In a 400-A main breaker panelboard fed from a transformer with a single conduit containing four 500 kcmil conductors, what is the smallest size copper wire permitted to be used as the main bonding jumper?

In a 400-A main breaker panelboard fed from a transformer with a single conduit containing four 500 kcmil conductors, what is the smallest size copper wire permitted to be used as the main bonding jumper?

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

Answer: (C)

Sizing the main bonding jumper is based on the service’s rating (the overcurrent protection at the service disconnect) and is guided by NEC requirements that the bond path between the neutral and the grounding system be capable of carrying fault currents. For a 400-amp service disconnect, the minimum copper conductor that can serve as the main bonding jumper is 1/0 AWG. This requirement is set to ensure the bond can carry the maximum fault current without overheating, independently of the size of the feeders in the conduit (the four 500 kcmil conductors) which do not determine the bonding jumper size. Using a smaller conductor would not meet the code minimum, while a larger copper size would still be permissible. If copper were replaced with aluminum, a different, typically larger size would apply per the aluminum bonding jumper guidelines.

Sizing the main bonding jumper is based on the service’s rating (the overcurrent protection at the service disconnect) and is guided by NEC requirements that the bond path between the neutral and the grounding system be capable of carrying fault currents. For a 400-amp service disconnect, the minimum copper conductor that can serve as the main bonding jumper is 1/0 AWG. This requirement is set to ensure the bond can carry the maximum fault current without overheating, independently of the size of the feeders in the conduit (the four 500 kcmil conductors) which do not determine the bonding jumper size. Using a smaller conductor would not meet the code minimum, while a larger copper size would still be permissible. If copper were replaced with aluminum, a different, typically larger size would apply per the aluminum bonding jumper guidelines.