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Knob and tube wiring is a style of electrical wiring that dates back to the late 19th century. Knob and tube (K&T) wiring was one of the first widely used methods of providing electricity to homes in the United States and other countries, and allowed homeowners to enjoy electric lighting in the home for the first time. While this wiring style can still be found in homes that were built before 1940, it has largely been replaced by more modern electrical systems.
In a knob and tube wiring system, single wires are run through the home inside walls or ceiling cavities. Each wire is protected by a ceramic or porcelain tube as it passes through beams and other building elements. To fasten the wires to the walls, electricians used a ceramic knob, which had an appearance similar to a roll of sewing thread. The wire was wrapped around the knob, and then the knob was nailed to the wall. Using this installation technique, installers could ensure the wire would stay separate from building components, which helped reduce the risk of electrical fires.
Many of the knobs used in knob and tube wiring had a groove running around the center to hold the wire in place. Others had wide edges that helped prevent the wire from slipping off the knob. The end of the knob could be placed flat against the wall, and a nail was hammered into the other end to secure the knob in place.
During the late 19th and early 20th centuries, knob and tube wiring was one of the only ways for the average family to bring electricity to the home. While more advanced wiring methods existed at the time, they were so expensive that they were impractical for most families. As long as the wires in this system were left uncovered, they could easily dissipate excess heat, and the risk of electrical fires remained low.
Today, K&T wiring is often considered a negative feature, and many believe it to be dangerous. While knob and tube wiring may have a bad reputation in some areas, this type of wiring is not inherently dangerous. If the wiring is still in good condition after years of use, it can often be kept in place and used to power modern homes.
Of course, modern wiring methods are often much safer and pose a much lower risk than older knob and tube wiring. This type of wiring is especially dangerous in homes with blown-in insulation, as contact between the wires and insulation could lead to fires. Homes with knob and tube wiring are often simply not up to the task of powering modern appliances, or handling the high electrical demands of the modern family. Many building codes require homeowners to update older wiring as part of a renovation or home improvement project. In addition, insurance companies may deny coverage to homeowners until they replace K&T wiring with safer, more modern electrical systems.