A service panel MDP with a 300-A main breaker and conduit containing 350 kcmil conductors. The minimum size main bonding jumper is ___ AWG or kcmil.

• A. 3
• B. 2
• C. 1
• D. 1/0

Answer: (B)

Bonding jumper sizing follows the service rating, not the conductor size in the raceway. The main bonding jumper connects the neutral to the grounding path at the service disconnect, and its size is set by the rating of the service equipment. For a service with a 300 A main breaker, the minimum bonding jumper size is copper #2 (or aluminum 1/0). The presence of 350 kcmil conductors doesn’t change that requirement—the MBJ is determined by the 300 A rating. If the conductors were aluminum, you’d use a larger size (1/0 kcmil).