National Electrical Installation Standards

Standards as High as Your Own

Wednesday, July 18, 2018


Re: CQD answer published Tuesday, July 10, 2018 - Concealed Knob & Tube Wiring

1) Charlie and Co., Thanks for all you do. Very professional and informative. Great resource for the electrical industry. Regarding the 7/10/2018 question, an understanding of knob and tube is helpful. After 50+ years of researching the Code and listening to a vast number of excellent electrical professionals, the understanding I have received is that the early Code experts knew the composite insulation on knob and tube wiring was inferior at best, and that it would not stand up to excessive moisture, heat or age. Hence the reason for separation from other conductors, objects and mounting surfaces. When the conductors were brought together for utilization or control equipment, the conductors needed a substantial extra layer of protective coating (the loom) to protect it from failure. This is also the reason there is minimal length of conductors in the box. The installers would install the loom at the box or attachment point, then they would attach the wires to the devices. They would then pull the wires back so the device was tight to the box and start installing knobs in the ceiling/wall cavities to keep proper clearances from mounting surfaces and other items such as plumbing pipes, heating ducts, etc. If the conductors were installed too close to adjacent surfaces or fished in, they would install the loom to reach from point to point for extra protection. As the wiring went through wooden portions of the structure the installers would use porcelain tubes to protect the wire and the wood. The splices were then soldered and taped while being concealed behind building finishes.

Adding insulation into these concealed spaces not only created detrimental heat for the composite insulation on the conductors, but also created overheating for the solder joints. Many electricians will be able to share experiences of removing insulation from an insulated knob and tube connection only to find a “carbonized ball” around the connection. Fortunately this “carbonized ball” cuts the oxygen out of the equation (for a while) helping to resist the surrounding wooden areas from catching fire. Over the years as energy costs escalated, people began insulating their buildings. We began to see the solder melting out of joints and resulting fires from overheated joints. 90.1 of the NEC begins the discussion with the requirement that electrical installations be safeguarded from hazards. Almost every jurisdiction I’ve been associated with and the Code experts in other jurisdictions have wording in their local Codes or Ordinances that have the same or similar language. The violation occurs when the knob and tube wiring becomes insulated. Upon discovery, it is up to the AHJ to provide safety to the occupants of the building by having the insulation removed and the knob and tube wiring checked for failures. Many insurance companies will not insure a building with knob and tube wiring.

Sorry for the long response, but I have explained this to many young, eager electricians who are looking to do the “right thing”. Hopefully our industry can continue to recruit these type of future Electrical Code Experts who are willing to pass down the information they are receiving. Thanks again Charlie and Co. for all you do for the industry. Bill Neitzel

2) Here in New Hampshire we have many really old buildings. Not sure if it is the realtors or insurance company's but usually when a building changes hands they insist on knob & tube being removed (and glass fuses). Thanks for making my mornings, Bob



Hey Bill and Bob, thanks for your comments with the additional information and the kind words. That is great information for our readers.

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CHARLIE TROUT: Charles M. Trout, better known as Charlie, was a nationally known NEC® expert and author. He served on several NEC® technical committees and is past chairman of CMP-12. In 2006 Charlie was awarded the prestigious Coggeshall Award for outstanding contributions to the electrical contracting industry, codes and standards development, and technical training. Even though Charlie passed away in October of 2015, his work continues in spirit. NECA continues to maintain this question forum for its many subscribers in memory and recognition of all his significant contributions to making the NEC what it is today.

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