Knob and Tube Wiring

November 20, 2013

Do you have knob and tube wiring?

If your home was built prior to 1950 then you may still have this type of wiring and not even know it. Most homes with knob and tube wiring have been at least partially upgraded to work with modern outlets and lighting fixtures. Sometimes homes will even have new wiring at the breaker panels and the original knob and tube wiring are used for lighting circuits on the second or top floor of the home.

What is knob and tube wiring?

It is the oldest residential wiring. Ceramic knobs support the wiring as it runs along its course, and then ceramic tubes are used to protect the wiring as it passes through floor joists and wall studs. This wiring was used until around 1950.

Is it Safe?

If the wiring has not been abused or tampered with then it should still be capable of carrying electricity throughout your home & should not be inherently unsafe. However, the safety really depends on the history and quality of the modifications and upgrades that have been done to the system. If you suspect you have knob and tube wiring then it is best to have a qualified electrician evaluate the system for safety.

Knob and tube wiring concerns and issues:

  • Improper Splices-  Splicing into this type of wire is not easy and requires a trained professional. A qualified electrician should be able to do a proper and safe splice, however, inspectors often find splices that were done incorrectly and are dangerous.
  • Not a grounded system– this means the system cannot be used to wire modern outlets. it was only designed and intended to be safely used with un-grounded applications
  • Brittle insulation-the insulation around the wiring was not designed to handle high-temperature environments such as those caused by modern ceiling-mounted light fixtures. If the wire is being used for inappropriate application
  • Don’t Cover with Insulation- Covering knob and tube wiring with thermal insulation is a major fire hazard. It is also a violation of the National Electrical Code to have knob and tube wiring in contact with thermal insulation. The wire suspended by the porcelain knobs cannot cool if it is covered with insulation. It’s also worth noting that switches on the knob and tube wiring were usually on the neutral wire instead of the hot wire. Doing that only shuts off the circuit and not the current, which can be a fire hazard too in the presence of thermal insulation
  • Limited Amperage- One of the downsides of knob and tube wiring is the limited amperage. Back when knob and tube wiring was popular, people did not use as much electricity as we do today. Knob and tube wiring can handle, at most, around 60 amps, whereas today the service panels of most houses today crank out at least 150 amps.Will insurance carriers decline to write homes with knob & tube wiring?

Yes, most home insurance carriers will not write a new policy for a home with knob and tube wiring. If you are buying a home with any knob and tube wiring then you will probably either have to replace all the wiring to qualify for home insurance, or you can ask that the seller upgrades prior to agreeing to buy the home. The home seller may have a hard time understanding why this is needed because they most likely are currently insuring the home. They may have insurance because they may have been with the same carrier for years and were not ever questioned on the original application about the wiring. Whereas, the insurance carriers over the years have become more and more reluctant to write home with this type of wiring due to the losses they have experienced from homes that have had this type of wiring improperly modified.

Cost of updating the wiring

If you only have a few circuits to replace then it should not be very expensive. However, if your home has knob and tube throughout then bringing the home up to modern standards and meeting the current building codes will be more expensive. Part of the reason for the greater cost is that more electrical outlets will likely need to be installed and the breaker panel may need to be upgraded to accommodate more circuits being added.