A very knowledgeable electrical contractor in the Phoenix area recently emailed me, stating he was having a National Electrical Code (NEC) discussion with a local electrical inspector. The discussion involved the sizing of equipment grounding and bonding conductors based on 250.122(B) and Table 250.122. The situation in question was a 20-ampere (A) branch circuit supplying metal boxes that was installed in concrete as deck boxes and supplied power for luminaires. PVC conduit was used from the panelboard to each deck box and luminaire. Due to the considerable length of the PVC conduit and distance to the boxes, the equipment grounding conductors were increased from 12 AWG to 10 AWG. The contractor asked whether the bonding conductors in each metal box could be 12 AWG or if the bonding jumpers had to be increased to 10 AWG.
To answer the question, the references for grounding and bonding the boxes would be 250.146 for connecting receptacle grounding terminals to metal boxes, 250.148 for continuity and attachment of the equipment grounding conductors to metal boxes, 250.122(B) for sizing up on the equipment grounding conductors due to long runs and Table 250.122 for sizing of equipment grounding conductors based on the size of the overcurrent device protecting the circuit. To determine the requirements for grounding and bonding of the metal boxes and the luminaires, a careful study of each section is required.
To start, an increase in the size of the equipment grounding conductor from 12 AWG to 10 AWG may be required by 250.122(B), based on the statement, “Where ungrounded conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation, where installed, shall be increased in size proportionately, according to the circular mil area of the ungrounded conductors.”
The difference in opinion between the contractor and the inspector was based on the increase in size of the equipment grounding conductor from 12 AWG to 10 AWG.
The electrical contractor based that increase, as he should, on 250.122(B), the length of the run and the proportionate increase in size of the equipment grounding conductors to the size increase in the ungrounded conductors. The increase in the size of the ungrounded conductor would be based on the voltage drop calculation for the branch circuit to ensure no more than a 3% voltage drop for the farthest lighting load based on 210.19(A) and Informational Note No. 4. The increase in the equipment grounding conductor size will ensure enough ground-fault current will pass through the overcurrent device to adequately clear that overcurrent device.
The requirement that the metal boxes supply power to the luminaires to be connected to the equipment grounding conductor is located in 250.146, based on grounding-type receptacles being used to connect the lighting units to the equipment grounding conductor. This section states, “An equipment bonding conductor shall be used to connect the grounding terminal of a grounding-type receptacle to a grounded box, unless grounded as in 250.146(A) through (D). The equipment bonding conductor shall be sized in accordance with Table 250.122 based on the rating of the overcurrent device protecting the circuit conductors.” Since none of the requirements in 250.146 (A) through (D) apply to this application, the base rule requiring the bonding jumper from the box to the terminal of the receptacle still applies. Since the circuit breaker is 20A, then Table 250.122 requires a 12 AWG bonding jumper; however, the installation in question did not use receptacles.
However, if circuit conductors are spliced within the boxes or terminated on equipment within or supported by a box—such as for luminaires—compliance with 250.148 is necessary for covering continuity and attachment of equipment grounding conductors to boxes. This section would require all of the equipment grounding conductors associated with any of the circuits within the box to be connected within the box or to the box with devices suitable for use in accordance with 250.8, assorted grounding methods and 250.148(A) through (E). In our discussion here, only 250.148(B) and (C) would apply. Section 250.148(B) requires that the disconnection or removal of any of the luminaires could not interfere with or interrupt the grounding continuity. However, 250.148(C) states a connection shall be made between one or more of the equipment grounding conductors and a metal box by means of a grounding screw that shall be used for no other purpose, equipment listed for grounding or a listed grounding device.
The EC used a listed 12 AWG bonding jumper and green equipment grounding screw, and since the luminaire was not connected to a receptacle, as covered in 250.146, the No. 10 equipment grounding conductor was required to be used to bond the boxes. The electrical inspector was correct in this case.