wires are bonding jumpers, not circuit conductors. The normal 1/0 AWG lower threshold for paralleled “phase, polarity, neutral, or grounded circuit conductors” [310.4(A)] does not apply in this case. For example, if the conduit fill consisted of sets of 350 kcmil, the Table 250.66 result for the bonding conductors would be 2 AWG, and they could be run in the conduit without being increased to 1/0 AWG.

The second sentence of part (C) sets minimum sizes of copper and aluminum service-entrance conductors above which a service bonding jumper must have a cross-sectional area “not less than 12½ percent of the area of the largest phase conductor.” And the rule states that if the service conductors and the bonding jumper are of different material (i.e., service conductors are copper, say, and the jumper is aluminum), the minimum size of the jumper shall be based on the assumed use of phase conductors of the same material as the jumper and with an ampacity equivalent to that of the installed phase conductors (Fig. 250-78). In this case, three sets of 750 kcmil aluminum at 75°C have a combined ampacity of 1155 A. A copper service equal to or above this number would be three sets of 600 kcmil, or 1,800,000 cm. 1,800,000 ÷ 8 (or, × 0.125) = 225,000 cm. Reviewing Table 8 in Chap. 9, the next large size wire would be a 250 kcmil copper bonding jumper.

Part (D) requires a bonding jumper on the load side of the service to be sized as if it were an equipment grounding conductor for the largest circuit

Fig. 250-78. Sizing a copper bonding jumper for aluminum service conductors. [Sec. 250.102(C).]

with which it is used. And sizing would have to be done from Table 250.122, as follows.

Figure 250-79 shows a floor trench in the switchboard room of a large hotel. The conductors are feeder conductors carried from circuit breakers in the main switchboard (just visible in upper right corner of photo) to feeder conduits going out at left, through the concrete wall of the trench, and under the slab floor to the various distribution panels throughout the building. Because the conduits themselves are not metallically connected to the metal switchboard enclosure, bonding must be provided from the conduits to the switchboard ground bus to ensure electrical continuity and conductivity as required by NE Code Secs. 250.4(A)(5), 250.86, 250.110(5), 250.120, and 250.134.

1. The single, common, continuous bonding conductor that bonds all the conduits to the switchboard must be sized in accordance with NE Code Table 250.122, based on the highest rating of CB or fuses protecting any one of the total number of circuits run in all the conduits.

2. Sizing of the single, common bonding jumper would be based on the highest rating of overcurrent protection for any one of the circuits run in the group of conduits. For instance, some of the circuits could be 400-A circuits made up of 500 kcmils in individual 3-in. (76-mm) conduits, and others could be parallel-circuit makeups in multiple conduits—such as 800-A circuits, with two conduits per circuit, and 1200-A circuits, with three conduits. If, for instance, the highest-rated feeder in the group was protected by a 2000-A circuit breaker, then the single, common bonding jumper for all the conduits would have to be 250-kcmil copper or 400-kcmil aluminum—determined readily from Table 250.122, by simply going down the left column to the value of “2000” and then reading across. The single conductor is run through a lug on each of the conduit bushings and then to the switchboard ground bus.

Fig. 250-79. Conduits in trench carry feeder conductors from switchboard at right (arrow) out to various panels and control centers. A single, common bonding jumper—run continuously from bushing to bushing—may be used to bond all conduits to the switchboard ground bus. [Sec. 250.102(D).]

In the case shown in Fig. 250-79, however, because the bonding jumper from the conduit ends to the switchboard is much longer than a jumper would be if the conduits stubbed up under the switchboard, better engineering design might dictate that a separate equipment grounding conductor (rather than a “jumper”) be used for each individual circuit in the group. If one of the conduits is a metric designator 78 (trade size 3) conduit carrying three 500-kcmil conductors from a 400-A CB in the switchboard, the minimum acceptable size of bonding jumper (or equipment grounding conductor) from a grounding bushing on the conduit end to the switchboard ground bus would be a 3 AWG copper or 1 AWG aluminum or copper-clad aluminum, as shown opposite the value of 400 A in the left column of Table 250.122. If another two of the metric designator 78 (trade size 3) conduits are used for a feeder consisting of two parallel sets of three 500-kcmil conductors (each set of three 500-kcmil conductors in a separate conduit) for a circuit protected at 800 A, a single bonding jumper could be used, run from one grounding bushing to the other grounding bushing and then to the switchboard ground bus. This single bonding jumper would have to be a minimum 1/0 AWG copper, from NE Code Table 250.122 on the basis of the 800-A rating of the feeder overcurrent protective device.

With such a long run for a jumper, as shown in Fig. 250-79, Code rules could be interpreted to require that the bonding jumper be subject to the rules of 250.122; that is, use of a bonding jumper must conform to the requirements for equipment grounding conductors. As a result, bonding of conduits for a parallel circuit makeup would have to comply with part (F) in 250.122, which requires equipment grounding conductors to be run in parallel “where conductors are run in parallel in multiple raceways. . . .” That would then be taken to require that bonding jumpers also must be run in parallel for multiple-conduit circuits.

That interpretation is incorrect. The rules in 250.122(F) assure an appropriate fault current path based on the possibility of backfeed into a fault from a downstream parallel bus connection, resulting in the maximum current flow from the overcurrent protective device. Therefore the equipment grounding conductor in each parallel raceway will be paralleled and will be based on the full trip setting of the breaker. When the parallel raceways reach the switchboard the equipment grounding conductors no longer need to be paralleled; they only need to remain sized based on Table 250.122 for the overcurrent device. This is entirely consistent with 250.122(C), which expressly allows a single equipment grounding conductor to protect any number of circuits as long as it is sized based on the highest rating of overcurrent protection on any of the associated conductors. However, it is certainly permissible to run individual conductors. Figure 250-80 shows the two possible arrangements.

Fig. 250-80. One jumper may be used to bond two or more conduits on the load side of the service. [Sec. 250.102(D).]

Part (E) of 250.102 follows the thinking that is described in 250.136 for external grounding of equipment attached to a properly grounded metal rack or structure. A short length of flexible metal conduit, liquidtight flex, or any other raceway may, if the raceway itself is not acceptable as a grounding conductor, be provided with grounding by a “bonding jumper” (note: not an “equipment grounding conductor”) run either inside or outside the raceway or enclosure PROVIDED THAT the length of the equipment bonding jumper is not more than 6 ft (1.8 m) and the jumper is routed with the raceway or enclosure.

Where an equipment bonding conductor is installed within a raceway, it must comply with all the Code rules on identification of equipment grounding conductors. A bonding jumper installed in flexible metal conduit or liquidtight flex serves essentially the same function as an equipment grounding conductor. For that reason, a bonding jumper should comply with the identification rules of 250.119—on the use of bare, green-insulated, or green-taped conductors for equipment grounding.

Note that this application has limited use for the conditions specified and is a special variation from the concept of 250.134(B), which requires grounding conductors run inside raceways. Its big application is for external bonding of short lengths of liquidtight or standard flex, under those conditions where the particular type of flex itself is not suitable for providing the grounding continuity required by 348.60 and 350.60. Refer also to 250.64.

The top of Fig. 250-81 shows how an external bonding jumper may be used with standard flexible metallic conduit (so-called Greenfield). If the length of the flex is not over 6 ft (1.8 m), but the conductors run within the flex are protected at more than 20 A, a bonding jumper must be used either inside or outside the flex. An outside jumper must comply as shown. For a length of listed

Fig. 250-81. Bonding jumper rules for standard flex and liquidtight flex. [Sec. 250.102(E).]

flex not over 6 ft (1.8 m), containing conductors that are protected at not more than 20 A and used with conduit termination fittings that are approved for grounding, a bonding jumper is not required.

The bottom of Fig. 250-81 shows use of an external bonding jumper with liquidtight flexible metallic conduit. If liquidtight flex is not over 6 ft (1.8 m) long but is larger than 1¼-in. trade size, a bonding jumper must be used, installed either inside or outside the liquidtight. An outside jumper must comply as shown. If a length of liquidtight flex larger than 1¼ in. is short enough to permit an external bonding jumper that is not more than 6 ft (1.8 m) long between external grounding-type connectors at the ends of the flex, an external bonding jumper may be used. BUT WATCH OUT! The rule says the jumper, not the flex, must not exceed 6 ft (1.8 m) in length AND the jumper “shall be routed with the raceway”—that is, run along the flex surface and not separated from the flex.

The exception at the end of this section provides some practical assistance to isolated steel sweeps that are frequently installed in runs of nonmetallic conduit to avoid problems associated with pulling ropes sawing through the inner radius of a nonmetallic sweep during a heavy pull. Since these sweeps are seldom located where a box could be conveniently located, making a connection to the enclosed equipment grounding conductor is problematic. In addition to the isolation provisions of 250.80 Exception and 250.86 Exception No. 3, the exception at this location provides help on a steel sweep positioned at the base of a pole. The exception also covers instances where not only the elbow is steel, but the first pipe section on the pole is steel as well, for mechanical strength. The exception allows a bonding jumper to be attached to the steel at a convenient location, and then run up the pole to a point where the enclosed equipment grounding conductor (load side of service) or grounded circuit conductor (supply side of service) is available for connection, even if the length exceeds 1.8 m (6 ft).

250.104. Bonding of Piping Systems and Exposed Structural Steel. This section on bonding of piping systems in buildings is divided into two parts—metal water piping and other metal piping. This section is a rather elaborate sequence of phrases that may be understood in several ways. Of course, the basic concept is to ground any metal pipes that would present a hazard if energized by an electrical circuit. In general, bear in mind that these bonding requirements apply to “piping systems.” With the increasing use of nonmetallic piping in many occupancies, controversies are arising over how short a section of piping has to be before it no longer qualifies as a “piping system.” This is a decision for the inspector, perhaps based in part on the proximity to other electrical equipment or wiring. However, it is quite clear that an isolated chrome faucet does not create a metal piping system, but metal piping covering an entire floor presumably is a system. The section concludes by correlating the grounding rules for separately derived systems in 250.30 with the bonding rules in this part of the article.

Part (A) requires any “metal water piping system(s) installed in or attached to a building or structure” to be bonded to the service-equipment enclosure, the grounded conductor (usually, a neutral) at the service, the grounding electrode conductor, or the one or more grounding electrodes used. All points of attachment of bonding jumpers for metal water piping systems must be accessible. Only the connections (and not the entire length) of water-pipe bonding jumpers are required to be accessible for inspection. This rule applies where the metal water piping system does not have 10 ft (3.0 m) of metal pipe buried in the earth and is, therefore, not a grounding electrode. In such cases, though, this rule makes clear that the water piping system must be bonded to the service grounding arrangement. And the bonding jumper used to connect the interior water piping to, say, the grounded neutral bus or terminal (or to the ground bus or terminal) must be sized from Table 250.66 based on the size of the service conductors. The jumper is sized from that table because that is the table that would have been used if the water piping had 10 ft (3.0 m) buried under the ground, making it suitable as a grounding electrode. Note that the “bonding jumper” is sized from Table 250.66, which means it never has to be larger than 3/0 AWG copper or 250-kcmil aluminum. Refer to the illustrations for 250.52 and 250.53, which also cover bonding of water piping.

Note that the status of the water piping system as a grounding electrode, particularly where compliance with 250.52(A)(1) Exception has been achieved, frequently extends very long distances into a building. Other sections of the NEC allow users to count on this when grounding other systems, including separately derived systems as covered in 250.30(A)(7)(1). If the appropriate bonding has not been performed and another system is connected, the result will be a hazard.

Part (A)(2) permits “isolated” metal water piping to be bonded to the main electrical enclosure (panelboard or switchboard) in each unit of a multitenant building—such as in each apartment of an apartment house, each store of a shopping center, or each office unit of a multitenant office building. See the top of Fig. 250-82. This rule is intended to provide a realistic and effective way to bond interior metal water piping to the electrical grounding system in multi-tenant buildings where the metal water piping in each tenant’s unit is fed from a main water distribution system of nonmetallic piping and is isolated from the metal water piping in other units. In apartment houses, multistory buildings, and the like, it would be difficult, costly, and ineffective to use long bonding jumpers to tie the isolated piping in all the units back to the equipment grounding point of the building’s service equipment—as required by the basic rule of Sec. 250.104(A). The objective of the basic rule is better achieved in such cases by simply bonding the isolated water piping in each occupancy to the equipment ground bus of the panelboard or switchboard serving the occupancy. The bonding jumper must be sized from Table 250.122 (not 250.66)—based on the rating of the protective device for the circuit supplying the occupancy.

Part (A)(3) covers instances where multiple buildings or structures under common ownership have water piping systems. In such cases the bonding follows the rules for isolated tenancies in one building, as just covered, except that the reference point in this case is the building disconnect, and the sizing applies Table 250.66 to the size of the feeder or branch-circuit conductors that supply the building, and not larger than the supply conductors. If there is no building disconnect the connection can be made to the equipment grounding

Fig. 250-82. Certain techniques are permitted as alternatives to the basic rules on grounding of metal piping systems. (Sec. 250.104.)

conductor of the supply circuit, or to the grounding electrode conductor for the building served.

Part (B) requires a bonding connection from “other” (than water) metal piping systems—such as process liquids or fluids—that “is likely to become energized” to the grounded neutral, the service ground terminal, the grounding electrode conductor, or the grounding electrodes. But, for these other piping systems the bonding jumper is sized from Table 250.122, using the rating of the overcurrent device of the circuit that may energize the piping. Since as a practical matter it is almost impossible to wire anything without an equipment grounding conductor in the circuit, and since equipment grounding conductors must be sized in accordance with 250.122, if there is any conductive connection between the equipment supplied by the branch circuit and the piping system, the equipment grounding connection will automatically bond the piping with no further effort required. Most solenoids and other control devices for piping qualify as bonded under this principle (see bottom of Fig. 250-82).

The phrase “likely to become energized” is used frequently in electrical standards. It basically means that it is capable of being energized if a single insulation failure occurs. For example, a motor supplied by a conventional motor circuit is “likely to become energized” because a single insulation failure on an ungrounded conductor in the usual metal termination box will energize the motor frame, subject to the operation of overcurrent protective devices. Double-insulated appliances are not “likely to become energized” because the supply conductors enter a nonmetallic housing, and two points of insulation failure (the conductor insulation and the nonmetallic connection housing) would have to fail before a voltage could be present on the surface of the equipment.

Part (C) of 250.104 requires exposed building steel that is not effectively grounded to be bonded to the grounding electrode system. The connection may be made at the service equipment, to the grounded conductor, to the grounding electrode conductor where the grounding electrode conductor is “of sufficient size,” or to any of the electrodes used. The reference to “sufficient size” for connection to the grounding electrode conductor is intended to indicate that such a conductor must be sized per 250.66 based on the size of the largest service phase conductor. But, where, say, 6 AWG copper is used for connection to a driven ground rod—as permitted in part (A) to 250.66—the bonding jumper from the nongrounded building must not be connected to that 6 AWG conductor, unless it also satisfies the basic rule in 250.66. The same concept applies to parts (B) and (C) of 250.66.

In part (D), this section puts forth the bonding requirements for separately derived systems in three parts. Paragraph (1) provides that the grounded conductor must be bonded to the interior metal water piping system within the area served by the separately derived system. This is in addition to the bonding connection required between the grounded conductor and the building steel, or where no building steel is available, the connection to the water pipe, not more than 5 ft from its point of entry. This local connection ensures that the metal water piping within the area supplied by the separately derived system is at the same potential with respect to ground as the metal enclosures and raceways associated with the separately derived system. Further, the electrical connections to both the bonding conductor and the grounding electrode conductor must occur at the same point. For example, if the grounding electrode conductor is connected in the secondary compartment of a transformer, then the bonding conductor must be connected in the same transformer compartment.

If the provisions of 250.52(A)(1) Exception successfully apply as far as the separately derived system location, then this bonding connection will also qualify as the termination point for the grounding electrode conductor. Since the wiring and sizing rules are the same for both, no adjustments are necessary. On the other hand, if 250.52(A)(1) Exception does not apply, then the grounding electrode connection and the bonding connection will usually be in two entirely different areas. In addition, if the grounding electrode for the derived system is local building steel, and the steel is bonded to the local water piping system, there is no point in making a second bonding connection to water piping in the area served.

Paragraph (2) covers structural metal framing that is exposed in an area supplied by a separately derived system. Here again the system grounded conductor and the conductive framing must be bonded in the area served, and both the bonding connection and the grounding electrode connection to this conductor must occur at the same point. For example, if in this case the grounding electrode conductor connection takes place in the main disconnect enclosure for the derived system, then the bonding connection must connect there and not in the transformer housing. If the local metal framing is the grounding electrode for the derived system, then an additional bonding connection is not required for obvious reasons. Likewise if the local water piping system is the grounding electrode for the derived system, and the water system and the metal framing are bonded in the area served by the derived system, then no further bonding is required.

Paragraph (3) covers bonding in areas where a common grounding electrode conductor is available, such as might be run vertically in a high-rise building, as discussed in 250.30(A)(4). In this case the derived system will be using the common grounding electrode conductor as its ground reference point. Both the local water piping (if any in the area served) and the local exposed structural metal framing (if any in the area served) must be bonded to the derived system. Presumably (although the NEC does not specifically say this) the connection should be made at the same location in the derived system as where the grounding electrode connection is made, in order to correlate this requirement with the other two. If these connections are separated, there is a possibility of a potential difference and circulating currents that would be objectionable under 250.6(A).

250.106. Lightning Protection Systems. Lightning discharges, with their steep wave fronts, build up tremendous voltages to metal near the air terminals and the down conductors. Although the grounding electrode systems will be bonded, the actual electrodes must be separated, and NFPA 780, Standard for Lightning Protection Systems has detailed information on grounding, bonding, sideflash distances, and required spacings for the safe operation of these systems. Former specific references to such dimensions as “1.8 m (6 ft)” have been removed from the associated notes in this section, because they are far too simplistic to be of any real value,

250.110. Equipment Fastened in Place or Connected by Permanent Wiring Methods (Fixed). The word “fixed” as applied to equipment requiring grounding now applies to “equipment fastened in place or connected by permanent wiring,” as shown in Fig. 250-83. And that usage is consistently followed in other Code sections. As noted in Exception No. 3, enclosures for listed equipment—such as information technology equipment and listed office equipment—operating at over 150 V to ground do not have to be grounded if protected by a system of double insulation or its equivalent. Since equipment grounding conductors travel with circuit conductors almost everywhere in modern wiring, item (5) alone on the list mandates an equipment grounding connection, even if the equipment does not meet the vertical or horizontal parameters in item (1).

Fig. 250-83. “Fixed” equipment is now clearly and readily identified for grounding rules. (Sec. 250.110.)

250.112. Fastened in Place or Connected by Permanent Wiring Methods (Fixed)—Specific. This section presents what amounts to an incomplete, yet binding, roster of equipment that must be connected to an equipment grounding conductor. The various subparts identify those items that are specifically required to be grounded.

Part (I) requires special commentary because of its close and often poorly understood connection with 250.20(A) and 250.162. If system grounding is required by either of these rules, then the remote control, signaling, or fire alarm circuit supplied must incorporate an equipment grounding conductor. For example, a 277-V duct heater with an integral Class 2 transformer to run the thermostatic control circuit, or some Class 2 or Class 3 lighting controls connected to 277-V branch circuits will qualify for mandatory system grounding, with the associated identified grounded circuit conductor.

Should such an application arise, there are additional considerations. Such a grounded system must be connected to a grounding electrode, although this is seldom a problem because the systems normally qualify as separately derived, and NEC 250.30(A)(3) Exception No. 3 generally allows the use of the equipment grounding conductor connection for the supply transformer (limited in the exception to 1 kVA) to function as a grounding electrode conductor in such cases. Another wrinkle is the requirement here that these systems incorporate an equipment grounding conductor. Such a conductor must normally be colored green; however, 250.119 Exception [erroneously written as though 250.112(I) didn’t exist] allows green wire for other than grounding purposes in power-limited cabling, so the color is at best unclear. What is clear is that some conductor in the cable must be an equipment ground, and green is obviously the best choice. Note also that the size of this wire can be the same as the others in the circuit because 250.122(A) never requires an equipment grounding conductor to be larger than the associated circuit conductors. In addition, the color white will be reserved for the grounded circuit conductor, because 200.7(B) reserves that color when, you guessed it, system grounding is required by 250.20(A). The likely result is two wires in the power-limited cable assembly (white and green) will have code required functions and connections at odds with the traditional color code for HVAC wiring diagrams. Take care to plan accordingly.

As required by part (L), motor-operated water pumps, including the submersible type, must have their metal frames grounded. This Code rule clarifies an issue that was a subject of controversy. It means that a circuit down to a submersible pump in a well or cistern must include a conductor to ground the pump’s metal frame, even though the frame is not accessible or exposed to contact by persons. This is why the parent sentence of the rule refers to exposed conductive parts on items (A) through (K), and all such parts of items (L) and (M). The water pump and the well casing must be bonded even if not exposed to direct contact.

Notice, too, that 250.112(M) makes grounding of metal well-casings mandatory for ALL types of occupancies. The grounding of metal well-casings was formerly specified for “Agricultural Buildings” in 547.8(D) until the issuance of the 1996 NEC. In recognition of the fact that the shock hazard exists where a metal well-casing is used—regardless of the type of occupancy or installation—this section requires grounding ALL metal well-casings used with “submersible pumps.” However, that rule in Art. 547 was eliminated because it was viewed as redundant. Remember, according to the rule of 90.3, any requirements put forth in Chaps. 1 to 4 apply to those special occupancies covered in Chap. 5, unless otherwise modified by the rules in Chap. 5. Therefore, extension of this requirement to other occupancies by inclusion in 250.112 made the wording in 547.8(D) superfluous and that section was deleted.

250.114. Equipment Connected by Cord and Plug. Figure 250-84 shows cord-connected loads that must either be operated grounded or be double-insulated. Except when supplied through an isolating transformer as permitted by the exception following (4)(g) of this section, the frames of portable tools should be grounded by means of an equipment grounding conductor in the cord or cable through which the motor is supplied. Portable hand lamps used inside boilers or metal tanks should preferably be supplied through isolating transformers having a secondary voltage of 50 V or less, with the secondary ungrounded. Code-recognized double-insulated tools and appliances may be used in all types of occupancies other than hazardous locations, in lieu of required grounding. Note that cord- and plug-connected equipment operating over 150 V to ground in instances where it consists of motors that are guarded, or if it consists of metal-framed electrically heated appliances exempted by special permission, and where the frames are permanently insulated from ground, is exempted from the usual equipment grounding requirement.

OSHA regulations have made NE Code 250.114 retroactive, requiring grounded operation of cord- and plug-connected appliances in all existing as well as new installations. Check on local rulings on that matter.

Fig. 250-84. Grounding cord and plug cap are required for shock protection. (Sec. 250.114.)

250.118. Types of Equipment Grounding Conductors. This section describes the various types of conductors and metallic cables or raceways that are considered suitable for use as equipment grounding conductors. And the Code recognizes cable tray as an equipment grounding conductor as permitted by Art. 392.

250.118(5), (6), and (7) recognize listed flexible metal conduit, listed liquidtight flexible metal conduit, and flexible metallic tubing, with termination fittings UL-listed for use as a grounding means (without a separate equipment grounding wire) if the total length of flexible methods is not over 6 ft (1.8 m) and the contained circuit conductors are protected by overcurrent devices rated at 20 A or less.

Standard flexible metal conduit (also known as “Greenfield”) must be listed by UL but the former allowance for flex listed as a grounding means has been deleted because none has, shall we say, a credible listing. However, part (5) permits flex to be used without any supplemental grounding conductor when any length of flex in a ground return path is not over 6 ft (1.8 m) and the conductors contained in the flex are protected by overcurrent devices rated not over 20 A and the fittings are listed as suitable for grounding (Fig. 250-85). Use of standard flex with the permission given in 250.102 for either internal or external bonding must be as follows:

1. When conductors within a length of flex up to 6 ft (1.83 m) are protected at more than 20 A, equipment grounding may not be provided by the flex, but a separate conductor must be used for grounding. If a length of flex is short enough to permit a bonding jumper not over 6 ft (1.83 m) long to be run between external grounding-type connectors at the flex ends, while keeping the jumper along the flex, such an external jumper may be used where equipment grounding is required—as for a short length of flex with circuit conductors in it protected at more than 20 A. Of course, such short lengths of flex may also be “bonded” by a bonding jumper inside the flex, instead of external. Refer to 250.102(E).

Fig. 250-85. Standard flex is limited in use without an equipment ground wire. (Sec. 250.118.)

2. Any length of standard flex that would require a bonding jumper longer than 6 ft (1.8 m) may not use an external jumper. In the Code sense, when the length of such a grounding conductor exceeds 6 ft (1.8 m), it is not a BONDING JUMPER BUT IS AN EQUIPMENT GROUNDING CONDUCTOR AND MUST BE RUN ONLY INSIDE THE FLEX, AS REQUIRED BY 250.134(B). Combining UL data with the rule of Sec. 250.102, every length of flex that is over 6 ft (1.8 m) must contain an equipment grounding conductor run only inside the flex (Fig. 250-86).

Fig. 250-86. Internal equipment grounding is required for any flex over 6 ft (1.8 m) long. (Sec. 250.118.)

In part (5)(c) it should be noted that exemption from the need for an equipment grounding conductor applies only to flex where there is not over 6 ft (1.8 m) of “total length in the same ground return path.” That means that from any branch-circuit load device—lighting fixture, motor, and so forth—all the way back to the service ground, the total permitted length of flex without a ground wire is 6 ft (1.8 m). In the total circuit run from the service to any outlet, there could be one 6-ft (1.8-m) length of flex or two 3-ft (900-mm) lengths or three 2-ft (600-mm) lengths or a 4-ft (1.2-m) and a 2-ft (600-mm) length—where the flex lengths are in series as equipment ground return paths. In any circuit run—feeder to subfeeder to branch circuit—any length of flex that would make the total series length over 6 ft (1.8 m) would have to use an internal or external bonding jumper, regardless of any other factors. Further, this principle extends to all varieties of flex covered here, taken in their collective lengths. For example, 600 mm (2 ft) of flexible metal conduit plus the same length of liquidtight flexible metal conduit plus the same length of flexible metallic tubing just equals the longest permitted length (1.8 m [6 ft]) without a separate equipment grounding conductor. If any one of the three wiring methods in this example were even slightly longer in the same fault current path, then a separate grounding conductor would be required.

In all cases, sizing of bonding jumpers for all flex applications is made according to Sec. 250.102, which requires the same minimum size for bonding jumpers as is required for equipment grounding conductors, or grounding electrode conductors for service applications as allowed in 230.43(15). In such cases, the size of the conductor is selected from Table 250.122, based on the maximum rating of the overcurrent devices protecting the circuit conductors that are within the flex. For service work, substitute Table 250.66 for obvious reasons.

In part (5)(d), the flex will require a separate equipment grounding conductor, regardless of whether it would otherwise qualify in terms of length or circuit protection, if “flexibility is required after installation.” A run to a swinging sign is a clear application of this principle. Somewhat more controversial, but seemingly still within the phrase “flexibility required” would be a run of flex used as a vibration isolator for a transformer. A short run at a motor to allow flexibility so a belt can be tightened would also void the exemption from running a separate equipment grounding conductor. The “after installation” provision is in the NEC in order to protect a run of flex that has been installed as part of a fixed raceway layout to get around an obstruction. Flexibility was clearly required to install the flex; that is why it was installed in the first place, but just as clearly after the installation is complete it will never move again, and there will be no motion stress applied to its connecting fittings.

Part (6) presents conditions under which liquidtight flexible metal conduit may be used without need for a separate equipment grounding conductor:

1. Both part (6) and the UL’s Electrical Construction Materials Directory (the Green Book) note that any listed liquidtight flex in metric designator 35 (trade size 1¼) and smaller, in a length not over 6 ft (1.8 m), may be satisfactorily used as a grounding means through the metal core of the flex, without need of a bonding jumper (or equipment grounding conductor) either internal or external (Fig. 250-87), depending on the rating of the overcurrent device ahead of it (next paragraph). The permitted length is exactly the same, and involves the other flexible metal raceways in exactly the same way, as flexible metal conduit, analyzed previously. In addition, there is an identical limitation for uses where flexibility is required after installation.

Where terminated in fittings investigated for grounding and where installed with not more than 6 ft (1.8 m) (total length) in any ground return path, liquidtight flexible metal conduit in the metric designators 12 and 16 (trade sizes and ½) is suitable for grounding where used on circuits rated 20 A or less, and the metric designators 21, 27, and 35 (trade sizes ¾, 1, and 1¼) are suitable for grounding where used on circuits rated 60 A or less. See the category “Conduit Fittings” (DWTT) with respect to fittings suitable as a grounding means.

Fig. 250-87. Liquidtight flex may be used with a separate ground wire. (Sec. 250.118.)

The following are not considered to be suitable as a grounding means:

a. The metric designator 41 (trade size 1½) and larger sizes

b. The metric designators 12 and 16 (trade sizes and ½) where used on circuits rated higher than 20 A, or where the total length in the ground return path is greater than 6 ft (1.8 m)

c. The metric designators 21, 27, and 35 (trade sizes ¾, 1, and 1¼) where used on circuits rated higher than 60 A, or where the total length in the ground return path is greater than 6 ft (1.8 m)

2. For liquidtight flex over metric designator 35 (trade size 1¼), UL does not list any as suitable for equipment grounding, thereby requiring use of a separate equipment grounding conductor installed in any length of the flex, as required by Code. If a length of liquidtight flex larger than this is short enough to permit an external bonding jumper not more than 6 ft (1.8 m) long between external grounding-type connectors at the ends of the flex, an external bonding jumper may be used. BUT WATCH OUT! The rule says the jumper, not the flex, must not exceed 6 ft (1.8 m) in length AND the jumper “shall be routed with the raceway”—that is, run along the flex surface and not separated from the flex.

3. If any length of flex is over 6 ft (1.8 m), then the flex is not a suitable grounding conductor, regardless of the trade size of the flex, whether it is larger or smaller than metric designator 35 (trade size 1¼). In such cases, an equipment grounding conductor (not a “bonding jumper”—the phrase reserved for short lengths) must be used to provide grounding continuity and IT MUST BE RUN INSIDE THE FLEX, NOT EXTERNAL TO IT, IN ACCORDANCE WITH 250.134(B).

250.119. Identification of Equipment Grounding Conductors. Although the default identification for equipment grounding conductors is green (next paragraph), what if equipment grounding is not required? This is indeed the case on many Class 2 and Class 3 control circuit applications supplied by the secondary of a 120-V transformer. Many air-conditioning and other applications use standard color codes for thermostat and related wiring that include the color green as an ungrounded control conductor, which technically violate and 250.119 here. A new exception allows this use on power limited circuits. However, the code-making panel failed to correlate this exception with the requirements in 250.20(A), 200.7(B), and 250.112(I). If you are dealing with a 120-V primary on the transformer, there should be no problem, but once the voltage goes above that there are very difficult correlation issues to contend with. Review the discussion at 250.112(I) for a detailed analysis of the code rules that intersect in this area.

Part (A) recognizes conductors of colors other than green for use as equipment grounding conductors if the conductor is stripped for its exposed length within an enclosure, so it appears bare, or if green coloring, green tape, or a green label is used on the conductor at the termination. As shown in Fig. 250-88, the phase legs may or may not be required to be “identified by phase and system” [see 210.5(C)]. If color coding is used, the phase legs may be any color other than white, gray, or green. The neutral may be white or gray or any other color than green if it is larger than No. 6 and if white tape, marking, or paint is applied to the neutral near its terminations. The grounding conductor may be green or may be any insulated conductor of any color if all insulation is stripped off for the exposed length. Alternatives to stripping the black insulated conductor used for equipment ground include (1) coloring the exposed insulation green or (2) marking the exposed insulation with green tape or green adhesive labels; however, any marking must “encircle” the grounding conductor. Relief

Fig. 250-88. Equipment grounding conductor larger than No. 6 may be a stripped conductor of any color covering. (Sec. 250.119.)

is granted to the marking rule on conductors larger than 6 AWG where they pass through conduit bodies that have no unused hubs and no splices.

Part (B) permits specific on-the-job identification of an insulated conductor used as an equipment grounding conductor in a multiconductor cable. Such a conductor, regardless of size, may be identified in the same manner permitted by 250.119(A) for conductors larger than No. 6 used in raceway. The conductor may be stripped bare or colored green to indicate that it is a grounding conductor. But such usage of multiconductor cables is recognized only for commercial, institutional-, and industrial-type systems under conditions of qualified maintenance and supervision.

250.120Equipment Grounding Conductor Installation. This rule contains a number of crucial requirements. Part (A) requires equipment grounding conductors in the form of raceways or cable assemblies to be installed with their code requirements. The last sentence, however, is a favorite with the inspection community: “All connections, joints, and fittings shall be made tight using suitable tools.” This is the requirement that directly reaches every untightened set-screw and every loose locknut. Attention to detail is absolutely critical in constructing an effective ground-fault current path required [250.4(A)(5)] on every part of every grounded electrical system, and the low-impedance path for fault current required [250.4(B)(4)] on every ungrounded system. A single poor connection can be the one that delays the operation of a circuit protective device such that a fire or even worse results.

This part also has a new note for the 2008 NEC pointing to equipment grounding conductors installed in a raceway containing other conductors with ceramifiable insulation that has been evaluated as part of some “Electrical Circuit Protective Systems.” These systems use special conductors, such as RHH but with nonstandard diameters, so the manufacturer’s information needs to be consulted for wire fill. This insulation, upon exposure to extreme heat, breaks down but the residue is akin to colored glass, not capable of significant movement but capable of maintaining the integrity of the circuit for a rated period of time. Some requirements relative to this part of the NEC include, but are not limited to (the full information is an entire page of very small print in the UL directory) the following items:

“The raceway should be connected together using the coupling type referenced in the system, such as steel set-screw type for EMT or threaded types of couplings for IMC and RMC. No other coupling should be used unless noted in the specific system.” This is because some of the tests involve a hose stream applied after the fire test, and compression couplings usually fail, and even set-screw couplings must have their screws driven very hard with a significant dimple or the raceway will pull apart and fail the test. Refer the “attention to detail” note above.

“The bare or insulated ground wire may be of special manufacture to be compatible with the system. The system will specify the manufacturer of an allowable ground wire. If not specified, the ground should be the same as the fire-rated wire described in the system.” This is because a separate equipment grounding conductor of random insulation type, under fire conditions, may be incompatible with the tested cables it is in intimate contact with; unless tested, this is cannot be predicted.

Part (B) correlates the equipment grounding rules for aluminum with the comparable rules for aluminum used for grounding electrode conductors, in 250.64(A). The analysis at that location also applies here.

Part (C) states the requirement that equipment grounding conductors must be protected within cable armor or raceway unless run within hollow walls or other locations where not subject to damage. The hollow-wall allowance primarily applies to where an old branch circuit with no equipment grounding conductor is being extended and a grounding conductor needs to be fished in to make it comply, as covered in 250.130(C).

250.122. Size of Equipment Grounding Conductors. When an individual equipment grounding conductor is used in a raceway—either in a nonmetallic raceway or in a metal raceway where such a conductor is used for grounding reliability even though 250.118 usually accepts metal raceways as a suitable grounding conductor—the grounding conductor must have a minimum size as shown in Table 250.122.

The basic rules are covered in 250.122(A), and one of the more important is this: In no case is an equipment grounding conductor required to be larger than its associated circuit conductors. This can come up on motor circuits, where the short-circuit and ground-fault protective device settings may be high enough to cause this problem. For example, a motor circuit using nontime-delay fuses for 12 AWG THHN wire [75°C ampacity, per 110.14(C)(1)(4) = 25 A] could be protected with a 80 A fuse per 430.52(C)(1) Exception No. 1. This circuit, by 250.122(A), should normally have an 8 AWG equipment grounding conductor run with the 12 AWG motor circuit conductors; this rule avoids that and would allow a 12 AWG grounding conductor for this purpose.

The minimum acceptable size of an equipment grounding conductor is based on the rating of the overcurrent device (fuse or CB) protecting the circuit, run in the same raceway, for which the equipment grounding conductor is intended to provide an effective ground-fault current path (Fig. 250-89). Each size of grounding conductor in the table is adequate to carry enough current to blow the fuse or trip the CB of the rating indicated beside it in the left-hand column.

Fig. 250-89. Size of grounding conductor must carry enough current to operate circuit overcurrent device. (Sec. 250.122.)

In Fig. 250-89, if the fuses are rated at 60 A, Table 250.122 shows that the equipment grounding conductor used with that circuit must be at least a 10 AWG copper or a 8 AWG aluminum or copper-clad aluminum.

Whenever an equipment grounding conductor is used for a circuit that consists of only one conductor for each hot leg (or phase leg), the grounding conductor is sized simply and directly from Table 250.122, as described. When a circuit is made up of parallel conductors per phase, say an 800-A circuit with two conductors per phase, an equipment grounding conductor is also sized in the same way and would, in that case, have to be at least a 1/0 AWG copper or 3/0 AWG aluminum. But, if such a circuit is made up using two conduits—that is, three phase legs and a neutral in each conduit—250.122(F) requires that an individual grounding conductor be run in each of the conduits and each of the two grounding conductors must be at least 1/0 AWG copper or 3/0 AWG aluminum (Fig. 250-90). Another example is shown in Fig. 250-91, where a 1200-A protective device on a parallel circuit calls for a 3/0 AWG copper or 250-kcmil aluminum grounding conductor.

Fig. 250-90. Grounding conductor must be used in each conduit for parallel conductor circuits. (Sec. 250.122.)

Fig. 250-91. Using equipment grounding conductors in parallel. (Sec. 250.122.)

Part (D) of 250.122 covers another concern for unnecessarily oversizing equipment grounding conductors. Because the minimum acceptable size of an equipment grounding conductor is based on the rating of the overcurrent protective device (fuse or CB) protecting the circuit for which the equipment grounding conductor is intended to provide a path of ground-fault return, a problem arises when a motor circuit is protected by a magnetic-only (a so-called instantaneous) circuit breaker. Because 430.52 and Table 430.52 permit an instantaneous-trip CB with a setting of 800 percent of (8 times) the motor full-load running current—and even up to 1700 percent for an instantaneous CB or MSCP (motor short-circuit protector), if needed to handle motor inrush current—use of those high values of current rating permitted in Table 430.52 would result in excessively large equipment grounding conductors. Because such large sizing is unreasonable and not necessary, the rule says when sizing an equipment grounding conductor from Table 250.122 for a circuit protected by an instantaneous-only circuit breaker or by an MSCP, there is a special calculation procedure which has significantly changed in the 2008 NEC.

Begin with the ampacity for the applicable size of the motor circuit conductors. Multiply the ampacity [using the 75°C column for all sizes, per 110.14(C)(1)(4)] by the percentage stated for a dual-element (time-delay) fuse in Table 430.52 (usually 175 percent). In accordance with 430.52(C)(1) Exception No. 1, round the resulting number up to the next higher standard sized fuse, as listed in 240.6(A), if it does not correspond to a standard size. This result, and not the much lower setting of the running overload protection as had been allowed in the NEC for the six previous cycles, is the one that you use to enter Table 250.122 (Fig. 250-92).

The last sentence of 250.122(A) points out that metal raceways and cable armor are recognized as equipment grounding conductors; Table 250.122 does not apply to them. However, they must still provide an effective fault current path, as covered in the note at the bottom of Table 250.122.

In 250.122(B), the Code places a mandatory requirement for equipment grounding conductors to be “increased in size” where the phase conductors are “upsized” for any reason, but traditionally to overcome voltage drop on long runs. Where any upsizing is provided to ensure adequate voltage at the point of equipment installation, the equipment grounding conductor must also be upsized to ensure adequate current flow under fault conditions. That is, if voltage drop presents a problem for the phase conductors, then it also presents a problem to the equipment grounding conductors, because the grounding conductors will be run for the same distance as the phase conductors.

The current rule to increase an equipment grounding conductor if the other circuit conductors, quite simply, are “increased in size” is unclear and requires field interpretation, particularly in the event the increase is simply to satisfy the minimum size requirements in the NEC. For example, suppose a 3-phase, 4-wire wye-connected feeder runs with significant harmonic loading. The ampacity of the conductors must be derated in accordance with 310.15(B)(4)(c) and 310.15(B)(2)(a). The conductor size must be increased to overcome the mandatory derating factor applied. Does this “increase in size” invoke a larger equipment grounding conductor? Perhaps the literal text would support that, but there does not seem to be any good technical substantiation to support it.

Fig. 250-92. These applications are covered by parts (C) and (D) of 250.122. (Sec. 250.122.)

It can be shown that when conductors are increased to counteract issues of ambient temperature and mutual conductor heating, the result tends to be a net decrease in the total impedance in the fault current path (i.e., from the fuse or circuit breaker out to a fault on a power conductor and back on an equipment grounding conductor), even when that conductor is taken straight from Table 250.122 with no adjustments. In Annex D, Example D3(a) there are comprehensive calculations that address problems of continuous loading, termination issues, etc. This issue is more fully examined there.

Example with methodology, if needed: Suppose you have a feeder that would, in its most basic application, use 1 AWG conductors, and they are being increased to 2/0 conductors. The Table 250.122 equipment grounding result based on the circuit protection is 6 AWG. Here is how to figure proportional increases to an equipment grounding conductor size if necessary. Perform the following calculation, using the numbers in Table 8 of NEC Chap. 9, where “CSA” means “cross-sectional area” and “EGC” means “equipment grounding conductor”:

Figure 250-92 shows details of a controversy that often arises about 250.122(C) and 250.134. When two or more circuits are used in the same conduit, it is logical to conclude that a single equipment grounding conductor within the conduit may serve as the required grounding conductor for each circuit if it satisfies Table 250.122 for the circuit with the highest rated overcurrent protection. The common contention is that if a single metal conduit is adequate as the equipment grounding conductor for all the contained circuits, a single grounding conductor can serve the same purpose when installed in a nonmetallic conduit that connects two metal enclosures (such as a panel and a home-run junction box) where both circuits are within both enclosures. As shown, a 12 AWG copper conductor satisfies Table 250.122 as an equipment grounding conductor for the circuit protected at 20 A. The same 12 AWG also may serve for the circuit protected at 15 A, for which a grounding conductor must not be smaller than 14 AWG copper. Such application is specifically permitted by part (C) of 250.122. Although this will have primary application with PVC conduit where an equipment grounding conductor is required, it may also apply to circuits in EMT, IMC, or rigid metal conduit when an equipment grounding conductor is run with the circuit conductors to supplement the metal raceway as an equipment grounding return path.

250.130. Equipment Grounding Conductor Connections. Part (A) requires that the equipment grounding conductor at a service—such as the ground bus or terminal in the service-equipment enclosure, or the enclosure itself—must be connected to the system grounded conductor (the neutral or grounded phase leg). The equipment ground and the neutral or other grounded leg must be bonded together and it must be done on the supply side of the service disconnecting means—which means either within or ahead of the enclosure for the service equipment (Fig. 250-93).

Part (B) requires the ground bus or the enclosure to be simply bonded to the grounding electrode conductor within or ahead of the service disconnect for an ungrounded system.

Fig. 250-93. Equipment ground must be “bonded” to grounded conductor at the service equipment. (Sec. 250.130.)

As shown at the top of Fig. 250-94, some switchboard sections or interiors include neutral busbars factory-bonded to the switchboard enclosure and are marked “suitable for use only as service equipment.” They may not be used as subdistribution switchboards—that is, they may not be used on the load side of the service except where used, with the inspector’s permission, as the first disconnecting means fed by a transformer secondary or a generator and where the bonded neutral satisfies 250.30 for a separately derived system.

Fig. 250-94. These are bonding and grounding details covered by 250.130(C).

Part (C) covers the retrofit of equipment grounding connections for receptacles connected to, or for extensions of, existing branch circuits that do not include any recognized equipment grounding conductor (Fig. 250-94, bottom), such as concealed knob-and-tube wiring (Art. 394). An equipment grounding conductor can be brought to these locations without rewiring the circuit, and used to make a grounding connection on a receptacle grounding terminal, or to the equipment grounding provisions of a wiring method being used to extend an existing branch circuit. For example, if a concealed knob-and-tube circuit were being extended with Type NM cable, which includes a bare copper wire for equipment grounding, an equipment grounding conductor could be fished to the location of the branch circuit being extended.

The equipment grounding conductor, presumably a 12 or 14 AWG wire, would be run to the location, probably using the provisions of 250.120(C) that give a waiver on raceway or cable armor protection in such cases for runs within hollow walls or joist cavities, or where otherwise protected from damage. It must originate from one of the following locations: (1) the grounding electrode system at an accessible point, or on the grounding electrode conductor; (2) the equipment grounding terminal bar within the enclosure where the circuit originates; (3) the service equipment grounding terminal bar (ungrounded systems) or the grounded service conductor within the service equipment.

At one time, this rule allowed connections to nearby water pipes, but not since the 1993 NEC. Now all the likely connection points are in a basement or the first level. You are unlikely to be searching for a method of grounding concealed knob-and-tube wiring in a steel-frame building. Rather you will be attempting this in old wood-frame buildings, probably residential. In such occupancies, even if the water supply lateral is metallic, the water piping system ceases to be considered as an electrode beyond 5 ft from the point of entry. This means fishing into the basement. If you can fish a ground wire down into the basement, you can fish a modern circuit up in the reverse direction and avoid all the problems. Meanwhile, the note at the end of this part points to the procedure in 406.3(D) that allows grounding-configured GFCI receptacles to be installed with no equipment ground provided. This entire procedure is of marginal significance today.

250.134. Equipment Fastened in Place or Connected by Permanent Wiring Methods (Fixed)—Grounding. This section requires that metal equipment enclosures, boxes, and cabinets to be grounded must be grounded by metal cable armor or by the metal raceway that supplies such enclosures (rigid metal conduit, intermediate metal conduit, EMT, flex or liquidtight flex), or by an equipment grounding conductor, such as where the equipment is fed by rigid nonmetallic conduit. Refer to 250.118. As illustrated at the bottom of Fig. 250-94, Exception No. 1 for parts (A) and (B) recognizes the accepted technique given in 250.130—for using grounding-type receptacles for replacement of existing nongrounding devices or for circuit extensions—on wiring systems that do not include an equipment grounding conductor.

In Sec. 250.134(B), the rule explicitly requires that when a separate equipment grounding conductor (i.e., other than the metal raceway or metal cable armor) is used for alternating-current circuits, it must be contained within the same raceway, cable, or cord or otherwise run with the circuit conductors (Fig. 250-95). External grounding of equipment enclosures or frames or housings is a violation for AC equipment. It is not acceptable, for instance, to feed an AC motor with a

Fig. 250-95. Equipment grounding conductor must be in raceway or cable with circuit conductors for AC equipment. (Sec. 250.134.)

nonmetallic conduit or cable, without a grounding conductor in the conduit or cable, and then provide grounding of the metal frame by a grounding conductor connected to the metal frame and run to building steel or to a grounding-grid conductor. An equipment grounding conductor must always be run with the circuit conductors.

The rule in Sec. 250.134(B) which insists on keeping an equipment grounding conductor physically close to AC circuit supply conductors is a logical follow-up to the rules of 250.4 which call for minimum impedance in grounding current paths to provide most effective clearing of ground faults. When an equipment grounding conductor is kept physically close to any circuit conductor that would be supplying the fault current (i.e., the grounding conductor is in the “same raceway, cable, or cord or otherwise run with the circuit conductors”), the impedance of the fault circuit has minimum inductive reactance and minimum AC resistance because of mutual cancellation of the magnetic fields around the conductors and the reduced skin effect. Under such condition of a “low-impedance circuit” the meaning of 250.4(A)(5) is best fulfilled—voltage to ground is limited to the greatest extent, the fault current is higher because of minimized impedance, the circuit overcurrent device will operate at a faster point in its time-current characteristic to ensure maximum fault-clearing speed, and the entire effect will be “facilitating the operation of the overcurrent device....”

The arrangement shown in Fig. 250-96 violates the basic rule of 250.134(B) because the lighting fixture, which must be grounded to satisfy 250.112, is not grounded in accordance with 250.134 and 250.118 or by an equipment grounding conductor contained within the cord, as noted in 250.138.

Note that 250.134(B) refers very clearly to an “equipment grounding conductor contained within the same raceway, cable, or otherwise run with the circuit conductors.” Except for DC circuits, replacement receptacles [250.130(C)], and isolated, ungrounded power sources [517.19(F) and (G)], an equipment grounding conductor of any type must not be run separately from the circuit conductors. The engineering reason for keeping the ground return path and the phase legs in close proximity (i.e., in the same raceway) is to minimize the impedance

Fig. 250-96. Supply to AC equipment must include equipment grounding conductor. (Sec. 250.134.)

of the fault circuit by placing conductors so their magnetic fields mutually cancel each other, keeping inductive reactance down, and allowing sufficient current to flow to “facilitating the operation of the overcurrent device....” as required by 250.4(A)(5).

The hookup in Fig. 250-96 also violates the rule of the last sentence in 250.136(A), which prohibits use of building steel as the equipment grounding conductor for AC equipment. And the rules of 250.136 often have to be considered in relation to the rules of 250.134.

Note: CARE MUST BE TAKEN TO DISTINGUISH BETWEEN AN “EQUIPMENT GROUNDING CONDUCTOR” AS COVERED BY 250.134 AND AN “EQUIPMENT BONDING JUMPER” AS COVERED BY 250.102(E). A “BONDING JUMPER” MAY BE USED EXTERNAL TO EQUIPMENT BUT IT MUST NOT BE OVER 6 ft (1.8 m) LONG.

250.136. Equipment Considered Grounded. This rule clarifies the way in which structural metal may be used as an equipment grounding conductor, consistent with the rule of 250.134(B) requiring a grounding conductor to be kept physically close to the conductors of any AC circuit for which the grounding conductor provides the fault return path.

Part (A) notes that if a piece of electrical equipment is attached and electrically conductive to a metal rack or structure supporting the equipment, the metal enclosure of the equipment is considered suitably grounded by connection to the metal rack, PROVIDED THAT the metal rack itself is effectively grounded by metal raceway enclosing the circuit conductors supplying the equipment or by an equipment grounding conductor run with the circuit supplying the equipment. An example of such application is shown in Fig. 250-97. Although this example shows grounding of lighting fixtures to a rack, the Code rule recognizes any “electric equipment” when this basic grounding concept is observed. It is important to note that if a ground fault developed in equipment so grounded (as at point A), the fault current would take the path indicated by the small arrows. In such case, although the fault-current path through the steel rack is not close to the hot conductor in the flexible cord that is feeding the fault—as normally required by 250.134(B)—the distance of the external ground path is not great, from the fixture to the panel enclosure or box. Because such a short external ground path produces only a relatively slight increase in ground-path impedance, 250.136(A) permits it. The permission for external bonding of flexible metal conduit and liquidtight flex in 250.102(E) is based on the same acceptance of only slight increase of overall impedance of the ground path.

The second sentence of 250.136(A) clearly prohibits using structural building steel as an equipment grounding conductor for equipment mounted on or fastened to the building steel—IF THE SUPPLY CIRCUIT TO THE EQUIPMENT OPERATES ON ALTERNATING CURRENT. BUT, structural building steel that is effectively grounded and bonded to the grounded circuit conductor of a DC supply system may be used as the equipment grounding conductor for the metal enclosure of DC-operated equipment that is conductively attached to the building steel.

It is important to understand the basis for the Code rules of 250.134(B) and 250.118 and their relation to the concept of 250.136(A):

Fig. 250-97. This use of metal rack as equipment ground is permitted. [Sec. 250.136(A).]

Note that 250.134(B) refers very clearly to an “equipment grounding conductor contained within the same raceway or cable or otherwise run with the circuit conductors.” Except for DC circuits (250.168) and for isolated, ungrounded power sources [517.19(F) and (G)], an equipment grounding conductor of any type must not be run separately from the circuit conductors. Keeping the ground return path and the phase legs in close proximity (i.e., in the same raceway) minimizes the impedance of the fault circuit by placing conductors so their magnetic fields mutually cancel each other, keeping inductive reactance down and allowing sufficient current to flow to “facilitating the operation of the overcurrent device....” as required by 250.4(A)(5).

The second sentence of 250.136(A) applies the concept of ground-fault impedance to the metal frame of a building and prohibits its use as an equipment grounding conductor for AC equipment enclosures. As shown in Fig. 250-98, use of building steel as a grounding conductor provides a long fault return path of very high impedance because the path is separated from the feeder circuit hot legs—thereby violating 250.4. Ground-fault current returning over building steel to the point where the building steel is bonded to the AC system neutral (or other grounded) conductor is separated from the circuit conductor that is providing the fault current. Impedance is, therefore, elevated and the optimum conditions required by 250.4 are not present, so that the grounding cannot be counted on for “facilitating the operation” of the fuse or CB protecting the faulted circuit. The current may not be high enough to provide fast and certain clearing of the fault.

Fig. 250-98. Building metal frame is not an acceptable grounding conductor for AC equipment. [Sec. 250.136(A).]

The first sentence of 250.136(A) accepts a limited variation from the basic concept of keeping circuit hot legs and equipment grounding conductors physically close to each other. When equipment is grounded by connection to a “metal rack or structure” that is specifically provided to support the equipment and is grounded, the separation between the circuit hot legs and the rack, which serves as the equipment grounding conductor, exists only for a very short length that will not significantly raise the overall impedance of the ground-fault path. Figure 250-99 shows another application of that type, similar to the one shown in Fig. 250-97. Although this shows a 2-wire cord as being acceptable, use of a 3-wire cord (two circuit wires and an equipment grounding wire) is better practice, at very slight cost increase.

Aside from the limited applications shown in Figs. 250-97 and 250-99, required equipment grounding must always keep the equipment grounding conductor alongside the circuit conductor for grounded AC systems. Of course, as long as required grounding techniques are observed, there is no objection to additional connection of equipment frames and housings to building steel or to grounding grids to provide potentials to ground. But the external grounding path is not suitable for clearing AC equipment ground faults.

Fig. 250-99. This satisfies basic rule of 250.136(A). [Sec. 250.136(A).]

250.138. Cord- and Plug-Connected Equipment. The proper method of grounding portable equipment is through an extra conductor in the supply cord. Then if the attachment plug and receptacle comply with the requirements of 250.138, the grounding connection will be completed when the plug is inserted in the receptacle.

A grounding-type receptacle and an attachment plug should be used where it is desired to provide for grounding the frames of small portable appliances. The receptacle will receive standard 2-pole attachment plugs, so grounding is optional with the user. The grounding contacts in the receptacle are electrically connected to the supporting yoke so that when the box is surface-mounted, the connection to ground is provided by a direct metal-to-metal contact between the device yoke and the box. For a recessed box a grounding jumper must be used on the receptacle or a self-grounding receptacle must be used. See 250.146 and 250.148.

Figure 250-100 shows a grounding-type attachment plug with a movable, self-restoring grounding member—as previously covered in an Exception to this section. Although previously recognized, availability of such a device is questionable, and use of such a product is no longer permitted.

Fig. 250-100. This type of plug cap is no longer recognized on cords for tools and appliances. However, the exception in 250.138(A) does recognize “moveable” ground pins on “grounding-type, plug-in” GFCIs where the voltage is not greater than 150 V. (Sec. 250.138(A) Exception.)

250.140. Frames of Ranges and Clothes Dryers. Prior to the 1996 edition of the NEC, the frame of an electric range, wall-mounted oven, or counter-mounted cooking unit could be grounded by direct connection to the grounded circuit conductor (the grounded neutral) and thus could be supplied by a 3-wire cord set and range receptacle irrespective of whether the conductor to the receptacle contains a separate grounding conductor.

The NEC prohibits such applications except on “existing branch circuits.” That wording doesn’t permit grounding of ranges or dryers with the neutral, unless the circuit itself—not the occupancy—is an existing circuit. For all new circuits and new construction, the neutral may not be used as a grounding conductor.

Where permitted to be so grounded parts (1) and (2) clarify the use of a No. 10 or larger grounded neutral conductor of a 120/208- or 120/240-V circuit for grounding the frames of electric ranges, wall-mounted ovens, counter-mounted units, or clothes dryers. This method is acceptable whether the 3-wire supply is 120/208 or 120/240 V. Normally, the grounded conductor must be insulated. However, a provision, applicable to both 3-wire supply voltages, does require that when using service-entrance cable having an uninsulated neutral conductor, the branch circuit must originate at the service-entrance equipment. The purpose of this provision is to prevent the uninsulated neutral from coming in contact with a panelboard supplied by a feeder and a separate grounding conductor (in the case of nonmetallic-sheathed cable). This would place the neutral in parallel with the grounding conductor, or with feeder metal raceways or cables if they were used. Insulated neutrals in such situations will prevent this (Fig. 250-101).

Fig. 250-101. Ranges and dryers may be grounded to the circuit neutral, but only on existing circuits. All new installations must provide an equipment grounding conductor with the 3-wire supply to the range or dryer. (Sec. 250.140.)

Wording of the rule that permits frames of ranges and clothes dryers to be grounded by connection to the grounded neutral conductor of their supply circuits also permits the same method of grounding of “outlet or junction boxes” serving such appliances. The rule permits grounding of an outlet or junction box, as well as cooking unit or dryer, by the circuit grounded neutral (Fig. 250-102). That practice has been common for many years but has raised questions about the suitability of the neutral for such grounding. However, the wording of this rule makes clear that such grounding of the box is acceptable. Figure 250-103 shows other details of such application. Without this permission to ground the metal box to the grounded neutral, it would be necessary to run a 4-wire supply cable to the box, with one of the wires serving as an equipment grounding conductor sized from Table 250.122, which is always required for other than existing branch circuits that supply dryers and ranges.

Important: As shown in the asterisk note under Fig. 250-102, if a nonmetallic-sheathed cable was used, say, to supply a wall oven or cooktop, such cable is required by 250.140 to have an insulated neutral. It would be a violation, for instance, to use a 10/2 NM cable with a bare 10 AWG grounding conductor to supply a cooking appliance—connecting the two insulated 10 AWG wires to the hot terminals and using the bare 10 AWG as a neutral conductor to ground the appliance. An uninsulated grounded neutral may be used only when part of a service-entrance cable. Where an existing branch circuit is made up with 10/2 Type NM cable, a new branch circuit with an equipment ground must be installed to supply the dryer or range.

Fig. 250-102. Neutral may be used to ground boxes as well as appliances. (Sec. 250.140.)

250.142. Use of Grounded Circuit Conductor for Grounding Equipment. Part (A) permits connection between a grounded neutral (or grounded phase leg) and equipment enclosures, for the purpose of grounding the enclosures to the grounded circuit conductor. The grounded conductor (usually the neutral) of a circuit may be used to ground metal equipment enclosures and raceways on the supply side of the service disconnect or the supply side of the first disconnect fed from a separately derived transformer secondary or generator output or on the supply side of a main disconnect for a separate building. The wording here includes the supply side of a separately derived system as a place where metal equipment parts or enclosures may be grounded by connection to the grounded circuit conductor (usually a neutral). It is important to note that, in the meaning of the code (as covered in 250.30 and in 250.24), the phrase “on the supply side of the disconnecting means” includes connection within the enclosure of the disconnecting means. Note that the supply side of the separately derived

Fig. 250-103. These techniques may be used to ground boxes on existing circuits only! (Sec. 250.140.)

system language correlates with the permission in 250.30(A)(1) to locate the system bonding jumper at a point from the source to the first system disconnecting means. Also the permission on outbuildings reflects a continuing, but now extremely limited, permission in those areas to reground a neutral on existing premises wiring systems only.

Figure 250-104 shows such applications. At A, the grounded service neutral is bonded to the meter housing by means of the bonded neutral terminal lug in the socket—and the housing is thereby grounded by this connection to the grounded neutral, which itself is grounded at the service equipment as well as at the utility transformer secondary supplying the service. At B, the service equipment enclosure is grounded by connection (bonding) to the grounded neutral—which itself is grounded at the meter socket and at the supply transformer. These same types of grounding connections may be made for CT cabinets, auxiliary gutters, and other enclosures on the line side of the service-entrance disconnect means, including the enclosure for the service disconnect. In some areas, the utilities and inspection departments will not permit the arrangement shown in Fig. 250-104 because the connecting lug in the meter housing is not always accessible for inspection and testing purposes. At C, equipment is grounded to the neutral on the line (supply) side of the first disconnect fed from a step-down transformer (a separately derived system).

Fig. 250-104. Using grounded circuit conductor to ground equipment housings on line side of service or separately derived system. (Sec. 250.142.)

Aside from the permission given in the three exceptions to the rule of part (B), and separately derived systems or main building disconnects (250.30 and 250.32), the wording of part (B) prohibits connection between a grounded neutral and equipment enclosures on the load side of the service. The wording supports the prohibition in 250.24(A)(5) of grounding connections. So aside from the few specific exceptions mentioned, bonding between any system grounded conductor, neutral or phase leg, and equipment enclosures is prohibited on the load side of the service (Fig. 250-105). The use of a neutral to ground panel-board or other equipment (other than specified in the Exceptions) on the load side of service equipment would be extremely hazardous if the neutral became loosened or disconnected. In such cases any line-to-neutral load would energize all metal components connected to the neutral, creating a dangerous potential above ground. Hence, the prohibition of such a practice. This is fully described in Fig. 250-12.

Fig. 250-105. Panel, switchboard, CB, and switch on load side of service within a single building. (Sec. 250.142.)

When a circuit is run from one building to another, it may be necessary, simply permissible, or expressly prohibited to connect the system “grounded” conductor to a grounding electrode at the other building—as covered by 250-32. Separately derived systems—as covered in 250.30—are also exempted from the basic requirement by the first sentence of 250.142(B).

Although this rule of the Code prohibits neutral bonding on the load side of the service, 250.130(A) and 250.24(A) clearly require such bonding at the service entrance. And the exceptions to prohibiting load-side neutral bonding to enclosures are few and very specific:

Exception No. 1 of 250.142(B) permits frames of ranges, wall ovens, countertop cook units, and clothes dryers to be “grounded” by connection to the grounded neutral of their supply circuit, but only for existing circuits (250.140).

Exception No. 2 to 250.142(B) permits grounding of meter enclosures to the grounded circuit conductor (generally, the grounded neutral) on the load side of the service disconnect if the meter enclosures are located immediately adjacent to the service disconnect, the service is not equipped with ground-fault protection, and the neutral is not less than the minimum required by 250.122, based on the rating of the service overcurrent device. This rule applies, of course, to multioccupancy buildings (apartments, office buildings, etc.) with individual tenant metering (Fig. 250-106).

Exception No. 3 permits DC systems to be grounded on the load side of the service disconnect as described in 250.164.

Exception No. 4 permits medium-voltage electrode-type boilers to operate with their neutral conductors bonded to the pressure vessel and with ancillary electrical equipment bonded to the vessel or to the equipment, as covered in Part V of Art. 490.

If a meter bank is on the upper floor of a building, as in a high-rise apartment house, or otherwise away from service disconnect, such meter enclosures would not meet the rule that they must be “immediately adjacent to” the service disconnect. In such cases, the enclosures must not be grounded to the neutral. And if the service has ground-fault protection, meter enclosures on the load side must not be connected to the neutral, even if they are “immediately adjacent to” the service disconnect.

Fig. 250-106. Grounding meter enclosures to grounded conductor on load side of service disconnect, with meters located “immediately adjacent” to the service disconnect. (Sec. 250.142.)

250.146. Connecting Receptacle Grounding Terminal to Box. The first paragraph requires that a jumper be used when the outlet box is installed in the wall (Fig. 250-107). Because boxes installed in walls are very seldom found to be perfectly flush with the wall, direct contact between device screws and yokes

Fig. 250-107. Bonding jumper connects receptacle ground to grounded box. (Sec. 250.146.)

and boxes is seldom achieved. Screws and yokes currently in use were designed solely for the support of devices rather than as part of the grounding circuit. Although the general rule states that a flush-type box, installed in a wall for a receptacle outlet, does require a bonding jumper from a grounded box to the receptacle grounding terminal, part (A) pertains to surface-mounted boxes and eliminates the need for a separate bonding jumper between a surface-mounted box and the receptacle grounding terminal under the conditions described. But, where such grounding is selected, at least one of the insulating washers must be removed from the device’s securing screws.

Although part (A) generally exempts surface-mounted boxes from the need for a bonding jumper from the box to the ground terminal of a receptacle installed in the box—because there is solid contact between the receptacle’s grounded mounting yoke and the ears on the box when installed—that is not applicable to a receptacle mounted in a raised box cover. (See Fig. 250-108.) There are several issues to consider with respect to mounting receptacles in raised covers. The first is 406.4(C), which requires (generally) no fewer than two screws to hold the receptacle in place. Contrary to common belief, this rule has nothing to do with grounding; it has to do with documented loss experience where a single screw holding the center of a duplex receptacle loosened to the point of allowing the receptacle to fall far enough behind the cover that it would begin to twist. If a Listed, raised covers with flat corners and “screw locking” means for receptacle attachments, no fewer than two points of support, now recognized for receptacle mounting without bonding jumpers.

Fig. 250-108. Typical applications where a surface box does and does not need a receptacle bonding jumper. (Sec. 250.146.)

plug were inserted at the time (and this happened number of times), and the twist occurred in the direction of the ungrounded blade on the plug, and if the plug were even slightly loose, the energized blade would contact the grounded cover at the edge of the duplex punch-out. The result, of course, would be the compete destruction of the plug, the receptacle, the cover, and quite a few items nearby. The double-screw rule has ended this problem.

The reluctance to recognize the cover as the grounding contact has more to do with the permanence of the entire setup. The language in Part A about a box and cover combination being listed as providing acceptable continuity has never been of much help; it went into the NEC to protect certain explosionproof receptacles from needing bonding jumpers to their heavily bolted supports. This problem has been addressed in the 2008 NEC, however, through additional language in Part A. Now, for the first time a raised cover is permitted to support receptacles without bonding jumpers to the box, provided it is of “crushed corner” construction. If the cover has totally flat corners, there will be no spring in the sheet metal, and the 8-32 corner screws on the box will hold securely. The cover must be listed and with no fewer than two fastening points for the receptacle. The support hardware must either be permanent, such as a rivet, or use “thread locking” or “screw locking” means. Thread locking means presumably a jam nut of some type, and it would be somewhat difficult to get hold of between the edge of the receptacle and the raised part of the cover. Screw locking seems to include the current raised cover designs, which come with knurled 6-32 nuts that grab the underside of the cover when the screw is tightened. Most inspectors seem to be accepting these designs which are proving to be a major convenience in comparison to attaching bonding jumpers.

Figure 250-109 illustrates a grounding device which is intended to provide the electrical grounding continuity between the receptacle yoke and the box on which it is mounted and serves the dual purpose of both a mounting screw and a means of providing electrical grounding continuity in lieu of the required bonding jumper. As shown in the sketch, special wire springs and four-lobed

Fig. 250-109. Self-grounding screws ground receptacle in recessed box without bonding jumper. (Sec. 250.146.)

machine screws are part of a receptacle design for use without a bonding jumper to box. This complies with 250.146(B).

250.146(C) permits non–self-grounding receptacles without an equipment grounding jumper to be used in floor boxes which are designed for and listed as providing proper continuity between the box and the receptacle mounting yoke.

Part (D) of 250.146 allows the use of a receptacle with an isolated grounding terminal (no connection between the receptacle grounding terminal and the yoke). Sensitive electronic equipment that is grounded normally through the building ground is often adversely affected by pickup of transient signals which cause an imbalance in the delicate circuits. This is particularly true with highly intricate medical and communications equipment, which often picks up unwanted currents, even of very low magnitude.

The use of an isolated grounding receptacle allows a “pure” path to be established back to the system grounding terminal, in the service disconnecting means, without terminating in any other intervening panelboard. In Fig. 250-110, a cutaway of an isolated grounding receptacle shows the insulation between the grounding screw and the yoke (top), and the hookup of the insulated grounding conductor to the common neutral-equipment-ground point of the electrical system (bottom).

The last sentence of part (D) permits an equipment grounding conductor from the insulated (quiet) ground terminal of a receptacle to be run, unbroken, all the way back to the ground terminal bus that is bonded to the neutral at the service equipment or at the secondary of a step-down transformer—but in no case may the isolated ground extend beyond the building in which it is used. That is, it must be bonded to a ground bus within the building it is run in, even if the “service equipment” is located in another building. Or the equipment grounding conductor may be connected to any ground bus in an intermediate panelboard fed from the service or transformer. But, the important point is to be sure the insulated ground terminal of the receptacle does tie into the equipment ground system that is bonded to the neutral.

This rule must be observed very carefully to avoid violations that have been commonly encountered in the application of branch circuits to computer equipment—where manufacturers of computers and so-called computer power centers specified connection of “quiet” receptacle ground terminals to a grounding electrode that is independent of (not bonded to) the neutral and bonded equipment ground bus of the electrical system. This practice developed to eliminate computer operating problems that were attributed to “electrical noise.” Such isolation of the receptacle ground terminal does not provide an effective return path for fault-current flow and, therefore, constitutes a hazard.

Any receptacle grounding terminal (the green hex-head screw)—whether it is the common type with the mounting yoke or the type insulated from the yoke—must be connected back to the point at which the system neutral is bonded to the equipment grounding terminal and to the grounding electrode, thereby providing the “effective ground-fault current path.” That common (bonded) point may be at the service equipment (where there is no voltage step-down from the service to the receptacle), or the common neutral-equipment-ground point may

Fig. 250-110. Receptacles with isolated ground terminal are used with “clean” or “quiet” ground. (Sec. 250.146.)

be at a panelboard fed from a step-down transformer (as used in computer power centers).

When an isolated ground connection is made for the receptacle ground terminal, the box containing the receptacle must be grounded by the raceway supplying it and/or by another equipment grounding conductor run with the circuit wires. And those grounding conductors must tie into the same neutral-equipment-ground point to which the receptacle isolated ground terminal is connected. The equipment grounding conductor that actually lands on the grounding terminal of the receptacle, in addition to passing through intervening panels without joining with other equipment grounding conductors, is also permitted (2008 NEC clarification) to pass through intervening boxes and other enclosures, also without bonding at those intermediate points. This was always implied in the rule but it is now expressly stated.

See comments that follow 408.40, Exception.

250.148. Continuity and Attachment of Equipment Grounding Conductors to Boxes. The basic rule requires that if circuit conductors enter a box and get spliced or terminated on equipment in or supported by the box, all equipment grounding conductors associated with those conductors must be connected as well, in accordance with five specific provisions (below). This wording means that when wires pass directly through a box, there is no requirement to break the continuity of any of the conductors. Of course, that does not relieve the requirement to connect the box to an equipment grounding conductor as required by 314.4. However, if a box is connected to a metal raceway, there is no reason to interrupt the continuity of the unbroken conductors. In addition, there is no reason to break the continuity of any equipment grounding conductors associated with circuit conductors that are unbroken in a box, even if conductors on or associated with a different circuit are spliced or terminated with the box and therefore subject to the five rules that follow:

Part (A) requires that all connections and splices meet 110.14(B), except that insulation is not required.

Part (B) requires that the grounding connections in a box be arranged so removing a device or other piece of equipment will not interrupt the continuity of the equipment grounding conductor to any other loads on the branch circuit. As a practical matter this means using pigtail connections so the grounding connection to any item can be released with the splice still intact.

Part (C) requires that for a metal box, a connection must be made between the one or more equipment grounding conductors that enter in association with conductors that are spliced or terminated (and that are subject to these five rules) and also a connection must be made between them and the box itself, using a grounding screw that is used for no other purpose, or other equipment or device listed for grounding.

Part (D) requires that equipment grounding conductors entering a nonmetallic box be arranged so a grounding connection can be made to any item in the box that requires the use of a grounding connection.

Part (E) forbids the use of connections that depend solely on solder.

Many issues follow from Part (C) above, beginning with the fact that grounding conductors in any metal box (for which the associated ungrounded conductors are spliced or terminated) must be connected to each other and to the box itself. Figure 250-111 shows a method of connecting ground wires in a box to satisfy the letter of 250.148(C). Note that the two ground wires are solidly connected to each other by means of a crimped-on spade-tongue terminal, with one of the ground wires (arrow) cut long enough so that it is bent back out of the crimp lug to provide connection to the green hex-head screw on a receptacle outlet (if required by 250.146). The spade lug is secured firmly under a screw head, bonding the lug to the box. Of course, the specific connections could be made in other ways. For instance, the ground wires could be connected to each other by twist-on splicing devices; and connection of the ground wires to the box could be made by simply wrapping a single wire under the screw head or by connecting a wire from the splice connector to an approved grounding clip on the edge of the box (Fig. 250-112).

Fig. 250-111. Both ground wires are solidly bonded together in the crimped barrel of the spade lug, which is screwed to the back of the metal box. (Sec. 250.148.)

In all the drawings here, connection to the box is made either by use of a screw in a threaded hole in the side or back of the box or by an approved ground clip device which tightly wedges a ground wire to the edge of the box wall, as shown in Fig. 250-113. Preassembled pigtail wires with attached screws are available for connecting either a receptacle or the system ground wire to the box.

Figure 250-114 shows connection of two cable ground wires by means of two grounding clips on the box edges (arrow). In the past, such use has been disallowed by some inspection authorities because the ground wires are not actually connected to each other but are connected only through the box. The text “...between the one or more... and a metal box by means of... equipment listed for grounding ...,” however, permits such practice as long as the clips are listed for grounding.

Figure 250-115 shows another method that has been objected to as clear violation of NE Code 250.148(C) which requires that a screw used for connection of grounding conductors to a box “shall be used for no other purpose.” Use of this screw, simultaneously, to hold the clamp is for “other purpose” than grounding. Objection is not generally made to use of the clamp screw for ground connection when, in cases where the clamp is not in use, the clamp is removed and the screw serves only the one purpose—to ground the grounding wires.

Fig. 250-112. All these techniques bond the ground wires together and to the box. (Sec. 250.148.)

The Exception to this rule eliminates the need for connecting an isolated grounding conductor to all other grounding conductors.

250.160. Size of Direct-Current Grounding Electrode Conductor. Figure 250-116 is a diagram of a balancer set used with a 2-wire 230-V generator to supply a 3-wire system as referred to in 445.12(D) and covered by part (A) of this section.

250.164. Point of Connection for Direct-Current Systems. On a 2- or a 3-wire DC distribution system, a neutral that is required to be grounded must be grounded at the supply station only.

Fig. 250-113. Ground clip is “identified” for use as called for by 110.14(B). (Sec. 250.148.) Note that the length of free equipment grounding conductor shown here is likely less than 150 mm (6 in.) and therefore in violation of 300.14. Contrary to widely held opinion, nothing in the wording of 300.14 limits it to current-carrying conductors; it applies equally to equipment grounding conductors.

As noted in part (B), an on-site supply for a DC system must have a required grounding connection made at either the source of the DC supply or at the first disconnect or overcurrent device supplied. Because the basic rule says a DC source (from outside a premises) must have a required grounding connection made at “one or more supply stations” and not at “any point on premises wiring,” an on-site DC source would be prohibited from having a grounding connection that might be required. This rule resolves that basic problem by referring to a “DC system source . . . located on the premises.”

Fig. 250-114. Each ground wire is connected to the metal box by a ground clip (one on each side at arrows). The first sentence of 110.14(B) permits ground wires to be “spliced or joined” by use of ground clips or ground screw terminals in the box. (Sec. 250.148.)

Fig. 250-115. This clearly violates 250.148(C) because the screw is also used to anchor the cable clamp. (Sec. 250.148.)

Fig. 250-116. Sizing a DC system grounding conductor. (Sec. 250.166.)

250.166. Size of the Direct-Current Grounding Electrode Conductor. These rules require the grounding electrode conductor to be no smaller than the neutral if there is a balancer set or balancer winding and not smaller than 8 AWG copper or 6 AWG aluminum. Otherwise the grounding conductor must be as large as the largest conductor supplied by the system. This rule is applicable to solar photovoltaic sources among other places. The rules also incorporate the special electrode sizing applicable to the same specific electrodes called out in 250.66, and with the same wire sizes specified.

250.184. Solidly Grounded Neutral Systems. Figure 250-117 shows the details of this set of rules. This section does permit a neutral conductor of a solidly grounded “Y” system to have insulation rated at only 600 V, instead of requiring

Fig. 250-117. Neutral of high-voltage system generally must be insulated for 600 V. (Sec. 250.184.)

insulation rated for the high voltage (over 1000 V). It also points out that a bare copper neutral may be used in such systems for service-entrance conductors or for direct buried feeders, and bare copper or copper-clad aluminum may be used for overhead sections of outdoor circuits.

250.190. Grounding of Equipment. All noncurrent-carrying metal parts of equipment, housings, and enclosures must be grounded, including associated fences and supporting structures. The exception covers equipment isolated from ground and placed so it would be impossible for anyone in contact with the ground to touch such metal parts when the equipment is energized. This correlates with 250.110 Exception No. 2 that exempts wooden-pole-mounted distribution equipment that is over 2.5 m (8 ft) high from the usual equipment grounding requirements.

The second paragraph requires that equipment grounding conductors that are not an integral part of a cable assembly must be no smaller than 6 AWG copper or 4 AWG aluminum. This requirement establishes the usual size of conductor that will be installed routinely in the field for transformer vaults, switchboards, and at other medium-voltage cable termination locations to make the required grounding connections to medium-voltage cable shielding.

ARTICLE 280. SURGE ARRESTERS, OVER 1 KV

Fig. 280-1. Surge (lightning) arresters in an electric substation serving an industrial plant are commonly used in areas where lightning is a problem. (Sec. 280.1.)

280.1. Scope. This article provides rules and regulations covering the application of “surge arresters” or, as they are more commonly known, “lightning arresters.” Here, as stated the Code provides general, installation, and connection requirements for lightning equipment and systems (Fig. 280-1). The major change in the 2008 NEC is that this article now only applies to over 1 kV applications. All other applications at conventional utilization voltages have been transferred to Art. 285.

Figure 280-2 shows a lightning arrester used on one of several high-voltage circuits serving the heavy electrical needs of a modern sports stadium.

Fig. 280-2. Lightning arrester (arrow) is a typical “surge arrester” and, where used, one arrester must be connected to each ungrounded circuit conductor—such as shown here for a 2400-V grounded circuit supplying a transformer for stepping voltage down to supply lighting at this athletic stadium. (Sec. 280.3.)

280.3. Number Required. A double-throw switch which disconnects the outside circuits from the station generator and connects these circuits to ground would satisfy the condition for a single set of arresters for a station bus, as covered in the second sentence of this section.

280.4. Surge Arrester Selection. Figure 280-3 shows the position of a choke coil where it is used as a lightning-protection accessory to an arrester.

Fig. 280-3. Using a choke coil as an accessory to an arrester. (Sec. 280.4.)

In 280.4(B), ratings of surge arresters are covered by the basic rule that applies to silicon-carbide-type surge arresters with a fine-print note pointing up the difference in voltage rating of metal-oxide-varistor (MOV)-type arresters. This addresses the high-technology operating nature of the metal-oxide surge arrester (MOSA) as applied to premises wiring systems. The concern is to make an effective distinction between gapped silicon-carbide arresters, widely used in the past, and the newer metal-oxide block arresters. Manufacturers’ application data on rating and other characteristics and the minimum duty-cycle voltage rating of an arrester for a particular method of system grounding must be observed carefully.

280.12. Routing of Surge Arrester Connections. This rule is particularly important because bends and turns enormously increase the impedance to lightning discharges and therefore tend to nullify the effectiveness of a grounding conductor.

280.24. Interconnections. These rules are aimed at ensuring more effective lightning protection of transformers. Lightning protection of a transformer cannot be provided by a primary arrester that is connected only to a separate electrode. Common grounding of gaps or other devices must be used to limit voltage stresses between windings and from windings to case.

280.25. Grounding. This section refers to Art. 250, which also covers connection of lightning arresters. The second sentence covers the need to keep grounding conductors electrically in parallel with their enclosing metal raceway. For instance, assume that a lightning arrester is installed at the service head on a conduit service riser, with the grounding conductor run inside the service conduit, bonded to the meter socket at the grounding lug, then run through a hole in the meter socket to the grounding electrode without a metal enclosure from the drilled hole to the electrode. In such a hookup, this rule requires the grounding conductor to be bonded to the conduit at the service head (Fig. 280-4).

Fig. 280-4. Arrester grounding conductor must be bonded to bth ends of enclosing metal raceway (or other enclosure). (Sec. 280.25.)

Ordinarily the meter enclosure has a threaded hub, which would mean the conduit would be in good electrical contact with the meter enclosure and would be bonded at the meter socket end. However, 280.25 requires that the grounding conductor, if in a metallic enclosure, be bonded at both ends. Therefore, bonding at the service head is necessary.

The reason given for putting that rule in the NEC was explained as follows:

When conducting lightning currents, the impedance of a lightning arrester grounding conductor is materially increased if run through a metallic enclosure, especially if of magnetic material. The voltage drop in this impedance may be sufficient to cause arcing to the enclosure, and in any event it reduces the effectiveness of the lightning arrester. Bonding of the conductor to both ends of the enclosure is necessary to eliminate this detrimental effect where metallic enclosures are used.

ARTICLE 285. SURGE-PROTECTIVE DEVICES, 1 KV OR LESS

285.1. Scope. The term “surge protective device” (SPDs) is now, as of the 2008 NEC, the term of art for devices that shunt voltage spikes to ground. At one time the term was “lightning arrester” but, effective with the 1981 NEC, the terminology changed to “surge arrester” in recognition that voltage surges had many origins in addition to lightning. The next step occurred in the 2002 NEC, when a new article recognized “transient voltage surge suppressors” in addition to surge arresters. These devices were normally located on the load side of the service equipment, but could be on the line side if special arrangements were made. This has now changed, and the terminology “surge arrester” is now reserved for medium-voltage (over 1 kV) applications exclusively. The term “surge protective device” covers the entire installation spectrum for 600 V and lower applications.

285.3. Uses Not Permitted. SPDs may not be used on circuits exceeding 600 V or ungrounded electrical systems. That is, if the system in question does not have one of its circuit conductors intentionally connected to earth, such as with a 480-V, ungrounded delta system, then use of surge suppressors is prohibited. In addition, an SPD must never be applied at a point where its rating is less than the maximum continuous phase-to-ground voltage at the point of application.

285.6. Short-Circuit Current Rating. Care must be taken when installing SPDs to ensure that the device, which is required to be listed by 285.5, has a listed fault-current rating that is at least equal to the fault current available at the point in the distribution system where it is installed. This rule doesn’t apply to receptacles.

285.12. Routing of Connections. As in the case of surge arresters running over 1 kV, this rule is particularly important because bends and turns enormously increase the impedance to lightning discharges and therefore tend to nullify the effectiveness of a grounding conductor.

285.21. Connection. As given in Secs. 285.23, 285.24, and 285.25, which cover permitted application locations of specific SPDs, such devices may be installed on the load side of the service OC protection, the load side of the OC protection at a main building disconnect, or on the load side of the first OC device fed from a separately derived system. Installation of such devices at other locations would constitute a violation of this rule.

The NEC recognizes three types of surge protective devices as suitable for field installations. Type 1 devices correspond to the old surge arresters and are suitable for installation on the line side of the service equipment. Type 2 devices can be installed at any point on the load side of a service disconnect. They are also permitted on the load side of the first overcurrent device in a building or other structure supplied by a feeder. When special arrangements are made for overcurrent protection, these devices can also go on the line side of the service. Type 3 devices, the least robust, are permitted only on the load side of branch circuit protective devices and with the further restriction that they are at least 30 ft, measured along the conductors, from the service or local feeder disconnect for the building.