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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Here’s links to a collection of our other blog posts about ELECTRICAL WIRING:

Which house appliances need a dedicated electrical circuit?

Can a short circuit cause a high electric bill?

What is the maximum spacing requirement for securing NM-cable (nonmetallic-sheathed cable)?

Is it alright to just put wire nuts on the end of unused or abandoned NM-cable or wiring?

What causes copper wires to turn green or black in an electric panel?  

What are typical aluminum service entrance wire/cable sizes for the electrical service to a house?

Why is it unsafe to bond neutral and ground wiring at subpanels?

Should I get a lightning rod system to protect my house?

Why is a strain relief clamp necessary for the cord connection to some electric appliances?  

Does a wire nut connection need to be wrapped with electrical tape?

What is the minimum clearance of overhead electric service drop wires above a house roof?

What are the requirements for NM-cables entering an electric panel box? 

What is the color code for NM cable (Romex®) sheathing?

How can I find out the size of the electric service to a house?

Can old electrical wiring go bad inside a wall? 

What is an open electrical splice?

What are the most common electrical defects found in a home inspection? 

What is the life expectancy of electrical wiring in a house? 

What is an "open junction box"? 

How dangerous is old electrical wiring? 

What is a ground wire? 

I heard that aluminum wiring is bad. How do you check for aluminum wiring?  

What is the code requirement for receptacle outlets in a closet?

   Visit our ELECTRICAL page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

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