Knob-and-tube wiring (sometimes abbreviated K&T) is a single-insulated copper conductor run within wall or ceiling cavities, passing through joist and stud about ½”-holes via protective porcelain insulating tubes, and supported along their length on nailed-down porcelain knob insulators. 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.
Elements Of Knob and Tube
Ceramic knobs were cylindrical and generally nailed directly into the wall studs or floor joists
Ceramic tubes were inserted into holes bored in wall studs or floor joists, and the wires were directed through them.
Compared to modern electrical wiring standards, these are the main technical shortcomings of knob-and-tube wiring methods:
never included a safety grounding conductor
permitted the use of in-line splices in walls without a junction box
Over time, the price of electrician labor grew faster than the cost of materials. 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.
Modern home buyers often find that existing K&T systems lack the capacity for today's levels of power use. First-generation wiring systems became susceptible to abuse by homeowners who would replace blown fuses with fuses rated for higher current. This overfusing of the circuits subjects wiring to higher levels of current and risks heat damage or fire.
Knob-and-tube wiring may also be damaged by building renovations. Its cloth and rubber insulation can dry out and turn brittle. It may also be damaged by rodents and careless activities such as hanging objects from wiring running in accessible areas like basements.
As existing K&T wiring gets older, insurance companies may deny coverage due to a perception of increased risk.