The main bonding jumper connects the grounded circuit conductor to the equipment grounding conductor, or the supply-side bonding jumper, or both, at the service.

Main bonding jumpers are sized from NEC^® Table 250.102(C)(1) which is also used to size grounded conductors, system bonding jumpers, and supply-side bonding jumper for alternating-current systems.

Table 250.102(C)(1) provides the size of the main bonding jumper which is based on the largest ungrounded conductor or equivalent area for parallel conductors.

Below is a preview of the NEC^®. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC^® edition of NFPA 70.

2020 Code Language:

Article 100.

Bonding Jumper, Main. The connection between the grounded circuit conductor and the equipment grounding conductor, or the supply-side bonding jumper, or both, at the service.