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Construction Concerns: Knob and Tube Wiring
Article and photos by Gregory Havel
In the early years of electrical service, the most common method used to install wiring inside buildings was known as "knob-and-tube" (photo 1), after the porcelain insulators (knobs) on which the wires were mounted, and after the porcelain tubes used to pass wires through wood joists and beams. This type of wiring could be exposed, as in basements, attics, and factories; or concealed inside the walls of homes, schools, and offices.
The two-piece knob and the tube were the most common insulators. The two-piece knob held the wire between the two pieces of porcelain (photo 2). This held the wire at least ½-inch away from the surface to which the insulator was attached with a nail or screw. Other types of knobs were equipped with clamps, to attach them to steel trusses and beams in factories. A porcelain tube (photo 2) supported the wire anywhere it passed through a wood joist, beam, or rafter. Another type of knob (cleat) held both wires of the circuit together and was held in place with two nails or screws (photo 3, below). Other styles and types of knobs for other purposes and higher voltages were also available.
The wires were single conductors, usually of copper; covered with rubber insulation and a cotton or muslin jacket; and originally intended for use in dry locations. A later development was a conductor for use in damp (not wet) locations. This was similar to the original type, except that the fabric jacket was saturated with paraffin. Wires could be spliced or tapped (the attachment of a branch line) anywhere in the run of wire. Another later development was the color-coding of each wire's fabric jacket (black, white, red, etc.), to make tracing wires easier.
According to the National Electrical Code (NEC) (1st Edition dated 1897, decades after electrical use and electrical incidents began), supports for wires could be no more than 4.5 feet apart, and wires up to 300 volts could be no closer than 2.5 inches. Any wire splice or tap was required to have the insulation stripped from the conductor; the conductors twisted together to make a mechanically secure connection; soldered; insulated with rubber tape; and covered with cloth "friction tape." Each wire passing into a metal box for a receptacle, fixture, or switch passed through the steel inside a porcelain insulating bushing. When the NEC was followed, knob and tube wiring was safe and reliable, unless remodeling was done without attention to detail (or mice chewed up the insulation too badly).
Since this method of wiring is labor-intensive and, therefore, expensive, it is no longer in common use today; it has been replaced with conduit systems and with metallic-sheathed and nonmetallic-sheathed cables, each containing one or more complete circuits. However, knob and tube wiring is still present in some older buildings-and this old system is often still energized and in use.
These photos were taken in August 2008 inside a 1895 building originally used to build wagons and, later, as an auto dealership, a plumbing and sheet-metal shop, a warehouse, a motorcycle dealership, and a video store. It is presently being converted into a candy store and kitchen.
