Rule 27 of 40 · Chapter IV — Conductors and Loads
Match conductor and terminal metals
Why this rule exists
Copper and aluminum are not interchangeable at a connection. Aluminum expands more with heat, oxidizes into a resistive film, and creeps under pressure, so aluminum wire on a copper-only terminal loosens and heats over time, a documented cause of house fires. Terminals and connectors are listed for the metals they hold, marked CO/ALR or AL-CU when they accept aluminum. Mixing metals or using the wrong connector makes a high-resistance joint that behaves fine at first and fails months later. The wire's metal has to match the rating of what holds it.
In practice
Identify the conductor metal before terminating; aluminum branch wiring is common in certain eras and needs specific handling. Use devices and connectors listed for the metal, CO/ALR receptacles and switches for aluminum, or listed pigtailing connectors to transition aluminum to a copper tail at the device. Apply the specified antioxidant compound where required and torque to spec. Do not land aluminum under a copper-only screw. When you find aluminum branch wiring at receptacles and switches, treat it as a known hazard needing correct devices, not a quick swap.
When it doesn't apply
Aluminum is entirely appropriate and common for larger feeders and service conductors; this rule targets the small-gauge branch-circuit connections that fail. Listed repair methods, approved pigtail connectors, are the accepted fix for aluminum branch wiring. If a connection's metal rating is uncertain, stop and use a listed method or call an electrician experienced with aluminum.