Knob and tube wiring

Knob and tube wiring in a 1930 home.

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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.^{[1] [2]} 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.

Elements

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.

Knob supporting a corner	Knob supporting a splice	Knobs supporting long runs of wire	Knob serving multiple functions
Ceramic tubes	Ceramic junction for light socket	Wiring running through tubes, and turning a corner supported by a knob. Notice the direct splice with more modern (1950s-era) NonMetallic sheathed cable	Splice with more modern power outlet

Wiring devices such as light switches, receptacle outlets, and lamp sockets were surface-mounted or flush-mounted within walls, using metal boxes to enclose the device.

Advantages

When 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 ^[3]. Flexible armored cable cost about twice the cost of K&T. Conduit cost about three times the cost for K&T in 1909 ^[4]. 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 very near 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, making it a safe wiring method. It is actually, when used correctly, safer than most modern wiring methods, since the wires are held away from the structural materials by ceramic insulators.

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.

Disadvantages

Historically, 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).

Knob and tube wiring at a museum display

Knob and tube wiring can be made with high ampacity. However, most existing residential knob and tube installations, dating to before 1940, lack the capacity that is desired today because of the paucity of circuits. 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. Home buyers often find that existing K&T systems lack the ampacity needed for today's levels of power use. Household power use increased following World War II (because more appliances were produced, and in use at the same time). First-generation wiring systems became susceptible to abuse by homeowners who would avoid repeatedly blowing fuses by using fuses with too large an amperage, thereby 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 cloth and rubber insulation may be dried-out, thus brittle when handled, or it may have been damaged by rodents or carelessness (for example, by hanging objects from 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.^[5] 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 unless an electrician has certified that the wiring is in good condition. Also, many institutional lenders are unwilling to finance a home with limited ampacity service (which, as noted above, often goes hand-in-hand with K&T wiring), unless the electrical service is upgraded.

References

1. ^ Terrell Croft and Wilford Summers (ed), American Electricans' Handbook, Eleventh Edition, McGraw Hill, New York (1987) ISBN 0-07013932-6 paragraphs 9-32 through 9-55
2. ^ Schneider, Norman H., Wiring houses for the electric light; together with special references to low voltage battery systems, Spon and Chamberlain, New York, 1916,chapters 2 through 4 - available from the Intenet Archive
3. ^ Schneider 1916, page 84
4. ^ Charles E. Knox, Electric Wiring Instruction Paper, American School of Correspondence, Chicago 1909, no ISBN, available from the Internet Archive, pages 12-14
5. ^ William Kibbel III. "Ask the Home Inspector: Knob and Tube Wiring". Old House Web. http://www.oldhouseweb.com/stories/Detailed/14214.shtml. Retrieved 2007-03-24.