Types of Old House Wiring

Knob and Tube

• Knob-and-tube wiring methods were used mainly before the 1930s. The wiring consisted of individual wires supported by porcelain insulators called knobs. The wires were suspended in air between the insulators, and the two conductors for each circuit were separated by 3 inches of space. Wherever the wires had to pass through the wooden house frame, holes were drilled for a porcelain tube, and the wires passed through the tube.

  Correctly wired knob-and-tube circuits had a number of features that made it very safe, but age, susceptibility to damage and rules for extending existing circuits now force replacement of knob-and-tube circuits with modern wiring, if changes are required.

Sheathed Cable

• A number of early forms of sheathed cable were introduced beginning in the early 20th century. Rubber-insulated wires were placed inside a flexible sheath, sometimes made of metal. Although touted as a replacement for knob-and-tube wiring, the cables were more expensive, and the same methods for connecting the wires were used. The wires were twisted together, soldered and then wrapped with nonconductive tape.

  Both knob-and-tube wiring and the early sheathed cables lacked a ground wire. Electrical codes would not require a ground wire until the 1960s, and then not in every case. The insulation on early sheathed cable became brittle with age and repeated heating and cooling.

Aluminum Wire

• In the mid 1960s, the price of copper rose suddenly. Seeking a less expensive conductor for wiring homes, builders began using sheathed cable with aluminum wire. Unfamiliar with the properties of aluminum wire, electricians used the same methods for making connections between wires and to terminals on switches, receptacles and other devices.

  Aluminum wire from the 1960s has two problems. It oxidizes easily, especially in the presence of brass or copper. It also expands and contracts more than copper, and this causes connected wires to loosen and terminal connections to loosen. The oxidation and loose connections may cause the wires to overheat, which in turn may cause a fire. Inspection of the wiring in homes from this era is recommended.

Wiring Methods

• Older homes may have circuits with unpolarized wiring. Although they turn a light on and off or provide power to a receptacle, power is always present at the terminals of the device even when it's switched off and can deliver a shock if touched. Outlets with two slots that are the same size identify an unpolarized outlet. Switches on neutral wires cause a problem similar to an unpolarized receptacle.

  Ungrounded wiring issues occur mainly in homes built before the 1970s. Electrical codes began requiring ground wires in the 1960s and were fully implemented by the 1970s. Ground wires route stray electrical current along a safe path away from humans.