What type of conductor is NOT suitable for 15- and 20-amp duplex receptacles marked with "CO/ALR"?

The type of conductor that is not suitable for 15- and 20-amp duplex receptacles marked with "CO/ALR" is non-metallic insulated conductors. These receptacles, marked with "CO/ALR," are specifically designed to be compatible with aluminum wiring that has been treated to minimize the risks associated with the use of aluminum. This designation indicates that the receptacle can safely handle the characteristics of aluminum conductors, which tend to have different thermal and conductivity properties compared to copper.

Non-metallic insulated conductors, commonly used for lower-voltage wiring and lighting circuits, generally do not have the same properties as metal conductors and are not suitable for the load handling requirements in applications associated with electrical receptacles designed for direct contact with aluminum conductors. This incompatibility can lead to poor conductivity and increased risk of overheating or failure at the connection points.

Moreover, receptacles with "CO/ALR" markings are engineered to provide a secure and safe connection with certain types of metal conductors, making non-metallic options inappropriate for this specification.