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Knob and tube wiring (sometimes
abbreviated K&T) was an early standardized method of electrical wiring in buildings, in common use in
North America from about 1880 to the 1930s. It consisted of single insulated copper conductors run within wall or ceiling
cavities, passing through joist and stud drill-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". The
first insulation was asphalt-saturated cotton cloth, then rubber became common. Wire splices in such installations were twisted
for good mechanical strength, then soldered and wrapped with rubber insulating tape and friction tape (asphalt saturated cloth),
or made inside metal junction boxes.Knob
and tube wiring was displaced from interior wiring systems because of the high cost of installation compared with use of power
cables, which combined both power conductors of a circuit in one run (and which later included grounding conductors).New knob and tube installation is permitted in the US only in a few very
specific situations listed in the National Electrical Code. The ceramic knobs were cylindrical and generally nailed directly into the
wall studs or floor joists. Most had a circular groove running around their circumference, although some were constructed
in two pieces with "pass-through" grooves on each side of the nail in the middle.By wrapping electrical wires around the knob, and securing them with tie
wires, the knob securely and permanently anchored the wire. The knobs separated the wire from potentially combustible framework,
facilitated changes in direction, and ensured that wires were not subject to excessive tension. Because the wires were suspended
in air, they could dissipate heat well.Ceramic
tubes were inserted into holes bored in wall studs or floor joists, and the wires were directed through them. This kept the
wires from coming into contact with the wood framing members and from being compressed by the wood as the house settled.Ceramic cleats, which were block-shaped pieces, served a purpose similar
to that of the knobs.Other ceramic pieces
would typically be used as a junction point between the wiring system proper and the more flexible cloth-clad wiring found
in light fixtures or other permanent, hard-wired devices. When a generic power outlet was desired, the wiring could run directly
into the junction box through an insulating sleeve called loom. AdvantagesWhen originally installed in the early 1900s, K&T wiring was less expensive than other wiring methods. For a
long time, electricians could choose between using K&T wiring on one hand and conduit, armored cable, and metal junction
boxes on the other. The conduit methods were known to be of better quality, but their cost was significantly higher than that
of K&T , about twice the cost for flexible armored cable and three times the cost for conduit in 1909 . Knob and tube
wiring persisted since it allowed owners to wire a building for electricity at low cost.Modern wiring methods assume two or more load-carrying conductors will lie against each other, as for instance in
standard NM-2 cable. Since the load-carrying wires are in close proximity, when they heat up the heating is shared across
the wires, limiting the overall current load they can support. Since the load-carrying wires in K&T wiring are widely
spaced, the wires are capable of carrying higher loads without risk of fire.K&T wiring was commonly insulated with cotton cloth and soft rubber, in addition to the porcelain standoffs.
Although the actual wire covering may have degraded over the decades, the porcelain standoffs have a nearly unlimited lifespan
and will keep any bare wires safely insulated. Porcelain standoffs are commonly used with bare wire electric fencing for livestock,
and such porcelain standoffs carry far higher voltage surges without risk of shorting to ground. DisadvantagesHistorically, wiring installation standards were less stringent in the age
of knob-and-tube wiring than today. Compared to modern electrical wiring standards, the main shortcomings of knob-and-tube
wiring are: knob-and-tube wiring never included a safety grounding conductor; did not confine switching to the hot conductor
(the so-called Carter System places loads across the common terminals of a three-way switch pair); and it
permitted the use of in-line splices in walls without a junction box (and thus exposing a potential fire hazard of an uncontained
spark caused by arcing following mechanical failure of the splice). Compared to modern thermoplastic wiring insulation, the
K&T wiring was less resistant to damage, but had a higher temperature rating.Knob and tube wiring can be made with great ampacity. However, most existing residential knob and tube installations,
dating to before 1940, lack the ampacity that is desired today because of the paucity of circuits and the fineness of the
wire gauge. Although these installations were adequate for the electrical loads at the time of installation, modern households
use a range and intensity of electrical equipment unforeseen at the time. Homebuyers often find that existing K&T systems
lack the ampacity needed for today's levels of power use. As household power use increased following the Second World
War (because more appliances per household were being plugged in), first-generation wiring systems became susceptible to abuse
by homeowners who would avoid repeatedly blowing fuses by overfusing the circuits, thus subjecting the wiring to heat damage
due to higher levels of current.Knob-and-tube
wiring may also have been damaged by building renovations. Its rubber insulation will be dried-out, thus brittle when handled,
or it may have been damaged by rodents or carelessness (for example, hanging objects off wiring running in accessible areas
like basements).Currently the United States
NEC forbids use of loose, blown-in, or expanding foam insulation over K&T wiring. This is because K&T is designed
to let heat dissipate to the surrounding air. As a result, energy efficiency upgrades that involve insulating previously uninsulated
walls usually also require replacement of the wiring in affected homes.As existing K&T wiring gets ever older, insurance companies may deny coverage due to increased risk. Several
companies will not write new homeowners policies at all unless all K&T wiring is replaced or an electrician has certified
that the wiring is in good condition.
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