JD’s Blog | The ABCs of Knob and Tube Wiring
Knob and tube wiring got its name from the ceramic knobs that were used for holding wires in their position and the ceramic tubes that were used to act as protective cases for the wires running through the floor and wall studs.
From the 1890s through late 1930s, knob and tube electrical wiring was the standard method for wiring in the United States. However, some electricians continued to apply this method into the 1970s. Knob & Tube method of wiring was outlawed by 1975 in all but some special applications & with permission. Knob and tube wiring only uses two wires; the grounding wire to discharge excess current found in modern wiring is missing. This difference explains why knob and tube wiring has two-pronged sockets and not three. Many electrical contractors opt to install ground fault circuit interrupters (GFCIs) so that the home can support a few three-pronged outlets. The GFCIs can sense imbalance in the hot and neutral wires and can trip on the slightest fluctuation. Even though they might not be grounded, GFCIs are assumed to be safe because of their ability to cut off excess current when needed.
Some History of Residential Wiring Practices in the U.S. By David A. Dini P.E.
Underwriters Laboratories Inc. – Copyright © 2006 Underwriters Laboratories Inc.
On December 31, 1879, Thomas Edison exhibited his newly invented electric lighting in a few houses along a residential neighborhood in Menlo Park, New Jersey. That New Year’s Eve night proved to be not only historical in terms of its significance to American ingenuity and invention, but it also signified the beginnings of residential electrification in the United States. Although originally available to only the wealthiest of families, by the turn of the century electricity in the home was becoming a reality for more and more people. This paper traces the history of some residential wiring practices from the early days of electricity into the 21st century. Wire and cable systems, over-current protection, raceways and boxes, wire splices and terminations, wiring devices, grounding, polarity and special protection devices are presented from the historical perspective of time, necessity, and technology. The influence of Code1 requirements and common trade practices are also presented.
Wire and Cable Systems:
In 1892 Thomas Edison was awarded a patent for what he called “electric conductor.” Edison described the object of his invention as effectively insulating wire so that it will be waterproof and fireproof. His electric conductor consisted of three parts (see Fig. 1); a) the conducting wire, b) a cotton braid separator over the wire, and c) an outer covering of rubber compound. Edison obviously knew that if his electric lamp was ever going to become a household item, the house itself had to be “wired” to accommodate the use these lamps, as well as countless other appliances that were soon to be envisioned. Edison also knew the dangers of electricity and fire, and in his patent he states, “… also fire-proof, so that if by accident the wire becomes red-hot the insulating-covering will not be set on fire and burned. The very earliest residential wiring systems were an open wiring system often referred to as “knob-and-tube.” The individual conductors were run spaced apart at least 2-1/2 inches (if exposed), but as the wires passed through walls and floors, they could be susceptible to dampness and abrasion, which could eventually lead to leakage currents and arcing fires. For protection in these places, “insulating tubes” were used. These tubes were made of porcelain, with a flange on one end and set on an angle to prevent the tube from sliding through the hole. To support the individual conductors in other places, a wide variety of insulators, including porcelain knobs and cleats were used (see Fig. 2). These were nailed to the wood structure, and would have a leather washer under the nail head to prevent the porcelain from being cracked when it was hammered in place. Although knobs had two grooves, they could not be used to support two wires of opposite polarity. However, cleats could be used when wires were run in parallel. In addition to keeping the wires spaced apart, these knobs and cleats also helped keep the conductors away from wood and other damp surfaces, as well as providing a degree of strain relief. Where free ends of wire attached to boxes, fixtures, and other devices, a special water resistant cotton braid tubing known as “loom” was used to cover the wire. Knob-and-tube wiring systems began being phased out in the 1930’s, probably because of the then growing popularity of non-metallic and armored cable systems for residential buildings.
Knob-and-tube wiring has not been permitted by the NEC for new installations since the mid-1970’s, however, is still described in the 2005 NEC in Art. 394 for existing installations and by special permission.
Safety Issues with Knob and Tube Wiring
Often, issues that arise with knob and tube wiring are due to old fittings wearing out or from some non-standard modification that was made to the electric system by either the previous owner or by an unprofessional electrician. This usually occurred when additional outlets were installed incorporating new Romex cable with old K&T wire. It’s like putting a square peg into a round hole.
Knob and tube wiring also has rubber insulation, which creates additional safety issues. As time passes, rubber degrades, exposing copper wires to moisture in the air, which greatly increases the chance of a short circuit or a fire. Edison invented the rubber insulation himself, so that’s probably how old it is!
Another issue with knob and tube wiring is it’s limitation with load demands. Very often, homeowners in need of extra outlets would hire electrical contractors to add said extra outlets to the existing K&T wires. These additions pose the risk of overloading the electrical systems, especially with today’s power-hungry appliances and devices.
Rewiring Your Home
If you are looking to replace knob and tube wiring in your home, hire a certified San Diego electrical contractor for the job. You should expect to pay anywhere from the range of $10,000 – $50,000 to rewire an average-sized home, depending on the scope of work. We have completely rewired some 2500-3000 sq. ft. homes in the price range of $50K. That price included everything electrical in the entire houses. We offer financing through GE Capital with zero to low interest if paid within 12 months.
As your trusted San Diego electrician, Point Loma Electric takes pride in providing quality services and superior results. Our team is experienced in dealing with all kinds of residential electrical problems, and we are experts at providing upgraded electrical wiring services. When you hire Point Loma Electric for your rewiring, you’ll be hiring fully licensed and highly insured electricians that provide quality work over & above what the NEC requires. Contact us today to schedule your rewiring service.