Knob & Tube Wiring: Is It A Fire Hazard In Your Home?

The Arts & Crafts era coincided with the introduction of residential wiring across America, which explains why many of the bungalows we live in were electrified with what is called 'knob and tube' wiring.

Used almost exclusively from the 1880s through the 1930s, knob and tube wiring is a controversial subject among homeowners, restorers and insurance companies, for no one seems willing to agree on whether or not it presents a fire hazard.

In contrast to modern wiring, in the knob and tube system the neutral and 'hot' wires are separated by no less than three inches of air space. Each strand was strung along a series of porcelain insulators, called 'knobs.' When the wire had to pass through a hole in a wooden joist, a porcelain 'tube' was first inserted to prevent the wiring from rubbing against the wood. The wires were insulated with either cotton cloth or a rubber coating.

The knob and tube system had several advantages. First, the two wires were kept far enough apart to prevent any dangerous sparks should the insulation wear off. Second, the knobs and tubes kept the wires away from wood framing. Third, the air circulating around the wires prevented any heat from accumulating to a dangerous level.

It also had its disadvantages. First, a protective ground wire was not included. Second, connections were not always made inside a metal junction box. Third, the thin insulation was prone to become brittle and could easily be eaten by rodents. Fourth, since the wires were suspended, they could easily be snagged, damaged or broken by someone crawling through an attic or working in the basement.

When knob and tube wiring was popular, insulation was not. When homeowners later added loose insulation to walls and attics, the old wires were often covered up. In some circumstances, this would prevent the wires from safely releasing the heat that electricity generates when it passes through a copper line.

Proponents will quickly point out that more fires have been caused by modern wiring than by knob and tube systems. Despite this, some insurance companies will not issue policies for homes that have knob and tube electrical systems in use.

Both sides agree that when properly installed and uncompromised by too many appliances, condition issues or insulation, the knob and tube system is safe.

They also agree that should a homeowner have a knob and tube system in their home, you should have it thoroughly inspected by a licensed electrician who can help you determine if it presents any danger of fire to your home.

- Bruce Johnson