A 175-A service with 2/0 AWG service conductors; the minimum MBJ size is ___ AWG.

• A. 1 AWG
• B. 4 AWG
• C. 2 AWG
• D. 3 AWG

Answer: (B)

The main concept is how the main bonding jumper (MBJ) is sized. The MBJ connects the neutral to the grounding electrode system at the service equipment and must be sized to carry fault current and to meet the equipment grounding conductors required for the service rating. NEC 250.122 and related rules specify the minimum size of bonding jumpers based on the overcurrent protection rating of the service.

For a service rated at 175 A, the copper bonding jumper must be at least 4 AWG. The fact that the service conductors are 2/0 AWG doesn't force the MBJ to be the same size; the jumper only needs to be large enough to satisfy the bonding/grounding requirements for that 175 A rating. Therefore, the minimum MBJ size is 4 AWG.