In a 277/480-volt, 3-phase, 4-wire system, the branch circuit conductors are on a 60-A circuit breaker. The load draws 52 A, and 1/0 AWG conductors would have been more suitable if the run were shorter, but voltage drop considerations require using an EGC of which size?

• A. 10 AWG
• B. 1/0 AWG
• C. 1 AWG
• D. 3 kcmil

Answer: (D)

The important idea is that the equipment grounding conductor (EGC) size is chosen to provide a low-impedance fault path based on the overcurrent protection device rating. For a branch circuit protected at 60 A, the NEC rule for EGC sizing sets a minimum size that ensures adequate fault current to trip the protective device in a timely manner, and this sizing also interacts with the voltage-drop considerations along the circuit’s return path. In a 277/480 V, 3-phase, 4-wire system, the long run means a larger EGC is required to keep the fault-path impedance low enough to meet performance and safety goals. The table that corresponds to a 60 A OCPD calls for a relatively large copper EGC, which in this case is 3 kcmil. Even though, for normal operation, a larger branch conductor like 1/0 AWG might be preferred to minimize voltage drop on the live path if the run were short, the long distance and the fault-current requirements drive the EGC size to 3 kcmil. This is why the correct choice is 3 kcmil.