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What is knob and tube wiring?

by Lauterborn Electric on February 22nd, 2011
  • Knob and tube or KT wiring was the method of wiring used in homes built from the late 1800’s to the mid 1940’s

 

Do you have Knob and Tube Wiring?

  • If your home was built prior to 1950, you may have knob and tube wiring and not know it. Most homes with knob and tube wiring have been at least partly upgraded. It is not unusual to find all new wiring at the breaker panel and old knob and tube wiring for the lighting circuits on the top floor of the home.

Is it Safe?

  • Knob and tube wiring is not inherently unsafe. Installations have to be evaluated on a case by case basis. Safety usually depends on the history of modifications and upgrades

Knob and tube wiring problems

  • No electrical ground is provided: The circuit is less safe than a modern grounded electrical circuit and appliances and devices that use a grounded plug should not be connected on an un-grounded circuit.
  • The knob and tube wiring may have become damaged by age, exposure to leaks, or to chewing rodents. In attics, for example, we often see that this wiring has been damaged by having been stepped-on or by chewing rodents.

 

  • Over time, knob and tube circuits have been modified or extended by building owners/occupants which is an improper practice that is not permitted in most codes. In some jurisdictions, extending an existing knob and tube circuit is not recommended, and is even an illegal installation. Adding load to the knob and tube circuit will risk overheating of the wiring and possibly cause a hazard.
  • Adding building Insulation changes the knob and tube wire game: The fire safety of knob and tube wiring relied on the fact that the wires were generally routed through the air, suspended by knobs and protected by a heavy ceramic tubes where passing through wood. Insulation installed or blown in years later can compromise the safety of knob and tube wiring. It can lead to overheating of knob and tube wiring in attics and walls.

 

 

 

  • Brittle insulation: The Insulation on knob and tube wiring cannot handle high temperature as those found in modern ceiling mounted light fixtures. If the wiring is used in such application the insulation around the wire will become brittle and break off, leaving exposed wiring.
  • If you have knob and tube wiring in your home and you want a free evaluation from a licensed electrical contractor, contact us at LauterbornElectric.com
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