What is the maximum size GEC required from the service equipment to the rebar in the bottom of the grade beam?

The maximum size Grounding Electrode Conductor (GEC) required from the service equipment to the rebar in the bottom of the grade beam is typically determined by the material and the size of the grounding electrode system being used, as well as the applicable codes and standards.

In this case, a size of 4 is appropriate based on the grounding electrode requirements outlined in the National Electrical Code (NEC). The NEC specifies that for concrete-encased electrodes, such as rebar within a concrete foundation like a grade beam, the minimum GEC size needs to be referenced. Generally, a copper conductor size is indicated where it must be no smaller than #4 AWG for such applications.

Using a larger size, such as 2/0 or 1/0, might exceed the necessity for the grounding application and would not adhere to the requirement for minimum sizes outlined in the code for performance and safety. Therefore, selecting a size of 4 aligns with regulatory standards while providing adequate grounding effectiveness for the service equipment and the encased rebar.