What is the minimum size copper grounding electrode conductor required to connect to a metal water piping system supplied by 500 KCMIL copper service entrance conductors?

The correct answer is 1/0 AWG, which is based on the National Electrical Code (NEC) requirements for grounding electrode conductors. The NEC specifies that the size of the grounding electrode conductor must be determined by the size of the service-entrance conductors supplying the system. For a service with conductors rated at 500 KCMIL, the code indicates that the grounding electrode conductor must be sized appropriately to ensure safety and adequate grounding.

According to NEC Table 250.66, for a copper grounding electrode conductor, when the largest ungrounded service-entrance conductor is 500 KCMIL, the minimum size for the grounding electrode conductor is indeed 1/0 AWG. This ensures that the grounding system can handle potential fault currents safely and effectively, creating a reliable path for electrical current in the case of faults.

The other options are not sufficient as they do not meet the NEC guideline for a service of that conductor size. Specifically, 2 AWG, 4 AWG, and 6 AWG are all smaller than the required minimum and would not provide an adequate level of protection or compliance with the applicable electrical codes for the system in question.