What is the smallest size copper wire permitted to be used as a main bonding jumper in a 400-A main breaker panelboard?

The smallest size copper wire permitted to be used as a main bonding jumper in a 400-A main breaker panelboard is 1/0 AWG. This is based on requirements set by the National Electrical Code (NEC), which specifies the minimum size for bonding jumpers based on the rating of the service equipment.

In addition to providing electrical connection to ground, bonding jumpers are essential in ensuring safety by maintaining a low-resistance path for fault currents. The 1/0 AWG copper wire size is suitable for a panelboard with a 400-A main breaker, ensuring that the wire can effectively handle potential fault currents without overheating or failing.

It's important to note that the sizes for bonding jumpers are determined in part by the ampacity of the systems they serve. For larger panelboards, such as one rated for 400 amps, larger gauge wires are necessary to safely accommodate the system's requirements. Thus, using 1/0 AWG copper meets the necessary criteria established by the NEC, while other options may not.