How to Safely Work with Knob-and-Tube Wiring

What is Knob-and-Tube Wiring?

Knob-and-tube wiring was commonly used in homes built before 1950. It consists of insulated wires that are run through the wall cavities, with ceramic knobs used to hold the wires in place and air gaps between the wires acting as insulation.

Some key characteristics of knob-and-tube wiring:

• Wires are separate hot and neutral wires, not bundled together like modern NM cable
• Wires are surrounded by open air, not insulation
• Wires are held in place by ceramic knobs
• Wires are connected by spliced joints twisted together and wrapped with cloth tape or rubber tape
• No ground wire present

While knob-and-tube wiring was suitable for the first half of the 20th century, it lacks safety features of modern electrical systems. The open splices and missing ground wire can pose fire and shock hazards.

Risks of Knob-and-Tube Wiring

Knob-and-tube wiring has largely been phased out of homes due to safety concerns, including:

• Fire hazard - The insulation on old wires can deteriorate over time, exposing bare copper. This creates a fire hazard.

• Shock hazard - No ground wire means there is no path to safely divert fault currents. You could get shocked by touching a shorted wire.

• Overload - Knob-and-tube circuits are rarely properly fused. Overloads can lead to unsafe overheating of wires.

• Unsafe splices - The old cloth-taped splices often fail over time, loosening up and creating hot spots.

• Insurance issues - Many insurance companies won't issue policies for homes with knob-and-tube wiring due to the fire risk.

While failures are uncommon in untouched wiring, any modifications or disturbances greatly increase the hazard.

Evaluating and Working With Knob-and-Tube Safely

If you have knob-and-tube wiring, here are some tips for living with it safely:

Inspection

• Have an electrician experienced with knob-and-tube inspect the system to evaluate condition and load.

• Use an infrared camera to check for hot spots on wires and connections.

• Check for any damaged insulation or exposed copper conductors.

Limited modifications

• It may be possible to tap into the existing wires to add grounded outlets in a limited capacity. This should be done by a professional electrician.

• Do not overload the system or make excessive modifications.

Leave undisturbed

• Do not run new wiring over top of knob-and-tube circuits. This could disturb or damage old insulation.

• Avoid damaging knob-and-tube during renovations. Remove and reinstall sections as needed by an electrician.

Fire precautions

• Take extra fire precautions by avoiding extension cords and keeping flammable materials away from knob-and-tube runs.

• Install smoke detectors and an AFCI outlet for additional protection.

Replacement

• If the system is in poor condition, has inadequate capacity, or you plan major changes, the best option is complete replacement by an electrician.

• Replacing knob-and-tube wiring will make your home safer and can help avoid insurance issues.

Working Safely With Old Wiring

Knob-and-tube wiring requires an abundance of caution. Here are some key tips if you need to work near or make any changes to knob-and-tube:

• Turn off power at the main breaker before beginning any work.

• Test wires to confirm they are de-energized.

• Wear personal protective equipment like non-conductive gloves and eye protection.

• Use insulated tools and ladders to avoid accidental contact.

• Be careful not to staple or damage the wiring.

• Check for asbestos insulation and take precautions if removing any old wiring.

• Always have work inspected by an experienced electrician when complete.

The risks of knob-and-tube can be managed with proper precautions, but replacement is the only way to bring wiring fully up to modern safety standards. Carefully consider your options when deciding how best to update the electrical system in an older home.