How the Obscure "Knob and Tube" Electrical Wiring System Could Be a Fire Hazard in Your Historic Home

What is Knob and Tube Wiring?

Knob and tube wiring was an early standardized method of electrical wiring used in North America from about 1880 to the 1940s. It consists of single insulated copper conductors run within wall cavities, passing through joists and studs. The wire is supported along its length with ceramic knobs and where wires enter a wiring device, fused panel, or junction box, they are attached to porcelain tube insulators.

This old system has knobs made of porcelain that act as spacers between the wires and the framing. The tubes are also porcelain and they guide the wire through openings in the framing. Unlike modern wiring systems, the hot and neutral wires are separate and they are often run through different joist cavities.

Key Features of Knob and Tube Wiring:

Why Knob and Tube Wiring Can Be a Fire Hazard

While knob and tube wiring was an improvement over previous methods when initially introduced, it does not meet modern electrical safety standards and can pose fire and electrocution hazards. Here's why:

Exposure to Airflow and Vibration

As knob and tube wires are run through open air without conduit, they are exposed to vibrations and air currents inside walls. This can lead to insulation breakdown over time. Bare wires touching wood framing is a fire hazard.

Cloth Insulation Dries Out

The cloth insulation used on knob and tube wires becomes brittle and cracks with age, exposing the copper conductor. Any abrasion or movement can strip away the deteriorated insulation.

Lack of Grounding

Knob and tube systems have no equipment grounding conductor. This can lead to an increased risk of electrocution and make lightning strikes more dangerous.

Unsafe Splicing and Capping

The connections and splices made between knob and tube wires are primitive by modern standards. Loose connections cause overheating and arcing. They lack twist-on wire connectors.

Inadequate Circuit Protection

Knob and tube wiring typically lacks the circuit breakers or fuses found in modern electrical systems. Overloads and short circuits can lead to overheating and fire.

Incompatible with Modern Electrical Loads

Lighting and appliances have much higher wattage demands than knob and tube systems were designed for. The undersized wires lead to overheating, voltage drops, and fire hazards.

Signs Your Home May Still Have Active Knob and Tube Wiring

Here are some signs that a home may still be relying on antiquated and potentially dangerous knob and tube wiring:

Other Clues Include:

If you see any of these signs in your historic home, it likely still has live knob and tube wiring that should be evaluated and upgraded by a licensed electrician. Don't take chances with this obsolete wiring!

Is Knob and Tube Wiring Ever Acceptable to Leave In Place?

Generally, it is recommended that any active knob and tube wiring be completely removed and upgraded in a home. However, in some cases, it may be impractical or cost-prohibitive to fully eliminate it. Here are some instances where it may be acceptable to leave some amount in place:

However, experts recommend the following if any knob and tube remains:

Ideally, as much knob and tube wiring as possible should be eliminated or replaced in any home. Continuing to use it in an active capacity is playing with fire.

Safest Options for Replacing Knob and Tube Wiring

If your home inspection or an electrician has identified that you still have active, live knob and tube wiring, the safest options are:

Replacing knob and tube wiring provides modern safety with features like proper grounding, circuit protection, and adequate wire gauges for larger electrical loads. Home insurance companies may also require its replacement.

In Summary

Knob and tube wiring poses a serious fire and shock risk in older homes. While once considered safe decades ago, it lacks the proper insulation, capacity, grounding, and overcurrent protection of modern electrical systems. Homeowners with this antiquated wiring should have it evaluated and upgraded before damage occurs. Replacing knob and tube wiring will make your historic home much safer.