Can I connect aluminum and copper wires together?

Yes, but only with connectors listed for the purpose. NEC 2023 110.14 requires terminals and splicing connectors to be identified for the conductor material, and it prohibits intermixing aluminum and copper in the same terminal unless the device is listed for that exact condition. A standard wirenut twisted over a copper and an aluminum conductor together is a violation and a genuine fire hazard.

Why the direct splice fails

Aluminum and copper are dissimilar metals. In contact, with a little moisture, they set up galvanic corrosion; aluminum also oxidizes instantly and its oxide layer is an insulator, and aluminum creeps under pressure over heat cycles. The joint loosens, resistance climbs, and the connection cooks. This is the story behind most 1960s and 70s aluminum branch circuit failures.

Connectors that are listed for it

Use a connector specifically listed for aluminum to copper transitions: insulated multi port mechanical lug connectors rated AL9CU (the common purple or black insulated lug blocks), listed AL/CU rated splice devices, or compression sleeves listed for the combination and installed with the right tool and die. The one commonly available twist on connector listed for aluminum to copper is rated for a limited combination set; follow its instructions exactly, including the pre twist and abrasion steps. For panel terminations, the breaker or lug must be marked AL/CU, and most modern ones are.

Workmanship details that matter

Abrade the aluminum conductor lightly to break the oxide, apply antioxidant compound where the connector instructions call for it, and torque to the listed value with a torque tool per 110.14(D). Never re terminate old work hardened aluminum without inspecting for damage. On old aluminum branch circuits, a listed copper pigtail transition at each device, using a connector listed for the purpose, is the recognized repair short of rewiring.