Knob and tube wiring was the standard method of electrical wiring in homes from about 1880 to the 1930s. This old-fashioned wiring system powered millions of homes for decades, but also created major fire hazards that many homeowners today are unaware of.
A Revolutionary System for Its Time
The knob and tube system was revolutionary when it was introduced. Up until that point, electrical wiring was something only businesses used. With knob and tube, even regular homes could have electricity and electric lighting.
Here's an overview of how the knob and tube system worked:
- Wires were run through the open spaces inside walls and attics, passing through holes drilled in joists and studs.
- The wires were separated from each other and held in place by ceramic knobs or tubes.
- Knobs held the wires away from surfaces. Tubes protected wires where they passed through holes.
- Wires were often coated with cloth insulation.
- Early versions had no ground wire, just a hot and neutral.
This open wiring had advantages over modern systems:
- Air could circulate around the wires, preventing overheating.
- It was easy to access wires for repairs.
- The separate wires minimized electromagnetic interference.
Popularity Peaked in the 1920s
Knob and tube wiring spread rapidly in the early 20th century. By the 1920s, it was the most popular wiring method for new construction.
- From about 1890 to 1930, knob and tube wiring was used in over 75% of new homes built.
- It was cheaper than other methods and didn't require tearing open walls.
- Even upper class homes were wired this way.
Millions of homes across America relied on knob and tube wiring up through the 1940s and 1950s. But it declined for new construction after the 1930s.
The Beginning of the End
Several factors led to the decline of knob and tube wiring:
- Nonmetallic sheathed cable (NM, or Romex) became a competitor in the 1930s. It was cheaper and easier to install than knob and tube.
- The National Electrical Code banned knob and tube in new construction in the 1950s.
- Insurance companies began refusing to insure homes with knob and tube wiring.
- Improvements in wiring insulation made Romex safer than uninsulated knob and tube.
So by the 1960s, knob and tube was obsolete in new construction. But millions of existing homes still relied on this outdated and potentially dangerous wiring.
Hidden Dangers of Knob and Tube Wiring
Knob and tube wiring might have been state-of-the-art in 1890. But it has serious safety issues by modern standards:
- The wiring is often ungrounded, with no ground wire to prevent shocks.
- The cloth insulation breaks down over time, leading to exposed live wires touching wood and causing fire hazards.
- Overloaded circuits get hot, starting fires in the walls.
- Rodents and insects can chew through the aged cloth insulation.
- Vibrations from slammed doors or heavy footsteps can degrade connections.
Fires linked to knob and tube wiring are common. The Consumer Product Safety Commission estimates it causes over 25,000 house fires per year.
Dangers Can Lurk for Decades
The scary thing about knob and tube wiring is that the hazards are often well hidden. Homeowners may have no idea their house wiring is outdated and dangerous.
Some signs you might have knob and tube wiring:
- Your home was built before 1950.
- You see old ceramic knobs or tubes inside walls.
- Wires are wrapped in degrading cloth.
- Light fixtures are on their own branches, separate from receptacles.
- You have ungrounded two-prong outlets.
- Fuses blow or circuit breakers trip for no apparent reason.
- You have cold spots or discolored walls where wires overheated.
If you suspect your home has knob and tube wiring, it's critical to have it inspected by a qualified electrician. Trying to repair or modify knob and tube wiring yourself is extremely dangerous due to the fire hazards.
Solutions for Homes Still Using Obsolete Wiring
If you have knob and tube wiring in your vintage home, here are your options:
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For a quick temporary fix, you can have individual damaged wires repaired. But this doesn't solve the underlying fire dangers.
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For a more thorough solution, you can have knob and tube wiring replaced completely with modern wiring. This eliminates the hazards but is expensive.
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Some electricians offer a "pigtailing" upgrade by splicing modern wiring onto the old. This is less expensive than a full rewire.
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Or consider upgrading the wiring little by little when you remodel or upgrade parts of your home over time.
Replacing knob and tube wiring is a major project, but it's a smart investment for your safety and peace of mind. Don't ignore obsolete electrical systems that could cause catastrophic fires.