Knob-and-tube wiring

Where conductors entered a wiring device such as a lamp or switch, or were pulled into a wall, they were protected by flexible cloth insulating sleeving called loom.

Ceramic tubes were inserted into holes bored in wall studs or floor joists, and the wires were directed through them.

When a generic power outlet was desired, the wiring could run directly into the junction box through a tube of protective loom and a ceramic bushing.

Wiring devices such as light switches, receptacle outlets, and lamp sockets were either surface-mounted, suspended, or flush-mounted within walls and ceilings.

In summary, K&T wiring that was installed correctly, and not damaged or incorrectly modified since then, is fairly safe when used within the original current-carrying limits, typically about ten amperes per circuit.

This removed the price advantage of K&T methods, especially since they required time-consuming skillful soldering of in-line splices and junctions, and careful hand-wrapping of connections in layers of insulating tape.

Household power use increased dramatically following World War II, due to the wide availability of new electrical appliances and devices.

Modern home buyers often find that existing K&T systems lack the capacity for today's levels of power use.

This overfusing of the circuits subjects wiring to higher levels of current and risks heat damage or fire.

[10] It may also be damaged by rodents and careless activities such as hanging objects from wiring running in accessible areas like basements or attics.

Currently, the United States National Electrical Code forbids the use of loose, blown-in, or expanding foam insulation over K&T wiring.

As a result, energy efficiency upgrades that involve insulating previously uninsulated walls usually also require replacement of the wiring in affected homes.

[13][14] As existing K&T wiring gets older, insurance companies may deny coverage due to a perception of increased risk.