Knob and Tube Wiring


Knob and tube wiring is a two wire, non-grounded wiring system that is supported by knobs where it attaches to framing members and runs through tubes where it penetrates framing. It was the main form of residential wiring until the early fifties, when “romex” wiring was introduced into the market.

There are two main types of knob and tube wiring. Early knob and tube systems (installed until around 1915) used a non-petroleum based insulation that tends to crumble when touched. The later versions used a petroleum-based insulation that holds up better but still degrades quickly when exposed to heat or weather.


Knob and tube wiring presents many problems, but the primary issue is that it does not have a ground wire. Grounding is very important, not only for personal safety, but also for the safety and longevity of modern appliances, including computers and other home electronics. A non-grounded circuit is a safety hazard when any device requiring a ground (having a 3-pronged cord) is plugged in to it. The National Electric Code does not permit a separate ground wire to be added to a knob and tube system, so that leaves little option but to rewire.

Another problem associated with knob and tube systems is that most do not have junction boxes for light fixtures. Instead, the wires are fed through a 1×6 board and connected directly to the light fixture. This makes the otherwise simple task of replacing an old light fixture with a modern unit not only difficult but unsafe.

A third frequent issue with knob and tube wiring is improper modifications. When knob and tube was originally installed the technique used to splice wires was solder and tape. To the layman it looks like the wires are simply twisted together and then taped and thus many do-it-yourselfers have done just that when making modifications to their home’s wiring. These types of improper connections can lead to arcing and eventually to a fire. Today’s code does not permit splicing the wires with solder and tape; instead, any splice should be made with wire nuts and enclosed in a junction box. Some jurisdictions do not permit adding-on to knob and tube circuits at all.

Finally, another issue that has become prevalent with knob and tube wiring is obtaining home insurance. Most insurance companies are very aware of the many issues associated with knob and tube wiring and have adjusted their policies accordingly. Some insurers will not sell a policy at all if the home has knob and tube; others will, but at an increased rate.

Knob and tube rewires

To many people a knob and tube rewire seems like a daunting task. Some contractors tell stories of having to cut slots in all the walls in order to get the new wiring in. Some say you will have to bring the entire house up to code as it relates to the number of receptacles, etc.

At Bowie Electric Service & Supplies, Inc., we approach knob and tube rewires with a different attitude, one developed from years of experience doing these rewires. We fish the new wiring in wherever possible using the latest tools, such as flexible drill bits and interconnecting fish rods. If we do need to cut a hole to get a wire in we use a hole-saw and we cut all holes the same size (making the patching much easier and therefore less expensive). Where a receptacle is very difficult to get to without damage our technicians may suggest a new location that requires little or no damage.

We work in occupied homes often and are comfortable doing so. We use tarps for protecting furniture and floors and special covers for our saws that collect the majority of the dust created when a cut becomes necessary. We have done hundreds of knob and tube rewires and our crews are well-qualified craftsmen.

If you have any questions or concerns about the knob and tube wiring in your home, please contact us for more information.

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2 Responses to Knob and Tube Wiring

  1. Electricians says:

    Thanks for sharing this informative post.

  2. Ceramic tubes were inserted into holes bored in wall studs or floor joists, and the wires were directed through them. This kept the wires from coming into contact with the wood framing members and from being compressed by the wood as the house settled. Ceramic tubes were sometimes also used when wires crossed over each other, for protection in case the upper wire were to break and fall on the lower conductor.

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