It is generally agreed that the best quality metal to use in household wiring is copper, although it is possible to use aluminum as well. Aluminum wiring is commonly used to transmit electricity through power grids. This is because it is lighter and usually cheaper than copper. For some time, starting in the 1960s, aluminum wiring was used in homes as a replacement for copper wire in new construction. This led to some of the problems which are typical of aluminum wiring if it is incorrectly installed, such as overheating that can lead to fires.
The most significant problem with aluminum wiring is referred to as cold creep. As metal heats up, it expands, and then contracts again when it is cooled. This can be dangerous over time where electrical connections are made. If it goes through enough heating and cooling cycles, which are caused by everyday use, aluminum wiring gradually becomes loose at the connection point. To make matters worse, aluminum oxidizes over time, sometimes causing additional heat buildup at the connection, which causes more electrical resistance, which in turn produces more heat.
This is not to say that aluminum wiring is inherently unsafe. When installed properly, aluminum wiring can be as safe and effective as copper. However, it also tends to be unforgiving of poor or careless workmanship. Some house fires have been caused by faulty aluminum wire connections, including some where people were killed. In almost every case, poorly connected wiring bore the blame for the fire, which suggests that even if the wiring were done with copper, the fire may have happened eventually anyway.
Because of the potential danger that aluminum wire presents, it has been phased out as a household wiring material in favor of copper. Apart from overheating, aluminum also presents other, more minor issues. Oxidation is the main one, which happens when aluminum oxide accumulates on the surface of aluminum wiring. This is the equivalent of rust, but on aluminum rather than iron or steel, and it is an electrical insulator, meaning it does not conduct electricity. This problem is worse at connection sites, and it may be necessary to clean the aluminum oxide from these connections if repairs need to be made.
Oxidation has few practical implications other than the corrosion that can occur at connection sites because of it. Another important point about aluminum is that even under the best of circumstances, it does not conduct electricity as well as copper. This translates into wires with a larger diameter as well as a slightly increased risk for overheating even away from connections. For all these reasons, aluminum wiring is not used in homes anymore, and some homeowner's insurance companies even charge customers higher premiums if their houses are wired with aluminum, because of the higher perceived risk.