What is Knob and Tube?

knob and tube wiring ottawa electrical ring electric 1930 hom
Knob and Tube Wiring in a 1930 home

From here

If your current home wiring looks like this, call Ring Electric at (613) 299-8239 for a free quote on upgrading your electrical.  This out dated method of wiring may not only result in your insurance company not insuring your home, but it is a fire hazard.  See the list of the other numerous disadvantages below for more information.

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).


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 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, 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 capacity 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 a current rating, 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, thusbrittle 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).

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 (current carrying capacity) service (which, as noted above, often goes hand-in-hand with K&T wiring), unless the electrical service is upgraded.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s