A 120/208 volt three phase service is fed with a single size 500 kcmil copper conductor for each phase. A copper grounding electrode conductor from the main service panel to the cold water pipe line must be at least size ____.

A 120/208 volt three phase service is fed with a single size 500 kcmil copper conductor for each phase. A copper grounding electrode conductor from the main service panel to the cold water pipe line must be at least size ____.