15- and 20-amp duplex receptacles not marked with "CO/ALR" are suitable for use with which conductors?

The correct answer indicates that 15- and 20-amp duplex receptacles not marked with "CO/ALR" are suitable for use with copper and copper-clad aluminum conductors only.

This is based on the fact that receptacles designed specifically for aluminum conductors must be marked "CO/ALR," meaning they are suitable for use with both copper and aluminum conductors, as well as specifically designed to handle the expansion and contraction characteristics of aluminum. When receptacles do not have this marking, it’s understood that they are not tested or rated for use with aluminum conductors and should only be used with copper conductors or copper-clad aluminum conductors, which have a thin layer of copper over aluminum, making them more amenable to use in traditional copper-rated devices.

Using copper conductors ensures the reliability and safety of the connections in these receptacles, as the electrical properties and thermal expansion characteristics of copper are different from those of aluminum. As a result, using receptacles that are not marked for aluminum conductors with aluminum would pose safety hazards, including potential overheating and failure of connections.