In a panel with an upper 250 A breaker feeding Load 1 and a lower 400 A breaker feeding Load 2, one conduit runs from the panel to a pull box with 250 kcmil conductors for the 250-A breaker and 500 kcmil conductors for the 400-A breaker. The smallest EGC permitted in this conduit is which AWG?

In a panel with an upper 250 A breaker feeding Load 1 and a lower 400 A breaker feeding Load 2, one conduit runs from the panel to a pull box with 250 kcmil conductors for the 250-A breaker and 500 kcmil conductors for the 400-A breaker. The smallest EGC permitted in this conduit is which AWG?

• A. 1/0
• B. 4
• C. 6
• D. 3

Answer: (D)

The main idea is that when multiple circuits share the same raceway, the equipment grounding conductor (EGC) must be sized for the largest overcurrent protection device in that run. In this conduit you have a 250 A circuit and a 400 A circuit, so the largest OCP is 400 A. Using NEC 250.122 for copper EGCs, the smallest copper size that can handle a 400 A OCP is 3 AWG. Therefore, the smallest EGC permitted in this conduit is 3 AWG copper. The large feeder conductors (250 kcmil and 500 kcmil) don’t change the EGC size—you size the EGC to the highest OCP, not to conductor size.

The main idea is that when multiple circuits share the same raceway, the equipment grounding conductor (EGC) must be sized for the largest overcurrent protection device in that run. In this conduit you have a 250 A circuit and a 400 A circuit, so the largest OCP is 400 A. Using NEC 250.122 for copper EGCs, the smallest copper size that can handle a 400 A OCP is 3 AWG. Therefore, the smallest EGC permitted in this conduit is 3 AWG copper. The large feeder conductors (250 kcmil and 500 kcmil) don’t change the EGC size—you size the EGC to the highest OCP, not to conductor size.