What is the minimum size copper supply-side bonding jumper for a metal conduit containing 250 KCMIL aluminum service entrance conductors?

The correct answer is 4 AWG because the National Electrical Code (NEC) provides guidelines for determining the minimum size of supply-side bonding jumpers, which are essential for ensuring the safety and integrity of electrical systems. When dealing with service-entrance conductors, the code specifies bonding jumper sizes based on the size and type of conductors involved.

In this case, with 250 KCMIL aluminum service entrance conductors, the NEC mandates that the bonding jumper must be sized according to the aluminum conductors' capacity. For aluminum conductors, the bonding jumper can be sized smaller in comparison to copper conductors, but you still need to meet minimum sizing requirements to ensure effective grounding and bonding.

Referencing NEC Table 250.66, for 250 KCMIL aluminum conductors, the minimum bonding jumper size required is indeed 4 AWG copper. This size adequately fulfills the bonding and grounding requirements to provide a safe and effective pathway for fault currents. Properly sizing the bonding jumper helps to prevent dangerous voltage differences that can occur during fault conditions, ensuring safety for both the electrical system and individuals who may interact with it.