What is knob and tube wiring?
Thursday, October 18, 2018
One answer to this question is: knob and tube wiring is an insurance no-no. Most insurance companies will not write a new homeowner’s policy for a house with knob and tube wiring.
It’s also sometimes called K&T, and part of the challenge we have when inspecting pre-1940 homes is determining if “active” K&T is present. Most homes of that era have had the wiring completely replaced, but sections of the abandoned, old knob and tube equipment are still in place in the attic and under the floor. Then again, some homes have had only part of the old wiring replaced, so some of the K&T is “live” and some is not. And, rarely, all the old knob and tube is still in use. The name comes from the white porcelain insulating knobs and tubes (shown above) used to mount and protect the single-insulated wires. K&T was the most cost-effective way to wire a home from about 1880 to the 1930s, then it began gradually being phased out through the 1940s and displaced by electrical cables that bundled hot and neutral, and eventually ground, wires in single flexible sleeve. We have not seen any homes built in the 1950s with K&T.
Here’s how K&T worked: wires were stretched between cylindrical knobs and wrapped around their grooved slot at turns, which kept them safely suspended and away from flammables. Porcelain tubes lined the holes drilled through wall studs and roof joists to let the wires pass through.
Part of the problem with knob and tubing wiring that is still functional in a home is simply it’s age. The insulation is at least 70-years old, brittle, and flaking off. Another problem is the low current-carrying capacity compared to modern wiring, and difficulty in safely splicing K&T with modern electrical cable. Also, the NEC does not allow it to be buried under insulation or covered by any other stored materials. We recently found an attic full of “live” knob and tube, most of it buried under insulation at a 115-year-old house in Alachua.
All knob and tube wiring systems are “two-wire,” meaning that they do not contain a third wire for grounding, which has been required for all residential electrical systems since about 1960. While K&T is now obsolete, and requires replacement when found during a home inspection, it’s worth noting that it was once state-of-the-art technology. We rarely see it anymore and, when we do, it’s in older downtown Gainesville neighborhoods like Duck Pond.
Also, see our blog post Why are old electrical components not always "grandfathered" as acceptable by home inspectors?
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