An electrical contractor called me with questions pertaining to the terminating of the neutral and equipment-grounding conductor (EGC) at a sub-fed panelboard. He was mainly concerned that the local inspector was allowing the grounded (neutral) conductor to be used as a current-carrying conductor as well as an EGC. The inspector also required the EGC, but not always the isolated EGC, to be connected at pull and junction boxes, et al., where installed in the raceway run. He said he knew the National Electrical Code (NEC) covers the permitted use of the neutral conductor and EGC means, but he did not remember where in the NEC the requirements were located.
Grounding of panelboards
Panelboard cabinets and frames, if constructed of metal, must make physical contact with each other, be connected to an EGC, and note, as outlined in NEC 408.40, a neutral conductor is not permitted for such purpose. If a nonmetallic raceway or cable is selected as the wiring method to enclose the circuit conductors, a terminal bar for connecting the EGCs must be provided and secured within the cabinet. It is important that this terminal bar is bonded to the metal of the cabinet and panelboard frame or connected to the EGC that is routed with the circuit conductors supplying the panelboard.
It is imperative that the EGCs are not connected to a terminal bar that is intended for the connection of the neutral conductors unless such a bar is identified for the purpose and is located where interconnection between EGCs and the neutral conductor is permitted. It is clear that the requirements in 408.40 prohibit the connection of the neutral conductor of a feeder circuit being connected at the sub-fed panelboard.
Section 408.41 contains a requirement for connecting the grounded conductor. The wording in the text plainly states that each neutral conductor must terminate within the panelboard to an individual terminal that is not used for connecting other conductors.
Load side use of neutral conductor
An explanation of how the neutral conductor is used on the load side of the service equipment is in order. The basic requirement of NEC 250.24(A)(5) is to prohibit connecting the neutral conductor to the metal of noncurrent-carrying parts of electrical equipment. To assist and help alleviate this problem, this section has been revised in the 2008 NEC and states, a “grounded conductor (neutral) shall not be connected to normally non-current carrying metal parts of equipment, to equipment grounding conductor(s), or be reconnected to grounding.” Notice this wording addresses the connecting of the grounding of the grounded conductor on the load side of the service main disconnecting means except as permitted in NEC 250.30(A), 250.32 and 250.142.
Frames of ranges and clothes dryers
Let’s not get confused where the NEC permits the metal frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for supplying these appliances to be connected to the neural conductor. For this type of installation to be permitted per NEC 250.142(B), Ex. 1, the conditions of the Ex. (1) through (4) to NEC 250.140 must be followed. However, there is not a permissive exception to NEC 408.40 that grants the neutral conductor to be used in this manner.
The exception to NEC 408.40 permits an EGC and isolated EGC as permitted per NEC 250.146(D) to be routed with the circuit conductors to reduce unwanted noise for cord-and-plug connected sensitive electronic equipment. Such an isolated EGC routed with the circuit conductors from the equipment served is permitted to pass through the panelboard without connecting to the equipment-grounding terminal bar.
Section 250.146(D) has been revised to address the grounding connections at wireways, auxiliary gutters, pull and junction boxes when, for example, complete isolation of grounding conductors is necessary. The revised wording to the last paragraph states, “Where installed in accordance with the provisions of this section, the EGC shall also be permitted to pass through boxes, wireways, or other enclosures without being connected to such enclosures.” Note that other revisions in the main section have been made to help clarify the intended use of the EGC and the isolated EGC when used for grounding and isolation of receptacles and sensitive electronic equipment. The rewrite clarifies that both the EGC and isolated EGC are permitted to pass through such enclosures as outlined in NEC 250.146(D).
In summary, the neutral conductor must not be connected to the cabinet or metal frame of the enclosure of a sub-fed panelboard. And the EGC and isolated EGC from the isolated receptacles can be routed through other enclosures without connecting them to the metal frames of such enclosures.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.