Knob + Tube

One of the more interesting aspects of the house we uncovered after the demolition was the knob and tube electrical wiring system.  Knob-and-tube wiring (sometimes abbreviated K&T) was an early standardized method of electrical wiring in buildings, that was commonly used in North America from about 1880 to the 1930s. Below are some images showing the components that made up the knob and tube system.

The individual conductors (wires) were run spaced apart at least 2-1/2 inches (if exposed), but as the wires passed through walls and floors, they could be susceptible to dampness and abrasion, which could eventually lead to leakage of current and potentially, arcing fires. For protection in these places, “insulating tubes” were used. These tubes were made of porcelain, with a flange on one end and set on an angle to prevent the tube from sliding through the hole. The below figure is an illustration showing the placement of tube through a stud and supporting a wire.

To support the individual conductors in other places, a wide variety of insulators, including porcelain knobs and cleats were used. These were nailed to the wood structure, and would have a leather washer under the nail head to prevent the porcelain from being cracked when it was hammered in place. 

Although knobs had two grooves, they could not be used to support two wires of opposite polarity. However, cleats could be used when wires were run in parallel. In addition to keeping the wires spaced apart, these knobs and cleats also helped keep the conductors away from wood and other damp surfaces, as well as providing a degree of strain relief. Where free ends of wire attached to boxes, fixtures, and other devices, a special water resistant cotton braid tubing known as “loom” was used to cover the wire. 

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). Compared to modern thermoplastic wiring insulation, the K&T wiring was less resistant to damage.

Knob-and-tube wiring systems began being phased out in the 1930’s, probably because of the then growing popularity of non-metallic and armored cable systems for residential buildings. Knob-and-tube wiring has not been permitted for new installations since the mid-1970’s.

Below are some pictures of the existing knob and tube wiling system in the house before it was removed to make way for a more updated up-to-code wiring system.

Durring the demolition we collected the porcelain tubes throughout the house.  Once we move into the renovated home we will figure out  way to highlight them as a small piece history from the house.