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posted: 4/13/2013 5:00 AM

Old wiring a concern in century-old home

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By Dwight Barnett

Q. The house we bought in 2005 was built in 1900. We have done our best to update the electricity by removing knob and tube wiring in the basement and all that is accessible in the attic. Our electrician advised us not to worry about the knob and tube buried in the walls and ceiling.

I, concerned for the safety of a 5-year-old in the home, worry about it. Should we go ahead and make the necessary investment to bypass and replace the remaining knob and tube in the walls and ceiling? If so, is it feasible to do this sort of thing, one room at a time? Should we upgrade to a 200-amp service at the same time since the breaker goes off whenever we use the microwave, dishwasher and toaster at the same time?

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A. Knob and tube (K&T) wiring is easily identified by the porcelain knob attached to floor joists and walls and ceiling joists to support a single strand of wire and by the porcelain tubes inserted in holes drilled in the sides of floor or ceiling joists to allow passage of a single strand of wire. Studies have shown the current-carrying capacity of K&T wiring is dependent on there being unobstructed air cooling of the wires, and fires have occurred when the wires were buried in thermal insulation.

Other studies have shown that simply coiling a wire upon itself three times and covering it with a cloth created enough heat for ignition.

The wiring you removed was neither insulated nor coiled upon itself, but there could be coils inside the walls where the wiring comes together near outlets and switches. Another reason for removing the K&T wiring is that the exterior walls cannot be safely insulated without creating a thermal blanket that could cause the wiring to overheat.

The K&T wiring can remain inside the walls, but it must be disconnected at the panel. New wiring can be added working from the basement or the attic, and in older homes such as yours there are usually no obstructions inside the wall cavities such as insulation and ductwork to interfere with the installation.

The wiring should all be done at the same time to save on the overall costs of the installation, and I would agree that the panel should be upgraded to a minimum of 100 amps or 200 amps if the electrician determines the additional amperage is necessary. By adding more circuits to the kitchen, even with a 100-amp panel, you can stop the bothersome tripping of the breakers. The kitchen should have at least two separate 20-amp circuits for the countertop outlets and additional separate circuits for the refrigerator, small appliances and the microwave.

• Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home improvement questions at C. Dwight Barnett at d.Barnett@insightbb.com.

Scripps Howard News Service

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