For a 277/480-volt service with a 3-phase main breaker panelboard, what is the minimum size wire needed for the main bonding jumper?

The correct choice is based on the requirements outlined in the National Electrical Code (NEC), which stipulates the necessary sizing of the main bonding jumper for electrical systems. Specifically, for a 277/480-volt, 3-phase service, the NEC provides guidelines that help determine the appropriate size of the bonding jumper based on the size of the service conductors and the overall system configuration.

In this case, the bonding jumper needs to effectively connect the grounded circuit elements to ensure a low-impedance path for fault current, thus satisfying safety and operational integrity. According to NEC Table 250.66, for services rated at 250 kcmil or larger conductors, the minimum bonding jumper size required is 1/0 AWG copper or equivalent aluminum size.

This indicates that the choice of 1/0 AWG is aligned with NEC standards to provide adequate protection against ground faults, ensuring compliance with safe operation practices in electrical installations. Therefore, for this specific 277/480-volt service scenario with a 3-phase configuration, 1/0 AWG is sufficiently sized to serve as the main bonding jumper.