The largest size copper GEC required to be used to building steel from a 400-A main breaker panelboard fed from a transformer with three parallel sets of 1/0 AWG is which AWG?

The largest size copper GEC required to be used to building steel from a 400-A main breaker panelboard fed from a transformer with three parallel sets of 1/0 AWG is which AWG?

• A. 1/0
• B. 3/0
• C. 2
• D. 1

Answer: (C)

Sizing the grounding electrode conductor is determined by the rating of the service overcurrent device that protects the building. In this case, even though the transformer feeds three parallel sets of 1/0 AWG, the main disconnect protecting the panel is 400 A. So you size the copper GEC for a 400 A service. Consulting the GEC sizing table for copper, a 400 A service requires a 2 AWG copper conductor. The parallel 1/0 conductors don’t raise the required GEC size because the rule uses the overcurrent device rating, not the total parallel conductor capacity. A 1/0 would be larger than needed, 3/0 would be more than required, and 1 AWG would be too small for a 400 A service.

Sizing the grounding electrode conductor is determined by the rating of the service overcurrent device that protects the building. In this case, even though the transformer feeds three parallel sets of 1/0 AWG, the main disconnect protecting the panel is 400 A. So you size the copper GEC for a 400 A service. Consulting the GEC sizing table for copper, a 400 A service requires a 2 AWG copper conductor. The parallel 1/0 conductors don’t raise the required GEC size because the rule uses the overcurrent device rating, not the total parallel conductor capacity. A 1/0 would be larger than needed, 3/0 would be more than required, and 1 AWG would be too small for a 400 A service.