Final answer:
A No. 4 copper wire should be selected to replace a No. 6 aluminum wire due to copper's better conductivity, requiring a smaller gauge (larger diameter) for the same current-carrying capacity and to account for differences in thermal expansion and oxidation behavior.
Step-by-step explanation:
To determine which copper wire should be selected to replace a No. 6 aluminum wire, we must consider the current-carrying capacity and the electrical resistance characteristics of both materials. Copper has a higher conductivity than aluminum, meaning for wires of the same length and resistance, the copper wire would have a smaller diameter than its aluminum counterpart. However, since safety and compatibility with existing connectors and devices are critical, especially how temperature changes affect the thermal expansion of the metals and the potential oxidation issues with aluminum, technical guidelines have been established to facilitate the appropriate selection.
In general, when replacing aluminum wire for copper wire in electrical systems, a rule of thumb is to go one or two gauges smaller since copper is a better conductor. Considering the information, and without specific details on connectors and loads, a No. 4 copper wire (smaller gauge, hence larger diameter) would be a conservative choice, ensuring that it has a similar or better current-carrying capacity than a No. 6 aluminum wire.