Code Q&A

Jul 22, 2005 8:16 AM, By Mike Holt

Q. How would you size the bonding wire to a motor that’s protected by an inverse time circuit breaker?

A. The equipment grounding (bonding) conductor must be sized in accordance with Table 250.122, based on the ampere rating of the circuit-protection device, but in no case is it required to be larger than the circuit conductors.

For example, a 5-hp, 208V, 3-phase motor with an FLC of 16.7A, and that’s wired with 12 AWG [430.22(A)], with a 40A short-circuit, ground-fault protection device [430.52(C)(1)] would have a 12 AWG equipment grounding (bonding) conductor. Since the circuit wires are 12 AWG, there is no reason to install a 10 AWG equipment grounding (bonding) conductor. Please review requirement in 250.122, which in part states "equipment grounding conductors shall not be smaller than shown in Table 250.122 but shall not be required to be larger than the circuit conductors supplying the equipment."

Got a question for Mike? E-mail him at mike@mikeholt.com.