Final answer:
The smallest size copper wire that can be the sole connection to a grounding electrode wire should have a diameter of at least 16.35 mm.
Step-by-step explanation:
The smallest size copper wire that can be the sole connection to a grounding electrode wire is determined by the current-carrying capacity of the wire. Copper wires are commonly sold using the American Wire Gauge (AWG) system, where a smaller gauge number represents a larger wire diameter. For power transmission, a 2/0 copper wire is commonly used as the service entrance wire, which has a diameter of approximately 16.35 mm. The grounding electrode wire should have a similar or larger diameter to ensure it can handle the same currents. Therefore, the smallest size copper wire that can be used as the sole connection to a grounding electrode wire should have a diameter of at least 16.35 mm, which is equivalent to around 5 AWG.