In a separately derived system with more than one enclosure, how is the system bonding jumper sized?

In a separately derived system with multiple enclosures, the sizing of the system bonding jumper is determined by the largest ungrounded feeder conductor that serves that enclosure. This approach ensures that the bonding jumper is adequate to handle potential fault current that might occur in the system.

The rationale behind this choice is that the bonding jumper must be capable of carrying the maximum fault current that could flow through it without overheating or causing damage. If the system bonding jumper is sized based on the largest ungrounded feeder conductor, it enhances the safety and effectiveness of the grounding and bonding system by ensuring that any electrical fault can be safely cleared. As such, a bonding jumper sized in accordance with the largest conductor allows the system to reliably perform its protective function, maintaining safety in the event of a short circuit or fault condition.

This method aligns with the requirements outlined in the National Electrical Code (NEC), which emphasizes proper bonding and grounding to protect equipment and personnel from electrical hazards.