What is the largest size copper wire required when installing a GEC to the main water line from a 200-A main breaker panel?

In the context of grounding electrode conductors (GEC), the size of the copper wire required can be determined by reference to the National Electrical Code (NEC) requirements. For a 200-A main service, the NEC specifies the minimum size for the GEC based on the size of the service conductors.

When looking at a 200-A service, the code dictates that the GEC must be sized according to the largest service-entrance conductors. For copper conductors, the required minimum size is 3/0 AWG when connecting to a metallic water pipe or main water line. This ensures that the grounding system is adequate to handle potential fault currents and provides effective grounding for the electrical service.

Hence, the choice of 3/0 AWG as the correct answer aligns with these standards, ensuring proper protection and compliance with electrical safety regulations.