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Old House Wiring

The original wiring in many if not most of the homes in Riverwest is called Knob and Tube. When inside a wall or floor space, this type of wiring will have white porcelain knobs nailed to the wood where the individual wires change direction and small (1/2″ diameter) tubes that insulate and protect the wires as they pass through holes in the wood joists or studs. When the wires are in visible locations like in the basement you may see long black flexible tubes run in pairs. Knob and Tube wiring is still legal to be used with some exceptions. It may not be run in insulation. This often happens when outside walls or attics are insulated. Old wiring was installed in open spaces so that it would stay cool. The insulation around the wires was made of rubber that burns at a relatively low temperature. If surrounded by house insulation, the wires will not cool and could heat up enough to burn. It is important that if an old house is to be insulated that any knob and tube wiring be re-wired first. Another prohibition is splicing another type of wiring to Knob and Tube. It can be difficult to replace Knob and Tube wiring used for overhead lights. In my opinion Knob and Tube wiring is safe to use for lighting only circuits. I’ll use some simple math to show why. If you live in a three bedroom flat with kitchen, bathroom, and dining room with lights in each room you have probably no more that ten overhead lights. Even if you love bright lights and use all 100 watt light bulbs, that adds up to 1000 watts. Knob and Tube wiring should be fused at 15 amps. Fifteen amps times 120 volts equals 1800 watts. So even if you have all the lights on at the same time, you don’t even come close to overloading the circuit. Using Knob and Tube wiring for wall outlets is where hazards are more likely to occur. Everyday items in the bathroom can quickly overload a 15 amp circuit. A 1500 watt hair dryer and a 300 watt curling iron plus a light bulb exceeds 15 amps. Knob and Tube wiring is a two conductor system; this means that there is no ground wire. Many things we plug in have a three prong plug. The large round prong is the ground. Those little three to two prong adaptors bypass the ground safety circuit. This is a particularly bad idea if you are plugging in a computer. Computers dissipate harmful static electricity down the ground. A cost effective strategy in old homes is to abandon any of the electric outlets wired with Knob and Tube and have new grounded outlets installed with modern wiring and keep the current Knob and Tube overhead lighting wiring in service. Riverwest Currents – Volume 2 – Issue 5 – May 2003