Which inscription on a circuit breaker indicates it is designed for use with both copper and aluminum wire?

The inscription "CU/AL" on a circuit breaker signifies that it is engineered for compatibility with both copper and aluminum wire. This designation is essential for electricians and anyone working with electrical systems, as it informs users that the breaker can adequately handle the characteristics of both types of conductors without risk of overheating or failure at the connections.

Copper and aluminum have different thermal expansion rates and electrical conductivity properties, so breakers designed to accommodate both are built with materials and designs that ensure safe and efficient operation with either type of wire. This is particularly important in installations where contractors or electricians might be mixing wire types due to existing wiring or specific applications.

The other options imply limitations to either copper or aluminum, which would not be suitable for applications requiring the flexibility to use both conductor types.