Question 1. Section 250-100 contains the following statement: 'Regardless of the voltage of the electrical system, the electrical continuity of noncurrent-carrying metal parts of equipment, raceways, and other enclosures in any hazardous (classified) location as defined in Article 500 shall be ensured by any of the methods specified for services in Section 250-94 that are approved for the wiring method used.'

An equipment grounding conductor (ground wire) has been included in the branch circuit for the lighting fixture. Assuming the knockouts in the 4-inch square box are not approved for bonding, are bonding fittings required on the raceway terminations in the 4-inch square box, or does the inclusion of the ground wire satisfy Section 250-100? — L.T.S.  

Answer 1. This question deals with the requirements for bonding in hazardous locations. It is important to utilize the proper Code sections and have a general working knowledge of the Code and the problem to properly apply the Code in this type of installation. While an equipment grounding conductor is installed in the metallic raceway, the bonding requirements in Chapter 5 of the NEC have not been satisfied based on the information and drawing provided. 

Let’s take a closer look at the Code requirements applicable to this installation and some of the important reasons why these minimum requirements are applicable. Grounding and bonding is essential for electrical safety in nonhazardous locations as well as in hazardous locations. In hazardous locations it is vital to have effective grounding and bonding to prevent an explosion. Under fault current conditions when heavy currents are flowing through metal conduit, every connection point in the raceway system is a potential source of sparks and ignition. If there is an arcing fault to a metal enclosure in a hazardous (classified) location, the external surface temperature of the metal enclosure at the point of the arcing fault will start to rise to temperatures that could cause ignition of the flammable vapors or accumulations of combustible dust. Under these fault conditions it is essential that the overcurrent device be caused to operate as quickly as possible to prevent a hot spot on the enclosure or even arcs that may burn through the enclosure from igniting the atmosphere, dust, or fibers and flyings on the outside of the enclosure. It is extremely important that all threaded joints be made up wrenchtight to prevent sparking at those threaded joints. If joints are other than the threaded type, such as locknuts and bushings or double locknuts and bushings at boxes, enclosures, cabinets, and panelboards it is essential that bonding be assured around those joints in the bonding path to prevent sparking and assure a low-impedance path for the fault current.

The bonding requirements are found in Part E of Article 250 of the NEC. Section 250-90 requires bonding where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed. Section 250-100 also includes the bonding requirements for hazardous locations and indicates that regardless of the voltage, the electrical continuity of noncurrent-carrying metal parts of equipment, raceways, and other enclosures in any hazardous (classified) location as defined in Article 500 shall be ensured by any of the methods specified for services in Section 250-94 that are approved for the wiring method used. Clearly, this requirement is pointing to the metal enclosures and raceway that enclose all of the conductors of the circuit, including an equipment grounding conductor.   

Chapter 2 of the NEC includes the general requirements, and Chapter 5 includes the requirements for hazardous locations. 90-3 indicates that Chapters 1-4 of the Code have general application, while 5,6,and 7 modify or amend the rules in Chapters 1-4. This question is a perfect example of a more restrictive requirement from Chapter 5 that must be complied with. Section 501-16(a), 502-16(a), and 503-16(a) cover the grounding and bonding rules required for wiring installed in hazardous (classified) locations. 

These sections indicate that locknut-bushing and double-locknut types of contacts shall not be depended upon for bonding purposes, but bonding jumpers with proper fittings or other approved means of bonding shall be used. Such means of bonding shall apply to all intervening raceways, fittings, boxes, enclosures, etc., between the classified location and the point of grounding for the service equipment or point of grounding for a separately derived system even if there are intervening overcurrent devices. This is to assure quick operation of the entire protective system. 

The illustration shows a light, a seal fitting, and a 4-inch square box in the branch circuit routed to the panelboard. If the panelboard is the service, then a bonding means in accordance with Section 250-94 is required to be provided for all metal raceways all the way back to the service grounding point. In the illustration, it appears as though the proper bonding methods are not complete through all intervening raceways back to the service. The inclusion of the equipment grounding conductor inside the raceway does not satisfy the requirements of Sections 501-16(a), 502-16(a), 503-16(a), or 250-100. Hope this helps paint a clearer picture of the need for boning of these metal enclosures and raceways when installed in these hazardous (classified) locations. — Michael J. Johnston, CMP-5

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Question 2. I have a separately derived system that is feeding panel “A” that has a feed-through on the load side of the main breaker in panel “A” that feeds panel “B.” Panel “B” is a main lug only panel next to panel “A.”

Would panel “B” be considered a sub-panel that would require the grounded conductor to be isolated from the enclosure and the grounded and grounding conductors separated?

Your assistance would be appreciated. I have a contractor that expressed an opinion that panel “B” is just an extension of panel “A.” — D.W.

Answer 2. The grounded conductor in panel "B" would have to be separated from the equipment grounding conductors. In panel "A," they can be together because that is where the bonding jumper is located in accordance with 250.30(A)(1) [2002 NEC]. Section 250.142(B) does not permit the connection of a grounded conductor to ground on the load side of the separately derived system overcurrent device which is located in panel "A." 

The question appears to indicate that the panelboards are next to each other or side by side. Remember, panel "A" could be located 100 feet away also because the panelboard and the conductors are properly protected already by the OCPD in panelboard 1. The issue is parallel return paths for the neutral current on the load side of the first OCPD. The first OCPD, in this case, is in panel "A" at the termination of the transformer secondary conductors. As always, the authority having jurisdiction has the — Michael Johnston, CMP-5

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Question 3. Section 408.3(E) gives a phase arrangement for busbars. Is this an arrangement for circuit breakers and fused disconnects? I ask because IAEI News, January/February 2002, page 39, shows from left to right, phase A, B, and C. Page 40, same issue, shows arrangement C, A, and B. Page 104 of 2002 NEC Analysis also shows C, A, and B. The diagram, on page 104 of the Analysis, does not show the equipment grounding conductor connected directly to the grounding conductor (same as page 40 of January/February issue of IAEI News). — J.J.

Answer 3. Your question is relative to the required phase arrangement specified in Article 408 regarding 3-phase switchboards and panelboards. The NEC requires a set phase arrangement for the buses of such equipment of A, B, C from left to right, front to back, and top to bottom within the equipment as viewed from the front of the equipment [see  408.3(E)]. Note this arrangement and requirement is for the buses only.

The drawing you are referring to in the January/February 2002 edition of the IAEI News on page 39 does not show the phase arrangement. The equipment in which the feeder conductors terminate is required to meet the phase arrangement required by 408.3(E). There are no phases of the transformers or panelboards identified in figures 3, 4, or 5 of the article. Figure 2, on page 38, identifies the phases of the delta transformer to indicate the voltages between the phase conductors of that transformer, but was not intended to relate to figures 3, 4, or 5.

Based on the NEC, phase arrangement does not apply to circuit breakers or fused switches but is applicable to 3-phase switchboards or panelboards. If the transformer secondary terminates in an overcurrent device in accordance with 240.21(C)(6) and 408.16, then the phase arrangement would be required to be A, B, C from left to right as viewed from the front of the equipment.

Figure 5 on page 40 of the IAEI News article and the graphic on page 104 of the 2002 NEC Analysis of Changes book do not indicate the phase identification or arrangement simply because that was not the emphasis of those particular drawings. It should be noted that the phase arrangement in the panelboards in both drawings would be required to meet the rule in 408.3(E).

The green conductor shown from the source enclosure to the first overcurrent device enclosure in both drawings is referred to by the Code as an equipment bonding jumper, and is supposed to show a connection between the transformer (source) enclosure and an equipment grounding terminal bar within the panelboard [see 250.30(A)(5)].

Phase arrangement of the panelboards in these drawings was not shown, but as a requirement of the Code and equipment standard, it would need to be as specified in 408.3(E). Typically the phase terminals or leads at the secondary side of transformers are identified as X1, X2, X3, and XO, and the equipment the conductors terminate in requires the phase identification of A, B, and C. I hope this provides some clarity relative to phase arrangement requirements set forth in the NEC. — Michael Johnston, CMP-5  

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Question 4. The local inspector has turned down my service because I took the grounding electrode conductor to the meter can instead of the fused disconnect. The service is a 400-amp service built on a pole to serve a ground water cleanup site. We installed the K meter on one side of the pole, and the disconnect on the other side. If I understood the Code correctly, then the grounding electrode conductor could be connected to the grounded conductor at any point between the service drop and the enclosure that contained the first means of disconnect. His contention is that the Code requires it to be in the service equipment and that the meter is not considered part of the service equipment. Could you help clear this up for us? — W.J.

Answer 4. The NEC permits the grounding electrode conductor connection to be made at any accessible location from the load end of the service drop, or lateral to and including the terminal or bus where the grounded conductor terminates in the service disconnecting means. In general, the common practice is to make this connection of the grounding electrode conductor to the grounded conductor within the disconnecting means enclosure. Some jurisdictions do not consider connections under a sealed utility company meter enclosure as accessible for the purposes of this section because it would violate the utility company requirements or laws in that particular area.

As a general rule, the NEC would permit the connection of the grounding electrode conductor in either of the locations mentioned in the question: the meter or the service disconnecting means. This is referenced in 250.24(A)(1) [NEC 2002]. 

Of course, this would be acceptable unless there are local codes that prohibit this connection from being made in meter socket enclosures. The authority having jurisdiction (AHJ) has the responsibility of interpreting the rules and enforcing them at the local levels. — Michael J. Johnston, CMP-5  

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Question 5. I am a building inspector with the city of Noblesville, Indiana. I have a question concerning 250.56 [NEC 2002]. May this section be applied to residential construction? Any help that you can provide will be most appreciated. — D.S.

Answer 5. Section 250.56 of NEC 1999 and 2002 would both be applicable to all types of construction, not just residential. The key factor is to remember that the 25-ohm requirement is to be applied to a single electrode of the rod, pipe, or plate type. Where the resistance to ground is questionable, ground resistance testing might be applied to demonstrate 25 ohms or less on the single electrode; or an additional electrode of the types specified in 250.52(A)(2) through (A)(7) is required to be installed to augment the single electrode. 

Where multiple electrodes are installed, the Code requires a minimum 1.8 m (6 ft.) separation between them. It is important to follow any manufacturer’s installation instructions that might be included with listed grounding electrodes. Those might require more distance for separation than the minimum distance given in the Code. Note that the 25-ohm requirement is not applicable to the grounding electrode system covered in 250.50. It is applicable to single electrodes of the rod, pipe, or plate variety.

It should also be noted that supplementary grounding electrodes installed in accordance with 250.54 are not required to meet the maximum 25-ohm resistance-to-ground rule specified in 250.56. They should be connected to and supplement the required equipment grounding conductor of the circuit. The earth shall not be permitted as the sole equipment grounding conductor. Additional information about supplementary and supplemental grounding electrodes is provided in an article on this topic in the September/October 2002 edition of IAEI News. Hopefully, this helps address your concerns. As always, the authority having jurisdiction has the responsibility for interpretation and enforcement of the applicable electrical code(s) and any local code amendments that may be adopted by that jurisdiction.   — Michael J. Johnston, CMP-5

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Question 6. I am an electrical inspector, and an IAEI member in the Central Pennsylvania Chapter. After reading Michael J. Johnston’s article on supplementary grounding electrode, I’m a little confused. Mr. Johnston states that the NEC does not require a ground rod at parking lot lights. I have been told by my managers that they are required, because they are considered a separate structure, and fall under 250.32(A). Could you shed some light on this subject? Thank you. — P.S.

Answer 6. Regarding your question to supplementary grounding electrodes, while a light pole base and the pole itself are technically structures by definition, the NEC does not require an electrode in these locations. Supplementary grounding electrodes overlay the required grounding and bonding safety system. The light pole base was just an example of one type of equipment where a supplementary grounding electrode is often installed as an option. There is an exception to 250.32(A) that should be referred to in this particular case you mentioned in the question. In accordance with the exception, a grounding electrode is not required for a structure or building that is supplied by a single branch circuit that includes an equipment grounding conductor for grounding the equipment.

The poles are often supplied by a branch circuit in accordance with the exception. A metal shade screen structure with a circuit supplying carport lighting is another example where the exception to this requirement is often used. Some designers or engineers actually specify a grounding electrode at light pole standard as part of the job specification. Other examples used in the article were supplementary grounding electrodes installed at computer equipment, machines, etc. The supplementary electrode connection establishes a connection from the light pole to the earth at that location. In the case of the light pole base, sometimes the base itself serves very well as a natural grounding electrode through the connection between the concrete and rebar and the earth.

The required equipment grounding conductor must always be connected to the equipment, and the earth shall not be permitted as the sole equipment grounding conductor. Local amendments or interpretations may come into play in your particular case. If your jurisdiction has chosen to require these electrodes at all pole bases that is their decision. As far as their being required at all light pole bases, this is not presently a requirement of the Code as indicated by the exception.

I hope this helps you with your particular question. Remember, the authority having jurisdiction has the interpretation responsibility and approving authority, and there might be local amendments or interpretations that may need to be considered. — Michael J. Johnston, CMP-5

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Question 7. Why is an equipment grounding conductor not required to be installed with the service-entrance conductors from the utility transformer to the service disconnecting means? — T.O.

Answer 7. For grounded ac systems operating at less than 1000 volts, the Code requires the grounded (usually the neutral) conductor to be brought to the service disconnect and bonded to the service disconnecting means enclosure [see 250.24(B)]. The grounded conductor is routed with the phase conductors of either a service lateral, or in the form of a service drop for overhead services. This grounded conductor is required to be sized not smaller than required to carry the maximum unbalanced neutral load current in accordance with 220.22, and in no case smaller than the required grounding electrode conductor specified in Table 250.66. Where the service-entrance conductors are larger than 1100 kcmil copper or 1750 kcmil aluminum, the grounded conductor (usually the neutral) must be sized at least 12.5 percent of the circular mil area of the largest ungrounded service-entrance phase conductor. 

To answer this question specifically, the grounded conductor is permitted for grounding and bonding on the supply side of the service disconnecting means as provided in 250.142(A) and serves two primary purposes. The grounded conductor installed with the service conductors serves to carry normal neutral current loads to the utility source in normal operation; and in fault conditions, it serves as the low impedance path to the source that facilitates overcurrent device operation. Equipment grounding conductors, on the other hand, are installed on the load side of overcurrent protective devices with feeders and branch circuits and are sized based on the rating of the overcurrent protective device using Table 250.122. Hopefully this helps address your concerns. As always, the authority having jurisdiction has the responsibility for interpretation and enforcement of applicable electrical code(s) and any local code requirements that may be adopted by that jurisdiction. — Michael J. Johnston, CMP-5

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Question 8. Are standard wire nuts acceptable as connectors on grounding conductors? I have been told that they are not and only "greenies and crimp devices" are acceptable on grounding conductor splices. — M. J.

Answer 8. Your question addresses the use of "wire nuts" or wire "pressure connectors" and whether they a permitted for use to splice equipment grounding conductors together in outlet boxes, junction boxes, and so forth. The simple answer is yes. Wire nuts (wire pressure connectors) are permitted for this purpose (see Section 250.8). Also take a look at Section 250.148. Here one can find the language that indicates splices in equipment grounding conductors shall be made in accordance with 110.14(B), except that insulation is not required for these types of splices. This is because they are not normally current-carrying conductors. They should only carry current during abnormal conditions or events such as ground faults. The equipment grounding conductors carry the ground-fault current for the time it takes an overcurrent device to open the faulted circuit. It should be noted that the same type of wire nut or wire pressure connector used for the ungrounded (hot) conductors is also exposed to the ground-fault current for the time it takes the overcurrent device to open and clear the faulted condition (see figure 1).

Section 110.14(B) addresses all types of splices for conductors and does not differentiate between ungrounded (hot) conductors or equipment grounding conductors. This section requires that the splice be insulated with insulation equivalent to the insulation of the conductor. As can be seen in 250.148, there is a modification that allows splices in equipment grounding conductors to be uninsulated.You mentioned "greenie" or "crimp devices" in the question. Those devices are certainly permitted also for splicing equipment grounding conductors if they are suitable for the use, but they are not the only devices suitable for the use as required by the wording of Section 250.148. The first sentence of 250.148 indicates that separate equipment grounding conductors shall be spliced or joined within the box. This can be accomplished with greenies, crimp sleeves, wire nuts, or other suitable devices in accordance with 110.14(B). The color of the device used for this purpose is not a requirement of the Code. The last part of the first sentence indicates that the equipment grounding conductors are required to be connected to the box with devices suitable for the purpose. This would be a listed grounding clip or suitable grounding screw for example [see also Section 250.148(A)]. Remember, sheet metal screws are not permitted to attach equipment grounding conductors to metal boxes as prohibited by Section 250.8.Another section to keep in mind here is Section 110.3(B). Basically this section requires equipment to be installed and used in accordance with the installation instructions and within the limits of the product listing. There is also valuable information in the General Information for Electrical Equipment, 2002 edition (UL White Book). See category (ZMVV) on page 133. Information about grounding and bonding equipment such as ground clamps, ground rods, grounding bushings, grounding and bonding locknuts, etc., is also included in the UL White Book under the category (KDER) on page 58.I hope this answers your question and clarifies which types of splicing devices are suitable for splicing equipment grounding conductors. As always, consult your local authority having jurisdiction, who has the responsibility of enforcement and interpretation of the rules. There may also be local amendments to the minimum requirements covered by the National Electrical Code. Thanks again for the question. — Michael Johnston, CMP-5

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Question 9. I am writing to your office as a poll of various jurisdictions on the subject of bonding (grounding) requirements for a raised cellular floor. I will offer a brief description of the system and the components and ask that you respond with your view on the requirement.

The system has: 2 ½ – 3" (finished height) raised floor; ¾" floor panel; 1 ¾ – 2 ¼" support cylinders; panels rest on 16 support cylinders; panels are secured by a flat washer in each corner; and the support cylinders are secured to concrete floor with "Mastic" at each corner.The panel components are: 24" x 24" cementatious tile (cement and wood particles); non-conductive; ¾" thick; the corner is recessed to accept flat washer; and they are supported on a bed of 16 cylinders. The fasteners components have: ¼-20 screw and 1" flat washer. The support cylinders have: stamped sheet metal; cylinders are approximately 2" round and 2" high; threaded hole to receive ¼-20 screw; and are not mechanically fastened to each other.The raised access floor is designed to be installed in general office areas as a means of distributing 120-volt power and network cabling to private offices and furniture stations.The void between the support cylinders and the panel provides a path for laying in armored cables (type MC or AC) for electrical loads and CAT5E for network cabling. Due to the low height of the finished floor, this raised floor is not to be used as an air handling space. Since the surface of the raised floor is non-conductive (cement) and all electrical cabling will have a bonding conductor (MC or AC cable), is bonding (grounding) the sheet metal support structure necessary? If so, at what interval? — R.G.

Answer 9. In general, there is no requirement in the NEC to bond such raised floor metal framing structures. Sometimes the framing structure used to support the raised flooring panels serves as a signal reference grid or is bonded to field-installed signal reference grid. Keep in mind that signal reference structures are a grounding and bonding enhancement often used in information technology room installations that overlay the required minimum equipment grounding and bonding conductors. Section 645.15 indicates that where signal references structures (grids) are installed (which could be the framing in some instances), they must be bonded to the equipment grounding system provided for the information technology equipment.

Note: It is really important to determine that Article 645 is even allowed to be applied to this situation. Article 645 of the NEC is only applicable to information technology equipment and rooms if all of the conditions set forth in 645.2 have been met. Signal reference grids are not a requirement of the NEC and the signal reference grid can be provided in a variety of forms such as the interlocked framing of a raised floor, a rebar mesh system in the concrete floor supporting the raised (platform) floor, a conductive network assembled under raised floors, or even the metal framing of a suspended ceiling grid system.

Your second question relates to any required bonding connection intervals, and is not applicable since the bonding of the framing is not generally required by the Code. If a bonding connection is required to the raised floor framing in accordance with 645.15 (last paragraph) because it is used as the signal reference structure, then the bonding and interconnecting of the framing used as the signal reference structure would be required to meet the bonding requirements of Part V of Article 250. Signal reference structures (grids) are generally a design issue, and an enhancement that reaches beyond the minimum requirements for grounding and bonding in accordance with the NEC minimums. I hope this helps provide some clarification. Always verify with the local AHJ for any local code rules that might be more restrictive than the minimum rules in the National Electrical Code. —Michael J. Johnston, CMP-5

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