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 passing through ceramic knobs and twisting through porcelain tubes, with connections and splices wrapped in cloth insulating tape.

While knob-and-tube wiring was considered very safe when it was introduced, it does not meet modern electrical standards. The outdated materials and installation methods used with knob-and-tube can make it a potential fire hazard. However, many older homes still contain original knob-and-tube wiring behind walls and ceilings.

Why Knob-and-Tube Wiring is Dangerous

There are several factors that make knob-and-tube wiring potentially unsafe:

While knob-and-tube can function reasonably well if undisturbed, any failures in the old, brittle insulation can have serious consequences. Minor arcs or sparks from damaged wiring can easily start fires inside walls and ceilings.

Signs You May Have Knob-and-Tube Wiring

It can be difficult to determine if a home still contains original knob-and-tube wiring. Some signs that suggest knob-and-tube may still be present include:

Having an inspection by a qualified electrician can conclusively determine if knob-and-tube wiring is present. They may also discover other unsafe conditions related to outdated wiring.

Dangers and Risks of Knob-and-Tube Wiring

Knob-and-tube wiring poses a variety of potential risks:

Fire Hazard

Fires are the greatest danger presented by aging, damanged knob-and-tube wiring. Faults in the wiring produce sparks that can easily ignite nearby wood framing, insulation, or debris inside walls and ceilings. Fires often spread undetected in hidden voids.

Shock Hazard

Exposed, damaged wiring can energize surrounding metal fixtures, surfaces, or materials. Coming into contact with these can deliver dangerous electric shocks. Ungrounded knob-and-tube provides no shock protection.


If faults produce exposed live conductors, it is possible to receive fatal shocks from damaged knob-and-tube wiring, particularly if moisture is present. Even low voltage can cause electrocution under certain conditions.

Equipment Damage

Electrical surges related to damaged, overloaded wiring can destroy sensitive electronic equipment, appliances, and devices. Surges can enter via connected circuits.

Insurance Issues

Many insurers refuse coverage for homes with knob-and-tube, or severely restrict coverage for fire and electrical damage. Lack of grounding and other hazards make claims more likely.

Dangers to Specific Groups

Some groups are at elevated risk from potential knob-and-tube hazards:

I always take extra precautions when dealing with older homes, as the hazards to me and my family are greatly elevated if old wiring is present. I advise others to do the same.

Recommendations for Homes with Knob-and-Tube

If you suspect your home has original knob-and-tube wiring, I strongly recommend the following:

Replacing knob-and-tube wiring will be expensive, but is worthwhile considering the potential risks. I advise all homeowners to take steps to ensure their home's electrical safety. Hidden electrical fires are devastating.


Knob-and-tube wiring, while once considered very safe, does not meet modern safety standards and can pose serious fire and shock hazards due to its age, lack of grounding, and outdated materials. Homeowners with old internal wiring may be at risk. There are often few signs of its presence. I highly recommend having a professional inspection and considering upgrades or replacement for this forgotten wiring system if found in your home. Taking action to identify and address electrical risks is essential to protect your home and family.