For a 400-A main panel with a 600 kcmil feeder, the smallest grounding electrode conductor to the ground rod (not subject to physical abuse) is ___ AWG.

For a 400-A main panel with a 600 kcmil feeder, the smallest grounding electrode conductor to the ground rod (not subject to physical abuse) is ___ AWG.

• A. 1/0
• B. 3
• C. 2
• D. 6

Answer: (D)

The key idea is that the size of the grounding electrode conductor (GEC) to a ground rod is set by the rating of the overcurrent device that protects the equipment—typically the main service disconnect or the feeder rating. This comes from NEC 250.66. The conductor must be large enough to safely carry any fault current that could flow between the system and the earth, but you don’t size it by the physical size of the feeder wire you ran to the panel; you size it by the protective device. In your scenario, the main panel is 400 A. For a 400 A service, the GEC to the ground rod, when it’s not going to be physically abused, has a minimum size of 6 AWG copper according to 250.66. The fact that there’s a 600 kcmil feeder doesn’t change that minimum—the GEC is sized to the rating of the protective device, not the feeder size. If the route to the rod were subject to physical damage, you’d need to upsize the GEC per the table, but for a non-abused route, 6 AWG is the smallest acceptable copper size.

The key idea is that the size of the grounding electrode conductor (GEC) to a ground rod is set by the rating of the overcurrent device that protects the equipment—typically the main service disconnect or the feeder rating. This comes from NEC 250.66. The conductor must be large enough to safely carry any fault current that could flow between the system and the earth, but you don’t size it by the physical size of the feeder wire you ran to the panel; you size it by the protective device.

In your scenario, the main panel is 400 A. For a 400 A service, the GEC to the ground rod, when it’s not going to be physically abused, has a minimum size of 6 AWG copper according to 250.66. The fact that there’s a 600 kcmil feeder doesn’t change that minimum—the GEC is sized to the rating of the protective device, not the feeder size. If the route to the rod were subject to physical damage, you’d need to upsize the GEC per the table, but for a non-abused route, 6 AWG is the smallest acceptable copper size.