The electric wiring most commonly installed in the houses that were built in the 60 year period between 1880 and 1940 is known as "knob-and-tube." The wires that ran parallel to floor joists were sandwiched between two ceramic discs (a "knob"); wires that were threaded through floor joists were sheathed in a ceramic tube (a "tube"). Although it required skilled labor to install, knob and tube was popular due to its relatively low cost compared to its alternative: conduit, armoured cable and junction boxes. Many of Toronto's older houses are still operating with knob and tube wiring.
When it was first introduced, knob and tube was a safe and economical way to wire Toronto houses. Over time, three problem areas came to light:
1) Deterioration of the flexible cloth insulation on the wire itself ("loom").
2) DIY homeowner repairs. Its accessibility tempted homeowners to do their own repairs rather than calling in an electrician.
3) An ever enlarging variety of power-hungry appliances this original wiring technology did not contemplate.
If your house has knob and tube, then your wiring is at least 70 years old. The soldering and splicing at the joins may have deteriorated with age. The loom on the wires may be worn leaving wires exposed. Damage may also have been caused by rodents or by renovation activity.
Because knob and tube doesn't end in a closed junction box, it is particularly susceptible to amateur repairs. Some homeowners "solved" the problem of overtaxed circuits that blow fuses by replacing 15 amp fuses with ones of 25 or 30 amps. This "fix" causes wires to overheat and insulating material to become brittle or burn. Another DIY fix is the patching of extra outlets into the existing system. Owners have been known to secure their handiwork with masking tape, scotch tape and Band-Aids!
The modern family's electrical needs far outweigh those of their 1940's predecessors. A knob and tube-wired house is not likely to have enough outlets to accommodate the wide variety of contemporary electrical appliances deemed essential by today's consumer. Knob and tube wiring also has no grounding wire and is considered unsafe in kitchens, bathrooms, laundry rooms, and the outdoors. The installation of ground fault detectors can ameliorate this situation.
Some newer stereo systems and computers are affected by electrical polarization, which knob and tube wiring cannot prevent. Finally, knob and tube is often placed between wall spaces, frustrating homeowner's efforts to "green" their houses by adding insulation.
Many insurance companies are reluctant to extend coverage to new owners of properties that contain knob and tube wiring. In order to maximize his choice of Insurers, a prudent Buyer will factor in the cost of re-wiring his purchase after closing when he is considering his options at the offer table. Knob and tube wiring is no reason not to buy a house that in other respects is a good fit, provided you budget for this and other remedial work that will be done after closing. If you think of such work as part of your closing costs, your ownership experience will be enhanced by the knowledge your wiring is safe and Code compliant!
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