Cables

Armored cables with two rubber-insulated conductors in a flexible metal sheath were used as early as 1906, and were considered at the time a better method than open knob-and-tube wiring, although much more expensive.

The first polymer-insulated cables for building wiring were introduced in 1922. These were two or more solid copper wires, with rubber insulation, woven cotton cloth over each conductor for protection of the insulation, with an overall woven jacket, usually impregnated with tar as a protection from moisture. Waxed paper was used as a filler and separator.

Rubber-insulated cables become brittle over time because of exposure to oxygen, so they must be handled with care, and should be replaced during renovations. When switches, outlets or light fixtures are replaced, the mere act of tightening connections may cause insulation to flake off the conductors. Rubber was hard to separate from bare copper, so copper was tinned, causing slightly more resistance.

About 1950, PVC insulation and jackets were introduced, especially for residential wiring. About the same time, single conductors with a thinner PVC insulation and a thin nylon jacket became common.

Aluminium wire was common in North American residential wiring from the late 1960s to mid 1970s, because of the rising cost of copper. Because of its greater resistivity, aluminium wiring requires larger conductors than with copper. For instance, instead of 14 AWG (American wire gauge) for most lighting circuits, aluminium wiring would typically be 12 AWG on a typical 15 amp circuit, though local building codes may vary.

Aluminium conductors were originally used with wiring devices intended for copper wires. This can cause defective connections unless all devices (breakers, switches, receptacles, splice connectors (i.e., wire nuts), etc.) were designed to address problems with junctions between dissimilar metals, oxidization on metal surfaces and mechanical effects that occur as different metals expand at different rates with increases in temperature. Because of improper design and installation, some junctions to wiring devices overheated under heavy current load and caused fires. Revised standards for wiring devices (such as the CO/ALR "copper-aluminum-revised" designation) were developed to reduce these problems.

Aluminium conductors are still used for power distribution and large feeder circuits because they cost less than copper wiring, especially in the large sizes needed for heavy current loads. Aluminum conductors must be installed with compatible connectors.

The simplest form of cable has two insulated conductors twisted together to form a unit; such unjacketed cables with two or three conductors are used for low-voltage signal and control applications such as doorbell wiring. In North American practice, an overhead cable from a transformer on a power pole to a residential electrical service consists of three twisted (triplexed) wires, often with one being a bare neutral and the other two being insulated for the line voltage.