Knob and Tube Wiring

The oldest wiring in an old house is knob-and-tube, installed between 1890 and 1910; however, some sources report installations as late as the 1930s.

knob and tube

Knob and tube wire is two wires, insulated with rubberized cloth, run independently of each other along beams from the basement through the center of the house. Where they run through the joists, they are encased in ceramic tubes to prevent the wire from chafing on the wood. The wires that run over joists are looped around ceramic knobs nailed to the joists, which is the reason they are called knob and tube.


Some of the dangers associated with knob and tube wiring

  • No ground: Only a hot and neutral wire are provided. Watch out: A knob and tube electrical circuit has no electrical ground path.
  • Wire insulation: The individual electrical wires are wrapped in a rubberized cloth. That was fine when the wires remained suspended in air and had not been chewed by a squirrel or surrounded with insulation
  • Knob and tube wiring connections or splices were made outside of electrical junction boxes. In normal practice knob and tube wiring splices are soldered and also taped.

A good way to make knob and tube wiring even more dangerous is to hang a few hub caps from it

  • No electrical grounding conductor is provided. Grounding conductors reduce the chance of electrical fire and damage to sensitive equipment. 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.
  • The safety of the knob and tube circuit may have been affected by building changes such as adding insulation and modifications to the original circuit

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 tube where passing through wood. We pose that the same insulation project may have filled wall cavities with insulation too, possibly leading to overheating of knob and tube circuits that run in the building’s exterior walls. We’re also concerned that where knob and tube wires have been run in an attic floor and later covered by insulation, there’s a good chance that someone walking in the attic has stepped-on and damaged the wires – a condition we’ve found often. Where such wires were routed in walls or in attic floors, and where later those building cavities have been insulated, the knob and tube wires are no longer suspended in air, can become hotter than intended, and may be a fire hazard for that reason.

knob_tube insulation

Another great way to cause a fire is to pack the knob and tube with insulation

Advice about improving the safety and reliability of knob and tube electrical wiring

  • Inspect the whole electrical: Inspect the condition of the building electrical wiring, including the wires, connections, devices like receptacles, switches, and overcurrent protection by fuses or circuit breakers.
  • Replace bad circuits: Knob and tube circuits that have been modified, damaged, or covered with insulation should be replaced with a modern grounded electrical circuit.

Knob and tube wiring and insurance:

Many insurance companies refuse to insure houses that have knob-and-tube wiring due to the risk of fire. Exceptions are sometimes made for houses where an electrical contractor has deemed the system to be safe.

Advice for those with knob and tube wiring:

  • Have the system evaluated by a qualified electrician. Only an expert can confirm that the system was installed and modified correctly.
  • Do not run an excessive amount of appliances in the home, as this can cause a fire.
  • Where the wiring is brittle or cracked, it should be replaced. Proper maintenance is crucial.
  • Knob and tube wiring should not be used in kitchens, bathrooms, laundry rooms or outdoors. Wiring must be grounded in order to be used safely in these locations.
  • Rewiring a house can take weeks and cost thousands of dollars, but unsafe wiring can cause fires, complicate estate transactions, and make insurers skittish.
  • Homeowners should carefully consider their options before deciding whether to rewire their house.
  • Carefully remove any insulation that is found surrounding knob and tube wires.

In summary, knob-and-tube wiring is likely to be a safety hazard due to improper modifications and the addition of building insulation. People with knob and tube wiring need to be wary of this old system and consult a qualified electrician to replace this hazard


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