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).
Many houses constructed pre 1950’s have what is called knob and tube wiring. One can determine if you have this type of wiring in your home, by closely looking at basement joists or attic rafters. To determine if your home is wired ” knob and tube”, look for ceramic knobs or tubes in which the wire gets attached to, or passes through, joists or studs. If the knob and tube wiring is not easily visible, you can usually tell by looking at your
electrical outlets and switches.
You may only have two prong outlets to plug into. Basically, no ground at each outlet or
fixture outlet means knob and tube wiring is present, likewise if you have older pushbutton switches, this is also a good sign you may have knob and tube. Nowadays, Home owners with knob and tube wiring may find it difficult or impossible to obtain insurance on their home because most insurance companies are not likely to insure a house they perceive as high risk. Insurance companies usually require a certificate of inspection and compliance from a licensed electrician, that all knob and tube has been removed and replaced with modern 3 wire grounded circuits before it will insure a home that previously had knob and tube wiring. After the electrician rewires your home, they give you a satisfactory assessment of your home, and the insurance company will consider giving an insurance policy for your house.
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 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 over fusing 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).
Advice for those with K&T wiring:
· Have the system evaluated by a qualified electrician. Only an expert can confirm that the system was installed and modified correctly.
· Do not run an excessive amount of appliances in the home, as this can cause a fire.
· Where the wiring is brittle or cracked, it should be replaced. Proper maintenance is crucial.
· K&T wiring should not be used in kitchens, bathrooms, laundry rooms or outdoors. Wiring must be grounded in order to be used safely in these locations.
· Rewiring a house can take weeks and cost thousands of dollars, but unsafe wiring can cause fires, complicate estate transactions, and make insurers skittish.
· Homeowners should carefully consider their options before deciding whether to rewire their house.
· The homeowner or an electrician should carefully remove any insulation that is found surrounding K&T wires.
· Prospective home buyers should get an estimate of the cost of replacing K&T wiring. They can use this amount to negotiate a cheaper price for the house.
