Four 500 kcmil conductors are installed from the transformer to the 400-A panel. The minimum size equipment grounding jumper for the FMC between the transformer and the 120/208-volt panel is which AWG or kcmil?

• A. 2/0
• B. 1/0
• C. 1
• D. 4

Answer: (B)

The main idea here is how the equipment grounding conductor (EGC) is sized. The EGC must be large enough to carry the fault current back to the source so the overcurrent protection device (the 400 A OCPD) can trip quickly. NEC 250.122 specifies the minimum EGC size based on the rating of that protection device.

For a circuit protected at 400 A, the minimum copper equipment grounding conductor size is 1/0 AWG. So the equipment grounding jumper between the transformer and the 120/208 V panel should be at least 1/0 copper to meet the requirement. Using a smaller size (like 1 AWG or 4 AWG) would not meet the minimum, while a larger size (like 2/0) is acceptable but not required—the question asks for the minimum. If the EGC were aluminum, the size would be different, but the options given are copper sizes.