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Understanding Knob & Tube


Arshad Khan, Certified Home Inspector - Qualityspec Inc.
Knob & Tube wiring is a type of wiring which was in common use until the 1940's. More than 1.5 million homes across Canada were built with knob and tube wiring. It was an early form of electrical wiring that was used up to 1945. So, many homes still have it but is now considered too risky and dangerous to be insurable, and for your own safety, and home resale value it should be replaced with copper wiring. Knob & tube wiring is comprised of insulated wires clamped on two-piece porcelain knobs; positive and negative are carried separately. Hollow porcelain tubes carry the wires, through beams and other combustible materials. Modern electrical codes do not permit this type of wiring. It can present a hazard if tampered with, particularly at unenclosed, soldered-and-taped junctions. The unsheathed insulation is subject to deterioration, particularly where it is exposed, that is, not enclosed in a wall or ceiling. Worn insulation, of course, presents a shock hazard The ESA's (Electrical Safety Authority) recommendation to insurance companies on knob and tube wiring is that they ask for inspections to make sure the wiring is safe. It does not recommend denying someone insurance simply because they have some knob and tube wiring, although individual insurance firms can refuse coverage for homes with knob and tube wiring as they see fit.

(This is an electrical panel using Knob and Tube wiring as well as fuses. The incoming service wires are knob and tube connected directly to the utility service wires that continue directly to the Meter. All circuit wires to lights, plugs, and switches in this home were still using the original Knob and Tube Wires)

What You Can Do
The first thing to determine is the safety of your system. Even if your system is determined safe, consider upgrading at least parts of it. The areas of a knob and tube installation that are most vulnerable to mechanical damage and trouble are also those areas that are exposed and easiest to change. An electrician can advise you on various upgrade options but don't attempt an upgrade yourself.

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