Aluminum Wiring

In the mid 1960s when the price of copper spiked, 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 a larger wire gauge 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 terminals, are usually made by wrapping the wire around screw terminals and tightening the screw. Over time, many of these terminations to aluminum wire began to fail due to improper connection techniques and dissimilar metals. These connection failures generated heat under electrical load and resulted in overheated connections.

Problems with aluminum wires

Aluminum wires have been implicated in house fires in which people have been killed.[1][2] There were several possible reasons why these connections failed. The two core reasons were improper installation and the difference between the coefficient of expansion between aluminum wire and the terminations used in the 1960s.

Aluminum oxidation

Most metals (with a few exceptions, such as gold) oxidize freely when exposed to air. Aluminum oxide is not an electrical conductor but rather an electrical insulator. Consequently, the flow of electrons through the oxide layer can be greatly impeded. However, since the oxide layer is only a few nanometers thick, the added resistance is not noticeable under most conditions. When aluminum wire is terminated properly, the mechanical connection breaks the thin, brittle layer of oxide to form an excellent electrical connection. Unless this connection is loosened, there is no way for oxygen to penetrate the connection point to form further oxide.

Coefficient of expansion

Aluminum's coefficient of expansion varies significantly from the metals common in devices, outlets, switches, and screws that were used before the mid-1970s. Many terminations of aluminum wire installed in the 1960s and 1970s continue to operate with no problems. However, many connections were not made properly when installed. Since the aluminum and steel both expand and contract at different rates under thermal load, these loose connections began to grow progressively looser over time. Likewise, a connection made with too much torque causes damage to the wire. Over time, this cycle results in the connection loosening slightly, overheating, and allowing intermetallic steel/aluminum alloying to occur between the conductor and the screw terminal. This results in a high-resistance junction, leading to additional overheating. Although many believe that oxidation was the issue, studies have shown that oxidation was not significant in these cases.

Joining aluminum and copper wires

Another issue is the joining of aluminum wire to copper wire. As aluminum and copper are dissimilar metals, galvanic corrosion can occur in the presence of an electrolyte and these connections can become unstable over 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 twist-on wire connectors use a special antioxidant paste to prevent corrosion of the connection. Lug type connectors similar to those used for larger gauge aluminum-aluminum and aluminum-copper connections are now available for branch circuit size wiring. These would appear to make a more reliable connection on the aluminum wire with its higher coefficient of expansion than wire nut-type connectors. These may have the same problem with enclosure space as the COPALUM system (described under "Upgrading aluminum-wired homes")or the AlumiConn connectors. A listed connector should always be used for connecting aluminum to copper wire.

Although aluminum wire smaller than 8AWG is not used in new house wiring, lots of aluminum wires are used all over North America. The larger sizes offer excellent options for terminations, since the most common termination in larger sizes is a dual-rated lug made of an aluminum alloy. Properly terminated aluminum wiring should be regarded as safe, since long-term installations have proven its reliability. Aluminum wire is often used in residential applications for service entrance and large branch circuit loads such as ranges and air-conditioning units.

Hazard insurance

In some states, home hazard insurance will not cover homes with aluminum wiring, and some insurance companies that claim to cover it charge a higher premium than for homes with copper wiring. Reputable and knowledgeable insurers should recognize the difference between AA-8000 series aluminum building wire and that used prior to 1972.