What is the required size main bonding jumper for a service entrance feeding a 400-A panelboard with copper conductors?

To determine the required size for the main bonding jumper for a service entrance feeding a 400-A panelboard, it is essential to refer to the National Electrical Code (NEC) guidelines. The NEC provides specific rules regarding bonding and grounding conductors, including those applicable to various service sizes.

For a service rated at 400 amperes, the NEC typically requires a copper bonding jumper that is sized based on the largest ungrounded conductor. Based on NEC Table 250.102(C)(1), the minimum size copper bonding jumper for a 400-A service is 2 AWG. This requirement ensures that the electrical system is adequately bonded, ensuring safe operation and maintaining system integrity, especially in fault conditions.

Therefore, 2 AWG copper is chosen because it meets the required size for safely handling the potential fault currents, minimizing the risk of excessive voltage on exposed conductive parts, which could pose a safety hazard. Selecting this size adheres to safety standards and provides adequate performance for the electrical system in question.