In a dual-conduit installation for a 3-phase system, with three 350 kcmil conductors in each parallel run and a 600-A circuit breaker, what is the smallest EGC gauge allowed to be installed in each conduit?

• A. 1 AWG
• B. 2 AWG
• C. 4 AWG
• D. 1/0 AWG

Answer: (A)

The rule being tested is how to size the equipment grounding conductor (EGC) based on the overcurrent protection device. For an overcurrent device rating of 600 A, NEC 250.122 requires a copper EGC of 1 AWG (or an aluminum EGC of 1/0 AWG). This sizing is tied to the protective device, not to the size of the current-carrying conductors, so even with three 350 kcmil conductors in each parallel run and two parallel conduits, the minimum EGC in each conduit must meet that 1 AWG copper requirement. In a dual-conduit setup, you still provide a ground path in each conduit with at least this size so a ground-fault current can return quickly enough to trip the 600 A breaker. Therefore, the smallest allowed EGC gauge in each conduit is 1 AWG copper. The other options are larger than the minimum and thus not the smallest permissible gauge.