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Why is Knob and Tube Wiring Dangerous?

Commonly used in homes across the country from as far back as the 1880s through to the 1940s, knob-and-tube (K&T) wiring is old news. So old in fact, many don’t even consider it may be lurking within the walls of their older home. Unfortunately, because wiring is tucked away in the basement, attic, or spider webbed within the walls of a home, the dangers of outdated knob-and-tube are often overlooked - despite the fact it still exists, hidden, in many of America’s historic homes.

What Does K&T Look Like?

This obsolete form of wiring is easily identifiable – if you can see it. Knob-and-tube is identifiable by its white, ceramic, spool-like knobs – hence the name: Knob-and-tube. Spools are typically nailed to joists, suspending and protecting the wires within from outside contact. Where they do enter a wiring device (switch, outlet, light), they (should be) protected by a flexible insulation called ‘loom.’

What Makes It Dangerous?

  • It’s Age
    Though that old K&T may have been ‘working’ in your home for a while, today’s homes require much more electricity than 1940s homes, before refrigeration, TVs, and smart stuff became standard fare. Thus knob-and-tube is frequently overloaded, posing a fire hazard. Loom can also breakdown, exposing wires over time, and in some cases, corroding copper wire due to insulation additives. Less resistant to damage than modern wiring, as components breakdown, stretch and sag over time, K&T can also suffer unintentional contact with surrounding materials, resulting in serious fire and electrical hazards.
  • It’s Lack of Ground Wire
    Incompatible with 3-prong appliances, K&T lacks grounding, providing only a hot and neutral wire. This puts electronics at increased risk of damage as voltage fluctuations and surges have nowhere to go, putting your valuables, family and home at risk of shock and fire.
  • It’s Incompatibility with Moisture
    Already ungrounded, knob-and-tube is not rated for moisture and exceedingly dangerous when used in wet locals such as kitchens, bathrooms, laundry and utility rooms, and outdoors. 
  • It’s Tendency to Be Overlooked
    Knob-and-tube is frequently buried with insulation and pushed into contact with building materials and storage clutter. Designed to dissipated heat freely into the air, this results in a fire hazard. National Electric Code (NEC) requires K&T not be covered by insulation, or used in the hollow spaces of walls, ceilings, or attics where insulating materials can come into contact with the wiring and conductors. Just one of the many reasons most insurance companies refuse to cover houses with knob-and-tube.
  • The Frequency of Improper Modifications with K&T
    Improper and unsafe DIY modifications are much more frequently found with knob-and-tube wiring than with modern wiring systems. Part of this is increased opportunity due to its age, as well as due to ease-of-access for splicing. Amateur modifications to K&T are so common, connections made with masking or Scotch tape (instead of electrical tape) are frequently encountered. Worse, the insufficiently trained frequently install fuses with an amperage too high for the wiring of K&T (to pair them with newer electronic appliances and devices), overloading and resulting in damage to wires, which over time breakdown, greatly increasing fire risk.

Live in an older home? Don’t assume safety. Schedule an electrical safety inspection with Mr. Electric today.

This blog is made available by Mr. Electric for educational purposes only to give the reader general information and a general understanding on the specific subject above. The blog should not be used as a substitute for a licensed electrical professional in your state or region. Check with city and state laws before performing any household project.