What type of conductors are suitable for use with 15- and 20-ampere duplex receptacles not marked "CO/ALR"?

The correct choice, which specifies copper and copper-clad aluminum conductors only, aligns with the requirements for ensuring safe and efficient electrical connections. Duplex receptacles rated for 15 and 20 amperes that are not marked with "CO/ALR" indicate that they are specifically designed for use with certain types of conductors.

Copper conductors are generally preferred for these applications due to their excellent conductivity and reliability in electrical installations. Copper-clad aluminum is also acceptable as it combines the lightweight properties of aluminum with the conductivity benefits of copper, making it versatile for various wiring conditions. This means that using either of these conductor types ensures good electrical contact and minimizes the risk of overheating or failure at the receptacle connection point.

In contrast, aluminum and aluminum-clad copper conductors may pose compatibility issues with receptacles not marked as suitable for those materials. The specific technology and materials used in the receptacles could lead to problems with connection integrity, potentially resulting in overheating or fire hazards. Therefore, ensuring that conductors are either copper or copper-clad aluminum is essential for maintaining safety and compliance with electrical standards.