For a period spanning roughly the mid nineteen sixties to the mid seventies, aluminium was widely used for branch circuit wiring in residential construction. Copper prices were high, aluminium was cheap, and the switch seemed straightforward. It was not.

The problem is not that aluminium conducts poorly. It conducts well enough that it remains standard for service entrance conductors and large feeders today. The problem is what happens at the terminations.

Aluminium expands and contracts more than copper as it heats and cools. Over thousands of heating cycles, a screw terminal that was tight when installed works loose. A loose connection has resistance. Resistance produces heat. Heat produces more expansion. The cycle accelerates.

Aluminium also oxidises readily, and unlike copper oxide, aluminium oxide is a poor conductor. So the surface of the connection degrades at the same time the mechanical connection loosens.

The result is a connection that can run hot for years and eventually fail, usually at an outlet or switch box where the conductor meets a device terminal.

Identifying it is generally straightforward. The conductors are silver rather than copper coloured, and the cable jacket is often printed with a marking indicating aluminium. Looking at the panel is usually enough for a first check, though the panel may have been partly rewired.

What is not recommended is simply replacing devices and hoping. Standard outlets and switches are not rated for aluminium conductors, and installing them on aluminium is the same mistake that created the original problem.

The recognised remediation approaches involve either replacing the branch circuits entirely, or using approved connectors to pigtail a short copper conductor onto the aluminium so that the device terminal only ever sees copper. The connector method is far less disruptive and is generally accepted when done with the correct hardware at every connection point.

Insurance is a practical consideration. Some insurers ask about aluminium branch wiring and either decline or require remediation, so it can matter at the point of sale or renewal rather than only as a safety question.

The important distinction is branch wiring versus service conductors. Aluminium feeding the panel from the meter is ordinary and not a defect.

Further reading: more from citiesofthedead.net