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disadvantage is that a detailed building wiring diagram is needed for other electricians to understand multiple interwoven circuits, especially if the wiring is not fully visible throughout its length. By contrast, modern electrical codes now require that all residential wiring connections be made only inside protective enclosures, such as junction boxes, and that all connections must remain accessible for inspection, troubleshooting, repair, or modification.
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In the early 1900s, K&T wiring was less expensive to install than other wiring methods. For several decades, electricians could choose between K&T wiring, conduit, armored cable, and metal junction boxes. The conduit methods were known to be of better quality, but cost significantly more than
677:
Knob-and-tube wiring can be made with high current carrying capacity. However, most existing residential knob-and-tube installations, dating to before 1940, have fewer branch circuits than is desired today. While these installations were adequate for the electrical loads at the time of installation,
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Modern electrical codes generally do not require a neutral fuse. Instead, they explicitly forbid configurations that might break continuity of the neutral conductor, unless all associated hot conductors are also simultaneously disconnected (for example, by using ganged or "tied" circuit breakers).
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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
617:
Over the K&T era multiple wire types evolved. Early wiring was insulated with cotton cloth and soft rubber, while later wiring was much more robust. Although the actual wire covering may have degraded over the decades, the porcelain standoffs have a nearly unlimited lifespan and will keep any
430:
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 tubes were sometimes also used when
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at all unless all K&T wiring is replaced, or an electrician certifies that the wiring is in good condition. Also, many institutional lenders are unwilling to finance a home with the relatively low-capacity service typical of K&T wiring, unless the electrical service is upgraded. Partial
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In many older K&T installations, the supply and return wires were routed separately from each other, rather than being located parallel to and near each other. This direct routing method has the advantage of reduced cost by allowing use of the shortest possible lengths of wire, but the major
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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. A leather washer often
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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. However, California, Washington,
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By wrapping electrical wires around the knob, and securing them with tie wires, the knob could be used to securely and permanently anchor the wire. The knobs separated the wire from potentially combustible framework, facilitated changes in direction, and ensured that wires were not subject to
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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
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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 or attics.
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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).
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Modern wiring methods assume that two or more load-carrying conductors will lie very near each other, as for instance in standard NM-2 cable. When installed correctly, the K&T wires are held away from the structural materials by ceramic insulators.
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Loom, a woven flexible insulating sleeve, was slipped over insulated wire to provide additional protection whenever a wire passed over or under another wire, when a wire entered a metal device enclosure, and in other situations prescribed by code.
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Wiring devices such as light switches, receptacle outlets, and lamp sockets were either surface-mounted, suspended, or flush-mounted within walls and ceilings. Only in the last case were metal boxes always used to enclose the wiring and device.
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Nebraska, and Oregon have modified the NEC to conditionally allow insulation around K&T. They did not find a single fire that was attributed to K&T, and permit insulation provided the home first passes inspection by an electrician.
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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.
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Historically, wiring installation requirements were less demanding in the age of knob-and-tube wiring than today. Compared to modern electrical wiring standards, these are the main technical shortcomings of knob-and-tube wiring methods:
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Ceramic cleats, which were block-shaped pieces, served a purpose similar to that of the knobs except that cleats were generally used in places where the wiring was surface mounted. Not all knob and tube installations utilized cleats.
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modern households use a range and intensity of electrical equipment unforeseen at the time. Household power use increased dramatically following World War II, due to the wide availability of new electrical appliances and devices.
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wiring layouts have now been banned, even for permissible new installations of K&T wiring. However, electricians must be aware of this older system, which is still present in many existing older electrical installations.
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bare wires safely insulated. Today, porcelain standoffs are still commonly used with bare-wire electric fencing for livestock, and such porcelain standoffs carry far higher voltage surges without risk of shorting to ground.
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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.
391:, 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
681:
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
593:(return) wire of an electrical circuit. The failure of a neutral fuse would cut off power flow through the affected circuit, but the hot conductor could still remain hot relative to
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permitted the use of in-line splices in walls without a junction box (however, this downside is offset by the strong nature of the soldered and taped junctions used at the time).
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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
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As existing K&T wiring gets older, insurance companies may deny coverage due to a perception of increased risk. Several companies will not write new
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with fuses rated for higher current. This overfusing of the circuits subjects wiring to higher levels of current and risks heat damage or fire.
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Another practice that was common (or even originally required) in some older K&T designs was the installation of separate fuses in
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Wiring running through tubes, and turning a corner supported by a knob. Notice the direct splice with more modern (1950s-era)
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wires crossed over each other, for protection in case the upper wire were to break and fall on the lower conductor.
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in buildings, in common use in North
America from about 1880 to the 1930s. It consisted of single-insulated
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Knob-and-tube wiring in a 1930 home. View looking upwards at upper wall stud bays and nearby ceiling joists.
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In retrofit situations electricians may place a higher value fuse on the neutral, so that fuse blows last.
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At present, new concealed knob and tube installations are permitted in the U.S. by special permission.
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Wiring houses for the electric light; together with special references to low voltage battery systems
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Ceramic bushings protected each wire entering a metal device box, when such an enclosure was used.
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upgrades, where low demand lighting circuits are left intact, may be acceptable to some insurers.
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964:"Electrical Knob & Tube Electrical Wiring Inspection, Evaluation, & Repair Suggestions"
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excessive tension. Because the wires were suspended in air, they could dissipate heat well.
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796:(2020 ed.). National Fire Protection Agency. 15 November 2019. p. 273.
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Ceramic junction for suspended light socket. Note deteriorated cloth insulation.
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Splice with more modern power outlet, a probable code violation as seen here
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Old
Electrical Wiring: Evaluating, Repairing, and Upgrading Dated Systems
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Written for professional electricians and advanced property owners.
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407:(asphalt saturated cloth), or made inside metal junction boxes.
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cushioned the ceramic, to reduce breakage during installation.
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did not confine switching to the hot conductor (the so-called
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Ceramic tubes protecting wires passing through ceiling joists
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through a tube of protective loom and a ceramic bushing.
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forbids the use of loose, blown-in, or expanding foam
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run within wall or ceiling cavities, passing through
821:. Newtown, Connecticut: Taunton Press. p. 136.
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susceptible to mechanical damage in accessible areas
915:"Knob and Tube Wiring & Home Owners' Insurance"
60:. Unsourced material may be challenged and removed.
1071:Insurance coverage issues affecting older homes
748:Croft, Terrell; Summers, Wilford, eds. (1987).
554:. This type of connection is forbidden by the
1129:Written for home owners and do-it-yourselfers.
990:"Ask the Home Inspector: Knob and Tube Wiring"
908:
906:
885:Nick Gromicko, Rob London and Kenton Shepard.
647:prohibited as of 1923 places electrical loads
144:Knob-and-tube wiring used in a textile factory
846:. Chicago: American School of Correspondence.
345:
8:
1098:(4th ed.). Newtown, CT: Taunton Press.
862:"Knob-and-Tube Wiring: What's the Big Deal?"
403:and wrapped with rubber insulating tape and
945:"Knob and Tube Wiring - an unbiased review"
771:
769:
558:, and a junction box should have been used.
1073:. Financial Services Commission of Ontario
1039:"Letters: Knob and Tube Not a Fire Hazard"
466:Knob supporting a wire change in direction
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147:
855:
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120:Learn how and when to remove this message
669:Knob-and-tube wiring at a museum display
664:
139:
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1137:(2nd ed.). New York: McGraw-Hill.
1045:. Energy Auditor & Retrofitter, Inc
1020:. Energy Auditor & Retrofitter, Inc
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211:
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776:Schneider, Norman Hugh (1916). "2-4".
752:(11 ed.). New York: McGraw Hill.
180:Regulation of electrical installations
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936:
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819:The complete guide to home inspection
371:) is an early standardized method of
7:
194:IEC 60364 IEC international standard
158:Wiring practice by region or country
58:adding citations to reliable sources
235:Mineral-insulated copper-clad cable
204:U.S. National Electrical Code (NEC)
490:Knobs supporting long runs of wire
199:Canadian Electrical Code (CE Code)
25:
844:Electric Wiring Instruction Paper
782:. New York: Spon and Chamberlain.
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502:Knobs serving multiple functions
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268:Switching and protection devices
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1014:"Knob-and-Tube Wiring Hang-ups"
45:needs additional citations for
866:Only Connect: Electrical FAQ's
817:Litchfield, Michael W (2015).
750:American Electricans' Handbook
572:Under the US electrical code,
1:
693:Currently, the United States
189:BS 7671 UK wiring regulations
260:Thermoplastic-sheathed cable
552:non-metallic–sheathed cable
387:drill-holes via protective
1198:
1133:Shapiro, David E. (2010).
1113:Shapiro, David E. (2001).
651:the common terminals of a
389:porcelain insulating tubes
220:AC power plugs and sockets
1117:. New York: McGraw-Hill.
842:Knox, Charles E. (1909).
245:Steel wire armoured cable
27:Type of electrical wiring
695:National Electrical Code
635:never included a safety
556:National Electrical Code
478:Knob supporting a splice
286:Electrical busbar system
152:Electrical installations
1094:Cauldwell, Rex (2009).
367:(sometimes abbreviated
306:Residual-current device
212:Cabling and accessories
171:United Kingdom practice
166:North American practice
1064:"Did you know that..."
887:"Knob-and-Tube Wiring"
794:National Electric Code
670:
564:Unusual wiring layouts
145:
137:
69:"Knob-and-tube wiring"
668:
589:the hot wire and the
143:
135:
1168:at Wikimedia Commons
1166:Knob and tube wiring
1043:Home Energy Magazine
1018:Home Energy Magazine
988:William Kibbel III.
919:Carson Dunlop Report
891:Inspecting the World
730:Western Union splice
365:Knob-and-tube wiring
54:improve this article
18:Knob and tube wiring
970:. InspectAPedia.com
707:homeowners policies
637:grounding conductor
397:mechanical strength
962:Friedman, Daniel.
671:
311:Distribution board
240:Multiway switching
230:Electrical conduit
146:
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1182:Electrical wiring
1164:Media related to
828:978-1-62710-480-7
377:copper conductors
373:electrical wiring
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321:Electrical switch
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43:This article
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1075:. Retrieved
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949:the original
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296:Disconnector
255:Ring circuit
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52:Please help
47:verification
44:
1077:2012-05-20
1049:2011-12-31
1024:2010-04-13
999:2010-04-13
974:2011-03-19
924:2011-03-19
896:2011-03-19
871:2011-03-19
736:References
699:insulation
605:Advantages
393:insulating
225:Cable tray
110:April 2019
80:newspapers
1176:Category
725:T-splice
714:See also
418:Elements
401:soldered
591:neutral
399:, then
369:K&T
94:scholar
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674:tape.
649:across
595:ground
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1067:(PDF)
684:fuses
655:pair)
381:joist
101:JSTOR
87:books
1139:ISBN
1119:ISBN
1100:ISBN
823:ISBN
798:ISBN
754:ISBN
587:both
385:stud
383:and
301:Fuse
281:ELCB
276:AFCI
73:news
56:by
1178::
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966:.
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1080:.
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1027:.
1002:.
977:.
927:.
899:.
874:.
831:.
806:.
762:.
353:e
346:t
339:v
123:)
117:(
112:)
108:(
98:·
91:·
84:·
77:·
50:.
20:)
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