Knob and Tube Wiring

If your home was built in 1950 or earlier, take a look in the basement. If you notice wires running through porcelain cylinders or “tubes” inserted in holes in the wooden floor joists, you have knob and tube wiring. You’ll also see porcelain “knobs,” which keep the wires secure, and prevent them from touching the wood along which the wires run. The wires are usually insulated with a rubberized cloth fabric.

One of the main differences between modern wiring and the old knob and tube, is that there is no ground wire. Therefore, this type of wiring cannot accommodate any electrical items with three pronged plugs, and the risk of shocks and fire is much greater. Also, the black and the white wires run separately, while in more modern wiring, you will see that the black wire, the white wire, as well as the ground wire, are all enclosed in a single cable. Another difference is the wire insulation. Modern wiring is insulated with plastic, while knob-and-tube uses rubber. The breakdown of the insulation over time on knob-and-tube wiring is often the reason it is replaced. It’s important to note that this is frequently the result of overheating or mechanical abuse.

You may have found it difficult to obtain home insurance if your home has k&t wiring. In fact, this type of wiring is not inherently dangerous. Problems arise when the insulation around the wires begins to deteriorate with age, or when home handymen have made alterations to the wiring. Knob and tube wiring should never run through insulation, especially blown-in insulation, as this type of wiring requires open space to keep cool. Any insulation surrounding the wiring can cause serious problems. In fact, this wiring is sometimes referred to as “Open Wiring” which helps to emphasize the importance of maintaining open airspace around the wire at all times to prevent any overheating.

There is nothing in the building code in Canada which states k&t wiring must be removed from existing homes, but it is considered obsolete, and can’t be used in any new construction.