What is the minimum size supply-side bonding jumper required with a 400-ampere feeder from an outdoor generator to the building disconnecting means?

The minimum size supply-side bonding jumper needed for a 400-ampere feeder is determined by the requirements outlined in the National Electrical Code (NEC). For an installation involving an outdoor generator and a building, the bonding jumper serves to connect the generator's frame and other metal parts to the grounding electrode system of the building.

In this case, the NEC provides tables that specify bonding jumper sizes based on the ratings of the feeders. For a feeder rated at 400 amperes, the minimum size supply-side bonding jumper required is indeed 1/0 AWG. This size ensures adequate conductivity to handle potential fault currents safely, helping to maintain a low impedance path to ground.

This bonding jumper plays a critical role in safety, helping to prevent electrical shock hazards and ensuring the proper operation of overcurrent devices during fault conditions. Thus, the choice of 1/0 AWG aligns with the NEC's emphasis on safe electrical installations for larger feeder ratings.