When is it required to size a system bonding jumper for a separately derived system?

Sizing a system bonding jumper for a separately derived system is essential for ensuring effective grounding and safety. The bonding jumper connects the non-current-carrying metal parts to the grounding system, and its size is crucial in ensuring that it can safely carry any fault current that may occur.

The correct approach is to size the bonding jumper using the largest ungrounded phase conductor size. This method ensures that the bonding jumper is adequately rated to handle potential fault currents that could flow through the system. By basing the size of the bonding jumper on the phase conductor, it aligns the grounding system's capacity with the load and fault potential of the system, maintaining a safe electrical environment.

Choosing an inappropriate sizing criterion, such as basing it on the voltage rating of the system, the length of the conductor, or proximity to the service equipment, does not account for the actual fault conditions that could arise during operation. These other factors do not directly relate to the requirements for bond sizing concerning fault current management and electrical safety. Thus, the most effective and compliant choice is to size the system bonding jumper according to the largest ungrounded phase conductor.