What is Knob and Tube Wiring?
Knob and tube wiring was an early standardized method of electrical wiring used in buildings in the United States from about 1880 to the 1940s. It consists of insulated copper conductors run within wall cavities, passing through joist and stud drill-holes via protective porcelain insulating tubes, and supported along their length on nailed-down porcelain knob insulators.
While knob and tube wiring was a major improvement over previous wiring methods, it has some serious drawbacks compared to modern electrical wiring that make it dangerous if still in use today.
Why Knob and Tube Wiring is Dangerous
There are several reasons why knob and tube wiring can be a fire and electrocution hazard:
Lack of Grounding
- Knob and tube circuits have no ground wire. This means if a short occurs, excess current has no safe path back, increasing risk of fire.
Insulation Degradation
- The rubber/cloth insulation on old knob and tube wiring becomes brittle and cracks over time. This allows wires to short and sparks to ignite fires.
Unsafe Wiring Practices
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Wires are often run through floor joists in knob and tube systems, exposing them to damage. This can cause shorts.
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Splices are not always properly contained in junction boxes. Exposed splices can short and arc.
Overloaded Circuits
- Knob and tube wiring was designed for simpler electrical loads. Today's homes have much higher electricity demand. Overloaded knob and tube circuits can overheat or short.
Inaccessibility
- Knob and tube wires are often sandwiched between structure walls/ceilings. This makes inspection or repair of failing wires impossible.
Dangers of DIY Rewiring Over Knob and Tube
Many homeowners attempt DIY rewiring projects to replace old knob and tube wiring. However, this often creates more safety issues:
Hiding Dangers
- Amateur rewiring projects frequently just hide the old wiring behind walls. This doesn't eliminate the risk of hidden shorts and fires.
Improper New Wiring
- New wiring installed incorrectly without proper materials or techniques can be an even bigger hazard than old knob and tube.
Working Live
- Rewiring requires turning off power. Live wires can electrocute. Fuses/breakers may not de-energize knob and tube circuits.
Structural Damage
- Fishing new wires often requires cutting holes in walls/ceilings. Patched holes can weaken structures, and allow fire to spread if ignited.
Creating Chaos
- Rewiring is complex, especially alongside old wires. The end result is often a tangled, hazardous mess if DIYed.
Dangers of Leaving Knob and Tube Wiring In Place
If you have knob and tube wiring, the safest option is complete professional rewiring. But many put this off due to the high cost. Understand the risks of leaving old systems in place:
Hidden Deterioration
- There may be extensive unseen corrosion and damage within walls and ceilings that can cause shorts and fires.
Insurance Issues
- Many insurers refuse to cover homes still containing knob and tube wiring due to the risk. This leaves you unprotected.
Resale Value Loss
- Most buyers won't purchase homes with knob and tube. Eliminating it before selling greatly increases property value.
Difficulty Renovating
- Old wiring severely limits renovations and additions. Rewiring is required before any major upgrades.
Electricity Capacity Limits
- Components like fuse boxes in knob and tube systems cannot handle increased loads from remodeling.
Safely Rewiring to Replace Knob and Tube
Replacing knob and tube wiring completely is the only sure way to eliminate the serious hazards it represents. Here are tips for doing this safely:
Turn Off Power
- The power company must disable electrical supply at the street before any work begins. Working on live knob and tube can kill.
Hire a Licensed Electrician
- Only experienced electricians have the skills to correctly and safely rewire a home from scratch. DIY is not an option.
Remove All Knob and Tube Components
- Don't abandon old wiring in walls. Complete elimination is required to prevent hidden hazards.
Install New Service Entry
- The power supply line, meter, and service panel/fuse box will need replacement to support rewiring.
Follow Codes and Use Copper
- All work must follow electrical codes, and new wiring must be standard copper with required sized conductors.
Add GFCI Protection
- Modern ground fault circuit interrupters (GFCIs) are essential for safety, especially in kitchens, bathrooms and outdoors.
Include AFCI Protection
- Arc fault circuit interrupters (AFCIs) help prevent fires and are now required in new bedroom circuits.
Confirm Safety
- Permit inspection, ground continuity testing, and infrared scans should be done to validate safety before re-energizing.
Replacing knob and tube wiring is a major undertaking, but essential for safety. Following proper procedures with licensed electrician supervision helps ensure it is done correctly. The investment protects your home and family.