For a 225-amp service using 4/0 AWG service entrance conductors, what is the smallest main bonding jumper allowed?

The smallest main bonding jumper size is determined by the size of the service entrance conductors and the rating of the service. In this case, for a 225-amp service using 4/0 AWG conductors, the National Electrical Code (NEC) specifies that the main bonding jumper must be sized appropriately to ensure effective grounding and bonding.

According to NEC Table 250.102(C)(1), the size of the main bonding jumper is based on the largest ungrounded service conductor. For a 225-amp service with 4/0 AWG conductors, the minimum size allowed for the main bonding jumper is 2 AWG copper or 1 AWG aluminum. This ensures that the bonding system can adequately carry the maximum fault current that could occur, providing a level of safety necessary for electrical systems in order to protect both life and property.

Therefore, the specified choice aligns with the code requirements based on the conductor sizes and service ratings in question, leading to a correct understanding of conductor sizing for bonding jumpers in electrical installations.