Originally Posted By: cbuell
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Knob and Tube Not a Fire Hazard
In "Retrofits We'd Rather Forget" (Jan/Feb '96), you made reference to insulating over knob-and-tube wiring as being a fire hazard. This statement is incorrect.
Legislation was enacted in Washington state to allow insulating over knob-and-tube wiring per Bonneville Power Administration (BPA) specifications. This resulted because there were no documented cases of a fire being caused by knob-and-tube wiring, whether insulation covered it or not. Because the two conductors of knob-and-tube wiring circuits are spaced some distance apart, it is nearly impossible to short out. Even when covered with flammable materials such as wood shavings, the only way you could get a short was if the insulating materials were wet ... then they won't burn, so you couldn't start a fire anyway.
Overheating the wire would be the only method of ignition for knob-and-tube. Nonmetallic sheathed cable (NMC) such as Romex, on the other hand, can short as well as be overheated, increasing the potential for fire. NMC has started fires, and we insulate over it.
As an extra safety measure, it is a good idea to use Type S fuses or breakers sized properly for the wire size, the same as you would for NMC. An inspection by the Washington State Electrical Inspector and the installation of proper fusing (or breakers) is a requirement when insulating over knob and tube wiring in Washington and where allowed in other northwest states that follow BPA specifications.
Energy Conservation Specialist
Grant County PUD
Moses Lake, WA
Editor's reply: It's true that insulating over knob-and-tube wiring does not generally create a fire hazard and that the nonmetallic sheathed cable electricians currently use could cause fires. (Knob-and-tube was the most common form of home wiring until about the mid-1940s. See "Knob-and-Tube Wiring Hang-ups," HE May/June '91, p. 7.)
Weatherization program policies concerning insulation in attics with knob-and-tube wiring vary, depending partly on electrical codes. Since 1987, the National Electrical Code has prohibited insulating walls, ceilings, and attics that contain knob-and-tube wiring, but several states have adapted their codes to allow insulation after a wiring inspecton. In the situation discussed in the article, the wiring was not inspected before insulating with cellulose.
Knob-and-tube wiring can be a fire hazard if the original fuses have been replaced with oversized fuses to handle larger electrical loads (an all-too-common practice). Type S fuses prevent homeowners from installing the oversized fuses, as long as the Type S fuses are not oversized. It's also good to check for bad connections or "hot spots," as described in "Assessing the Integrity of Electrical Wiring" (HE Sept/Oct '95, p. 5). Once the wiring has been inspected and proper fuse sizing assured, it should be at least as safe after insulating as before. However, some weatherization programs require a sign to be posted to warn attic-goers of the danger of electrocution from concealed wires.
It is easier to change direction than it is to forget where one has been.