If a service consists of more than one enclosure, how is the main bonding jumper sized?

The main bonding jumper is sized according to the largest ungrounded service conductor when a service consists of more than one enclosure. This requirement ensures that the bonding jumper can adequately handle the maximum fault current that could flow through it during a short circuit event. By sizing the jumper to the largest ungrounded conductor, it minimizes the risk of overheating and ensures effective grounding throughout the entire service.

Choosing the largest ungrounded service conductor provides a reliable reference point to accommodate the maximum capacity of the system, maintaining proper safety standards and overall integrity of the grounding system across multiple enclosures. This practice aligns with the grounding and bonding principles outlined in the National Electrical Code (NEC), which aims to ensure safe operation and reduce the risk of hazardous conditions.

Sizing based on the smallest ungrounded conductor or following arbitrary size rules would not meet these safety standards, while suggesting that all enclosures share one bonding jumper does not address the varying sizes of conductors that may exist within the service.