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|The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (September 2010)|
Aluminum wire is a type of wiring used in houses, power grids, and airplanes. Aluminum provides a much better conductivity to weight ratio than copper, and therefore is used in power wiring of some aircraft. 
Utility companies have used aluminum wire for transmission of electricity within their power grids since the early 1900s. It has cost and weight advantages over copper wires. Aluminum wire in transmission and distribution applications is still the preferred material today.
In North American residential construction, aluminum wire was used to wire entire houses for a short time from the late 1960s to the late 1970s during a period of high copper prices. Wiring devices (outlets, switches, fans, etc.) at the time were not designed with the particular properties of aluminum wire in mind and there were problems with the properties of the wire itself. Older wiring devices not originally rated for aluminum wiring present a fire hazard. Revised manufacturing standards for wiring devices were required.
In the mid-1960s when the price of copper spiked, aluminum wire was manufactured in sizes small enough to be used in homes. Aluminum wire requires a larger wire gauge than copper to carry the same current.
When first used in branch circuit wiring, aluminum wire was not installed any differently from 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 having different resistances and different coefficients of thermal expansion. These connection failures generated heat under electrical load and resulted in overheated connections.
In the late 1960s, a device specification known as CU/AL was created that specified standards for devices intended for use with aluminum wire. Because of more rigorous testing, larger undercut screw terminals were designed to hold the wire more suitably. 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 screw terminals that have even deeper undercuts and are 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.
The first 8000 series electric conductor alloy, still widely used, was developed and patented in 1972 by Aluminum Company of America (ALCOA). This alloy, along with AA-8030 (patented by Olin in 1973) and AA-8176 (patented by Southwire in 1975 and 1980) perform mechanically like copper. Unlike the AA-1350 series, these 8000 series alloys retain their tensile strength after the standard current cycle test or the Current Cycle Submersion Test (CCST); both tests are described in ANSI C119.4:2004. Depending on the annealing grade, AA-8176 may elongate up to 30% with less springback effect and possesses a higher yield strength (19.8 KSI for a coldworked AA-8076 wire).
Building wire in US jurisdictions now uses the new 8000 alloy of aluminum as specified by the National Electrical Code (NEC). Contractors are also using a larger gauge aluminum building wire for low voltage feeders where the savings over copper is significant due the lower weight. Aluminum building wire will have half the weight of copper even though the aluminum conductor must have 50% greater area than copper to carry the same current. The aluminum conductors used for building wire may be compacted in such a way that the overall diameter of the aluminum wire is approximately the same as copper.
This alloy, when used with CO/ALR devices and aluminum-rated twist-on connectors, can be just as safe as copper wiring. However it is extremely rare in branch circuit wiring, and most twist-on connectors in typical branch-circuit sizes, even those designed to connect copper to aluminum wiring, are not rated for aluminum to aluminum connections (an exception is the Marette 63 and Marette 65). A home with aluminum wiring installed prior to 1972 probably has the older 1350 series alloy that was designed for power transmission. Due to their undesirable mechanical properties, most 1350 alloys were not suitable for branch wiring.
Aluminum wires have been implicated in house fires. There are several possible reasons why these connections failed. The two main reasons were improper installation and the differences in the coefficient of expansion between aluminum wire used in the 1960s and the terminations.
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.
Aluminum wire used before the mid-1970s has a coefficient of expansion that varies significantly from the metals common in devices, outlets, switches, and screws. Many terminations of aluminum wire installed in the 1960s and 1970s continue to operate with no problems. However, problems can develop in the future and some connections were not made properly when installed, including not wrapping wires around terminal screws and inadequate torque on the connection screws. There can also be problems with connections made with too much torque as it causes damage to the wire.
Aluminum and steel both expand and contract at different rates under thermal load, so a connection can become loose, and loose connections get progressively worse 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. The problems related to aluminum wire are typically associated with older pre-1970s solid wire smaller that No. 8 AWG, as the properties of that wire result in significantly more expansion and contraction than modern day AA-8000 series aluminum wire. Older solid aluminum wire also had problems with a property called "creep" which resulted in the wire permanently deforming or relaxing over time under load.
Although aluminum wire smaller than No. 8 AWG is typically not used in new house wiring, the use of larger stranded aluminum wires are fairly common in much of North America. The larger size stranded aluminum wires don't have the same historical problems as solid aluminum wires, and most common terminations for larger sizes are dual-rated lugs made of an aluminum alloy. Proper termination of larger stranded aluminum wiring can be regarded as safe, since long-term installations have proven its reliability. Larger aluminum wire is often used in residential applications for services and large branch circuit loads such as ranges and air-conditioning units.
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 twist-on 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. They are not recommended for permanent retrofit of entire homes.
COPALUM connectors are a special crimp type connection used to attaching aluminum and copper wires. However the connection requires special tools and training, and there can be limited space in existing enclosures for these connectors. Also as least one manufacturer, AlumiConn, offers UL/CSA listed lug type connectors for smaller solid aluminum branch circuit size wiring similar to those used for larger gauge aluminum-aluminum and aluminum-copper connections. These would appear to make a more reliable connection than wire nut-type connectors with aluminum wire due to its higher coefficient of expansion. These connectors may have the same problem with limited enclosure space as the COPALUM system (also described under "Upgrading aluminum-wired homes"). A listed connector should always be used for connecting aluminum to copper wire.
In some states of the United States, home hazard insurance will not cover homes with any aluminum wiring, and some insurance companies that claim to cover it charge a higher premium than for homes with copper wiring.
There are several upgrades or repairs available for homes with pre-1974 aluminum branch circuit wiring:
The Consumer Product Safety Commission (CPSC) recommends that for a permanent repair, the pigtailing be done with special crimp connectors called COPALUM, or a special miniature lug-type connectors called AlumiConn connectors. And any repairs should be done by qualified electricians familiar with aluminum wire problems and repair methods.
COPALUM connectors are a sophisticated crimping system that creates a cold weld between the copper and aluminum wire, and is regarded to be a permanent, maintenance-free repair. These connections are sometimes too large to be installed in existing enclosures. Surface enclosures or larger enclosures may be installed to remedy this problem. COPALUM connectors can be costly to install and must also be done using special tools by electricians certified in its use, and it can be difficult to find local certified electricians.
As of April 2011, the CPSC has also approved the AlumiConn miniature lug connector as an alternate to the COPALUM connectors for a permanent repair. The AlumiConn pigtail connectors are easier for local electricians to install as they only require the electrician to use a special torque screwdriver.
CPSC considers the use of pigtails with wire nuts a temporary repair, and there have been some problems reported. In some cases using wire nuts can increase the risks of a fire. However it is believed that the problems are primarily related to improper installation procedures or products. Ideal, the manufacturer of one brand of wire nut often used for wire nut pigtails, has stated that special procedures are required when using the wire nuts to pigtail older pre-1970s ("old technology") aluminum wire, such as pre-twisting and slightly abrading the wires, and the manufacturer of another brand has stated that they are not to be used for retrofitting older aluminum wiring, even though some websites recommend their use. Presently Ideal No. 65 wire nuts, which are a distinctive purple color, are the only wire nuts listed by UL for repairing aluminum wiring. However these are described by their manufacturer as not suitable for permanent retrofit of aluminum wiring in testimony to the CPSC leaving the COPALUM and AlumiConn systems as the only systems approved by the CPSC and unequivocally recommended by their manufacturers as safe for permanent long term retrofit of older aluminum wiring. Properly installed wire nuts by qualified electricians using approved products and special installation procedures can offer a viable option for a temporary repair to improve the safety of older aluminum wiring, but should not be considered a permanent correction of the problem.
Also some incorrectly believe that repairing older pre-1970s aluminum can be done by replacing electrical devices (switches, outlets, etc.) with ones that are rated for use with aluminum wire (CO/ALR rated devices). The problem is that these modern devices are tested and listed using modern AA-8000 series alumimum wire, which is very different than older pre-1970s aluminum wire which expands and contracts much more, and is more susceptible to creep. So the CPSC considers using CO/ALR devices for older aluminum wiring only an emergency repair.