This image (above) is a perfect example of the problems that are associated with Aluminum wiring. Note letters A and B (the insulation jackets are melted), which I will explain "Why" in the third paragraph and C (mixing copper wiring with aluminum wiring on the same breaker). Aluminum wiring can not be mixed with copper wiring and breakers can not be double tapped with two wires, unless designated by manufacturer and the breaker connnection will contain a special clip for double tapping. Aluminum wiring is very easy to identify due to it's obvious aluminum color. Aluminum wiring is not to be confused with tin coated copper which looks similar to aluminum wiring but aluminum wiring has distinguishing characteristics and is a solid conductor.
During the 1970's, aluminum (instead of copper) wiring became quite popular and was extensively used throughout the United States. Since that time, aluminum wiring has been implicated in a number of house fires, which caused jurisdictions to no longer permit aluminum wiring in new installations. I highly recommend that you do not use aluminum wiring for any type of new installation. But don't panic if your house does contain aluminum wiring. Aluminum wiring, when properly installed, can be just as safe as copper wiring. Aluminum wiring is, however, very unforgiving of improper installations. I will cover a bit of the theory behind potential electrical problems, and what you can do to make your wiring (in your home) safe.
The main problem that exists with aluminum wiring is a phenomenon known as "cold creep". When aluminum wiring warms up, it expands. When it cools down, it contracts. Unlike copper, when aluminum goes through a number of warm/cool cycles it loses a bit of it's tightness over time. To make the problem worse, aluminum oxidizes (or corrodes) when in contact with certain types of metal, so the resistance of the connection will go up. Which causes the aluminum wiring to heat up and corrode/oxidize even more. Eventually the wire may start to become very hot and melt the insulation jacket (shown in the picture above) or the fixture that it's attached to, and possibly even cause a fire.
Since people usually encounter aluminum wiring when they move into a house that was built in the 70's, I will cover the basic points of safe aluminum wiring. I suggest that, if you're considering purchasing a home with aluminum wiring or have discovered aluminum wiring after moving in, that you hire a licensed electrician to inspect the wiring for the following:
1) Fixtures (eg: outlets and switches) directly attached to aluminum wiring should be rated for it. The device will be stamped with "Al/Cu" or "CO/ALR". The latter supersedes the former, but both are completely safe. These fixtures are somewhat more expensive than the ordinary fixtures.
2) Wires should be properly connected (at least 3/4 way around the screw in a clockwise direction). All connections should be tight. While repeated tightening of the screws can make the problem worse, during the inspection it would pay off to snug up each connection.
{Note that stranded aluminum wiring is still often used for the main service entrance cable at your main panel. It should also be inspected.}
3) The "push-in" terminals are an extreme hazard with an aluminum wires. Any connections using the push-in terminals should be upgraded with the proper screw connections immediately.
4) There should be no signs of overheating: darkened connections, melted insulation, or "baked" fixtures. Any such damage should be repaired by a licensed Electrician and the connection should be upgraded.
5) Connections between aluminum and copper wire need to be handled specially. Current codes require that the connectors used must be specially marked for connecting aluminum to copper. The NEC requires that the wire be connected together using special crimp devices, with an anti-oxidant grease. The tools and materials for the latter are quite expensive - not practical to do it yourself unless you can rent the tool.
{Note that regulations are changing rapidly in this area. Suggest that you discuss any work with an Electrical inspector if you're going to do more than one or two connections.}
6) Any non-rated receptacles can be connected to aluminum wiring by means of a short copper "pigtail". See #5 above.
7) Shows reasonable workmanship: neat wiring, properly stripped (not nicked) wire etc.
If, when considering purchasing a home, my inspection of the exposed wiring (in your prospective home) shows no problems, you can consider the wiring safe. If there are signs of electrical problems in many places (which will be noted on your home inspection report), I suggest you consider a complete electrical inspection and possibly upgrading all branch wiring throughout the house. If the wrong receptacles are used, you can replace them with the proper type, or have the Electrician use pigtails. Having this professionally done by a licensed Electrician can run close to $10.00 per receptacle/switch plus hourly labor.
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