Back in the day when homes were first wired for electricity, knob and tube wiring was state of the art. All in all, a pretty good system… prior to the 1930’s. On many of the home inspections I do on older homes, I come across knob and tube wiring that is still live but should be removed and replaced. Unfortunately, over the years, this type of system is prone to problems due to age, damage, improper alterations and deterioration.
As I said, this type of wiring was fairly common in homes prior to the 1930’s. The system was made up of porcelain insulators, (knobs and tubes), wiring with a protective cloth insulative covering, and cloth electricians tape. The knobs were used to route wires through unobstructed areas, and the tubes were used to route wiring through studs and joists.
Knob and Tube wiring did have a few safety features:
Suspending the wiring in open air via the knob allows heat to be efficiently dissipated.
Porcelain tubes provided additional protection to wiring passing through studs and joists.
It was common to install the wiring near the center of studs and joists, away from potential nail damage
The hot and neutral wires were typically separated by at least 3 inches with the exception connections at junctions or fixtures.
Additional protection was added in the form of insulative sleeves from the last knob to the fixture where 3 inch separation could not be maintained.
Splices were formed by wrapping one wire around the other and soldering them together and wrapping the exposed wire with electricians tape. Knobs were then placed 4-6 inched from the splice to eliminate stress to the connection.
Wiring was typically pulled tight and run at right angles. Wires were not to be placed across the tops of joists where they could be damaged.
The downside of the Knob and Tube system was:
The system did not include a ground conductor
Switches were typically placed on the neutral wire, allowing the circuit to be switched off, but not the current.
Exposed wiring was subject to damage in areas used for storage in attics and basements.
Through the years however, this type of wiring is subject to a variety of problems. Improper alterations made to the original system can pose significant safety issues. From a safety standpoint, unfortunately the electrical system is one of the systems in a home that can have installations that are wrong or dangerous and till work.
Addition of new branches and fixtures to the original system is a common problem. Added lights and outlets require more current. More current blows fuses. The solution: Bigger fuse. Bigger fuse results in increased heat generation in the conductors (wires). This increased heat breaks down the insulation, making it brittle, eventually disintegrating or falling off. The heat generated by ceiling fixtures, and higher temperatures in unvented attics also contribute to the degradation of the insulation of Knob and Tube wiring. It also seems that critters like the chewing on and gathering the insulation for their nests. At a recent home inspection I had entered an attic which had Knob and Tube wiring. I turned around in a cramped area and noticed that I nearly came in contact with bare wires running along an entire wall.
As we became more energy conscious, and homeowners got tired of drafty houses and high heating and cooling costs, homeowners started adding additional insulation to their attics and walls. Refer back to the list of safety features for Knob and Tube wiring. You will see that the first one suspending the conductors in open air dissipates heat. Adding loose, blown or batt insulation over Knob and Tube wiring counteracts the open air heat dissipation. Adding insulation on top of Knob and Tube wiring is a fire hazard.
In conclusion, there seems to be a growing concern amongst homeowners insurance companies with respect to this old wiring. My recommendation when finding Knob and Tube wiring at a home inspection is that the client have a qualified Electrician inspect the wiring for safety and potential replacement.