Solid aluminum branch-circuit wiring from the 1960s and 1970s raises which specific concern?

Aluminum is a perfectly good conductor — utility service drops are aluminum — but solid aluminum used for small branch circuits behaves badly at terminations, and that is where the fires start. Four properties combine. Aluminum expands and contracts more than copper with temperature, so screw terminals work loose over cycles. It creeps, deforming slowly under the sustained pressure of a tightened screw, so a joint that was tight relaxes. Its oxide is electrically resistive, unlike copper's, so any exposed surface adds resistance. And that added resistance generates heat, which accelerates all three of the others. The result is a joint that deteriorates progressively at devices and splices, heating inside boxes where the evidence is concealed. The remedy addresses terminations specifically — copper pigtails with listed connectors, or replacement — which is why tightening is not a fix.

Aluminum's thermal expansion, creep under pressure and resistive oxide all act at screw terminations, so connections loosen and heat over time. The conductor itself is adequate; the failure is concentrated at the joints.

Solid aluminum branch wiring used in mid-century construction is associated with overheating at terminations due to expansion, creep and oxide formation. Approved remediation includes copper pigtails with connectors listed for the purpose, or replacement of the circuits.

The Wire Is Fine, the Ends Are Not. Expansion, creep and oxide all attack the screw terminal — which is why tightening only resets the clock.

Look for the visible signs at devices: discoloured terminals, scorched insulation and warm cover plates. Those confirm the process is already underway.

• •Treating aluminum as unable to carry residential loads
• •Recommending tightening as a durable remedy
• •Asserting that replacement is required before a sale