Receptacles rated 20 amps or less and directly connected to aluminum conductors must be marked with what designation?

Receptacles that are rated for 20 amps or less and are directly connected to aluminum conductors must be marked with the designation that indicates their compatibility with aluminum wiring. The "CO/ALR" marking signifies that the device is specifically listed for use with both copper and aluminum wire, which is crucial due to the differences in the thermal expansion and conductivity properties between these two types of metal.

Using a receptacle without the proper designation could lead to safety issues, primarily due to the risks associated with the connection points overheating if unsuitable materials are used together. The "CO/ALR" marking helps to ensure that electricians and consumers are aware of the appropriate combination for safe use, thereby reducing the potential for electrical failures or fires.

Options that do not provide this specific designation could lead those using the receptacles to make improper assumptions about compatibility, which may endanger both equipment and safety.