Back in the mid 1950s, utility companies began using aluminum wire for transmission of electricity within their power grids. It had advantages over the typical copper wire in that it was lighter and more flexible. Also important, it was less expensive. Aluminum wire in power grid applications was very successful and is still used today.

In the mid 1960s, the price of copper spiked due to economic reasons and as a result, aluminum wire was manufactured in sizes small enough to be used in homes. One thing that was known at the time was that aluminum wire requires one wire gauge larger than copper to carry the same current. For example, a standard 15-amp 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. Typical connections from electrical wire to electrical devices, also called terminations, are usually made by wrapping the wire around screw terminals and tightening the wire. Over time, many of these terminations to aluminum wire began to fail. These connection failures generated heat under electrical load and the result was devastating in some cases. Some deaths due to fires were reported.

There are actually many reasons why these connections failed. The two core reasons are oxidation of the aluminum itself, and aluminum's coefficient of expansion.

All metals oxidize freely when exposed to open air. The only problem with aluminum is that its oxide is not an electrical conductor, but rather an electrical insulator. Consequently, the flow of electrons through the oxide layer can be greatly impeded. This is known as a high resistance connection and it will produce significant heat under electrical load. The oxidation of aluminum wire can be accelerated when it makes contact with a dissimilar metal. In many cases, the screw terminals initially used with aluminum wire were the same as that used with copper, and they were often made of brass, which is a dissimilar metal.

Aluminum's coefficient of expansion is also a problem. It is possible to form a reasonably gas tight connection at termination points simply by ensuring that it is torqued properly. However, aluminum works against this because it expands and contracts under load. As the wire expands under the screw terminal, it begins to deform slightly. Aluminum does not have any elastic memory, and when the wire contracts again, it will not return to the original shape that was once secure. Over time, this cycle results in the connection loosening slightly and allowing oxidation to occur on the wire between itself and the screw terminal.

In the late 60s, a device specification known as CU-AL (or AL-CU) was created that specified standards for devices intended for use with aluminum wire. Larger screw terminals designed to hold the wire more suitably were the general improvement here. Unfortunately, CU-AL switches and receptacles failed to work well enough with aluminum wire, and a new specification, called CO/ALR (meaning copper-aluminum revised) was created. These devices employ indium-plated screw terminals which are designed to act as a similar metal to aluminum, to expand at a similar rate, and to form a gas tight seal against oxidation. CO/ALR applies only to standard light switches and receptacles; CU-AL is still an acceptable rating for high voltage receptacles and circuit breakers.

Another issue with aluminum wire is the joining of aluminum wire to copper wire. As aluminum and copper are dissimilar metals, oxidation occurs and these connections can become unstable after a very short time. Special connectors have been designed for the purpose of joining aluminum to copper wire, such as the Marrette No. 63 and No. 65 and the Ideal Twister No. 65. These wire nuts use a special antioxidant paste to prevent oxidation of the connection. There is some debate over whether or not these products are effective.

In the early 1970s, a new aluminum alloy was created, known as 8000 series or "ACM" aluminum wire that is the current aluminum wire used today. It is also sometimes referred to as "NUAL." Many believe that this alloy of wire, when used with proper CO/ALR devices and aluminum rated twist-on connectors can be just as safe as copper wiring. It is, however, EXTREMELY rare in branch circuit wiring. Most likely, if you encounter a home with aluminum wiring, it is the older 1350 series alloy. This alloy was specifically designed for power transmission purposes and did not suit branch circuit use very well.

Larger aluminum wires (No. 8 gauge) and larger are generally not regarded as a problem in any installation. The larger sizes offer better options for terminations and due to their use primarily in high voltage applications, are most often worked on by professionals. Properly terminated large-gauge aluminum wiring should be regarded as safe. It is still sometimes used today in homes and condos to power large appliances such as electric ranges and dryers.

There are several "upgrades" that are commonly done to homes with aluminum branch circuit wiring. The simplest is to ensure that all devices are rated for use with aluminum wire. It is surprising how many are not--either by predating the CO/ALR specification, or by replacement by unaware persons. The next is called "pigtailing" and involves splicing a short length of copper to the original aluminum wire. The thought here is that this connection could potentially be more stable than the direct termination of the aluminum wire. There is a lot of discussion over whether or not this works. The third, is a proprietary system called COPALUM. COPALUM is a sophisticated crimping system that is regarded to be a permanent, maintenance-free repair. The last "upgrade" is to completely rewire using copper.

Always call electrical professionals when aluminum wire is found. The risk of fire with aluminum wire is said to be 55 times higher than that of copper.