If 15- and 20-ampere duplex receptacles are not marked "CO/ALR," they are suitable for use with what conductors?

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

This is because receptacles that do not specifically indicate compatibility with aluminum conductors (designated by the "CO/ALR" marking) are not designed or tested for safe long-term use with aluminum wiring. The "CO/ALR" marking indicates that the receptacle has been designed and tested to handle aluminum conductors, which have different thermal and conductivity properties than copper.

Copper and copper-clad aluminum conductors are compatible with these receptacles because they conform to the expected electrical and thermal performance requirements of the receptacle design. Copper conductors are widely used and have good conductivity, while copper-clad aluminum offers a balance of weight and conductivity, making it acceptable for use in this context.

Other options, particularly indicating standalone use of aluminum conductors or a mix of copper and aluminum without proper marking, are not acceptable as they do not meet safety standards and could lead to issues like overheating and failure at the contact points within the receptacle.