What elements are electrical wiring made out of?

Almost all electrical wiring is made of copper. For larger sizes to keep the cost of the installation down the wire used is aluminum.

Sometimes the copper wire is tinned with solder, and sometimes there is some silver in it.

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For over 100 years utility companies have been using aluminum wire in their power grids. It has advantages over copper wire in that it is lighter, more flexible, and less expensive. Aluminium wire in power grid applications was very successful and is still used today.

Wiring in homes and buildings is another matter. In the '60s when the price of copper skyrocketed, aluminum wire was manufactured in sizes small enough to be used in homes. Aluminium wire requires a larger wire gauge than copper to carry the same current. For example, a standard 15 A branch circuit wired with No. 14 gauge copper requires No. 12 gauge aluminum.

When first used in branch circuit wiring, aluminum wire was not installed any differently than copper, and many of these connections failed due to bad connection techniques and dissimilar metals. These connection failures generated heat under electrical load and resulted in overheated connections.

Most metals oxidize when exposed to air. Aluminium oxide is an electrical insulator. The aluminum in a slightly loose conenction oxidizes and over time will fail.

In the late 1960s, the CU/AL specification was created that specified standards for devices intended for use with aluminum wire. Larger screw terminals were designed to hold the wire more suitably. Unfortunately, CU/AL switches and receptacles failed to work well with aluminum, and a new specification called CO/ALR (copper-aluminum, revised) was created. These devices employ screw terminals designed to act as a similar metal to aluminum and to expand at a similar rate. CO/ALR applies only to standard light switches and receptacles; CU/AL is the standard marking for circuit breakers and larger equipment.

Aluminium wires have been implicated in house fires in which people have been killed. There were several reasons why these connections failed. The main reasons were improper installation, the differences in coefficient of expansion between aluminum wire and the terminations used in the 1960s.

Aluminium's coefficient of expansion varies significantly from the metals common in devices, outlets, switches, and screws that were used before the mid-1970s. Since aluminum and steel both expand and contract at different rates under thermal load, loose connections began to grow progressively looser over time.