What is the allowance for the smallest main bonding jumper that can be used given the largest ungrounded conductor is 3/0 AWG?

The correct allowance for the smallest main bonding jumper when the largest ungrounded conductor is 3/0 AWG is 4 AWG. This determination is based on the National Electrical Code (NEC) guidelines which provide specifications for the sizing of bonding jumpers based on the size of the largest ungrounded conductor.

In this context, the bonding jumper is crucial because it establishes a low-resistance path for fault currents, ensuring that the metal parts of an electrical installation are effectively grounded. According to NEC Table 250.102(C)(1), the minimum size of the main bonding jumper must be determined by the size of the largest ungrounded conductor.

For a 3/0 AWG ungrounded conductor, the NEC specifies that the minimum size for the main bonding jumper should be 4 AWG copper or 2 AWG aluminum. This requirement ensures safety and proper functioning of the grounding system in the electrical installation. Therefore, since 4 AWG is the smallest allowed size that meets code for this scenario, it is the correct answer.