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 together 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 eventually 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).
At present, new knob and tube installations are permitted in the US only in a few very specific situations listed in the National Electrical Code, such as certain industrial and agricultural environments. (Source: http://en.wikipedia.org/wiki/Knob_and_tube_wiring)
We have a quiz question for you about knob and tube wiring. See if you can answer it (answer provided below).
Where knob-and-tube conductors pass through wood cross members in plastered partitions, conductors shall be protected by noncombustible, nonabsorbent, insulating tubes extending not less than _____ beyond the wood member.
a) 2 in.
b) 3 in.
c) 4 in.
d) 6 in.
The correct answer is: b) 3 in.
Article 394 covers the use, installation, and construction specifications for concealed knob-and-tube wiring. As noted in 394.17, “Conductors shall comply with 398.17 where passing through holes in structural members. Where passing through wood cross members in plastered partitions, conductors shall be protected by noncombustible, nonabsorbent, insulating tubes extending not less than 3 in. beyond the wood member.”
Finish this article here: http://ecmweb.com/code-quiz-day/protection-knob-and-tube-wiring?page=1
When originally installed in the early 1900s, K&T wiring was less expensive than other wiring methods. For several decades, electricians could choose between using K&T wiring, compared to conduit, armored cable, and metal junction boxes. The conduit methods were known to be of better quality, but their cost was significantly higher than that of K&T. In 1909, flexible armored cable cost about twice as much as K&T, and conduit cost about three times the price of K&T. Knob and tube wiring persisted since it allowed owners to wire a building for electricity at lower cost.
Knob and tube wiring