In a 120/240-volt, 1-phase, 3-wire service with a 125-A main breaker, what is the smallest main bonding jumper that can be installed?

To determine the smallest main bonding jumper that can be installed in a 120/240-volt, 1-phase, 3-wire service with a 125-A main breaker, it's important to refer to the National Electrical Code (NEC) guidelines.

The main bonding jumper is a critical component in grounding and bonding electrical systems to ensure safety and proper operation. The size of the bonding jumper is determined based on the size of the service entrance conductors and the rating of the overcurrent protection device (i.e., the main breaker).

For a service with a 125-A main breaker, the NEC provides specific tables that dictate the size of the bonding jumper required for different amperage ratings. According to NEC Table 250.102(C)(1), for a main bonding jumper connected to a system with a 125-A service, the appropriate sizing indicates that a bonding jumper size of 1/0 AWG copper is acceptable.

This size is determined based on the minimal requirements for effective grounding and bonding that can safely handle possible fault currents while ensuring sufficient conductivity for any ground fault path. The use of a 1/0 AWG bonding jumper meets these requirements, making it the smallest acceptable size for this application.

Thus, choosing 1/0