Aluminum wire

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Aluminum wire is a type of wiring used in houses and power grids.

Contents

[edit] History

[edit] Usage within utilities

Since the early 1900s, utility companies have been using aluminum wire for transmission of electricity within their power grids. It has advantages over the older copper wire in that it is lighter, more flexible, and less expensive. Aluminum wire in power grid applications was very successful and is still used today.

The latest market to embrace aluminum is building wire due to the rapidly rising price of copper. Electrical contractors have switched from copper to aluminum alloy building wire using the new 8000 alloy as specified by the National Electrical Code (NEC). Contractors are using larger sizes of aluminum building wire for low voltage feeders where the savings over copper is significant due the higher 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 as copper. The aluminum conductors used for building wire are compacted in such a way that the overall diameter of the aluminum wire is approximately the same as copper. A popular choice is to use aluminum Metal Clad (Type MC) where the cable is preassembled in interlocked aluminum armor and does not require pulling the wire in a conduit. The cable can be installed in one step saving on labor in addition to savings over copper.

[edit] Increased copper prices

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 wire. 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.

In the late '60s, 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 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 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.

[edit] ACM, or NUAL, wire

In the early 1970s, a new aluminum alloy was listed by UL, known as AA-8000 series or "ACM" aluminum wire, which is the current aluminum wire used today. It is also sometimes referred to as "NUAL" in Canada and "STABILOY" in the U.S. 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, a home with aluminum wiring that was installed prior to 1972 has the older 1350 series alloy. This alloy was specifically designed for power transmission purposes. Due to its mechanical properties the 1350 alloys were not suitable for use in branch circuitry.

[edit] Problems with aluminum wires

Aluminum wires have been implicated in house fires in which people have been killed, although there are no confirmed cases.[1] Reports of fires with aluminum wiring generally show that poor workmanship led to the failure. Poorly made connections were often the cause. 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.

[edit] 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.

[edit] Coefficient of expansion

Aluminum's coefficient of expansion varies significantly from the metals common in devices, outlets, switches, and screws that were used before 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.

[edit] 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"). It should be noted that 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.

[edit] Hazard insurance

In some States, Hazard Home Insurance will not cover homes with aluminum wiring, and some insurance companies that claim to cover it, have a higher premium than homes with copper wiring. Check with your insurance company before purchasing a home with aluminum wiring.

[edit] Upgrading aluminum-wired homes

There are several "upgrades" that are commonly done to homes with pre-1974 aluminum branch circuit wiring:

  • Ensuring that all devices are rated for use with aluminum wire. Many are not, since they do not meet the CO/ALR specification.
  • "Pigtailing", which involves splicing a short length of copper to the original aluminum wire for use with devices not CO/ALR rated.
  • COPALUM, a sophisticated crimping system that creates a cold weld between 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.
  • Completely rewiring the house with copper instead.

When deciding to repair or replace any electrical installation, a qualified professional should be consulted. The majority of homes wired with the general purpose circuits wired with aluminum are now over 30 years old. The likelihood of experiencing any problems unique to having aluminum wiring is slight.

ANY electrical system should be evaluated every 10 years by a qualified professional to determine if it is likely to operate safely under the increased loads in different rooms being used differently, e.g., home office or bathrooms with larger hair dryers.

[edit] References

  • Hunter, Christel. (2006). Aluminum Building Wire Installation and Terminations. [2]. Accessed on August 10, 2006.