Knob-and-Tube Wiring. A Brief History
Knob-and-Tube Wiring systems was an early standardized method of electrical wiring in buildings. Its common use was in North America from about 1880 to the early 1940s. It consists of single-insulated copper conductors that run within wall or ceiling cavities, passing through joist and stud drill-holes via protective porcelain insulating tubes, and supported along their length on nailed-down porcelain knob insulators.
Where conductors entered a wiring device such as a lamp or switch, or were pulled into a wall, they were protected by flexible cloth insulating sleeving called loom. The first insulation (loom) consisting of an asphalt-saturated cotton cloth, then rubber became common. Wire splices in such installations were twisted together for good mechanical strength, then soldered and wrapped with rubber insulating tape and friction tape (asphalt saturated cloth), or made inside metal junction boxes.
Knob and tube wiring was eventually displaced from interior wiring systems because of the high cost of installation compared with use of power cables, which combined both power conductors of a circuit in one run (and which later included grounding conductors).
At present, new knob and tube installations are permitted in the U.S. only in a few very specific situations listed in the National Electrical Code, such as certain industrial and agricultural environments.
Facts about Knob-and-Tube
These systems, per say, are not inherently dangerous, however, the dangers from this system arise from:
- Its age
- Loose connections and sagging wiring
- Improper modifications
- Situations where building insulation envelops the wires.
These systems possess no ground wire and thus cannot service any three-pronged appliances (ground not required until 1963 – NEC). Grounding conductors reduce the chance of electrical fire and damage to sensitive equipment.
While the product is considered obsolete, to date there is still no code that requires its complete removal. However, it is treated differently in different jurisdictions. In some areas, it must be removed at all accessible locations, while others don’t.
Unsafe modifications are a far more common danger. Given that the product is so old, more opportunity has existed (over the years) for improper modifications by homeowners, handymen, and electricians alike. K&T wiring is often spliced with modern wiring incorrectly by amateurs. This is perhaps due to the ease by which K&T wiring is accessed.
The insulation (cloth loom) that envelopes the wiring also posed a fire hazard.
Building Insulation and K&T Systems
Knob-and-Tube Wiring systems are designed to dissipate heat into free air. Insulation coverage will disturb this process. Installation of insulation around K&T wires will cause heat to build up, thus creating a fire hazard. The National Electrical Code (as of 2008) requires that K&T systems NOT be covered by insulation. Specifically, “the wiring system (Knob-and-Tube) should not be in hollow spaces of walls, ceilings and attics where such spaces are insulated by loose, rolled or foamed-in-place insulating material that envelops the conductors.”
Local jurisdictions (Authority Having Jurisdiction – AHJ) may or may not adopt the NEC’s requirement. The California Electrical Code, for instance, allows insulation to be in contact with knob-and-tube wiring, provided that certain conditions are met, such as, but not limited to, the following:
- A licensed electrical contractor must certify that the system is safe.
- The certification must be filed with the local building department.
- Accessible areas where insulation covers the wiring must be posted with a warning sign. In some areas, this sign must be in Spanish and English.
- The insulation must be non-combustible and non-conductive.
Normal requirements for insulation must be met.
Insurance coverage and Knob-and-Tube
Many insurance companies refuse to insure houses that have knob-and-tube wiring due to the risk of fire. Exceptions are sometimes made for houses where an electrical contractor has deemed the system to be safe.
Advice for those with K&T wiring:
- Have the system evaluated by a qualified electrician. Only an expert can confirm that the system was installed and modified correctly.
- Do not run an excessive amount of appliances in the home, as this can cause a fire.
- Where the wiring is brittle or cracked, it should be replaced.
- Proper maintenance is crucial.
- K&T wiring should not be used in kitchens, bathrooms, laundry rooms or outdoors. Wiring must be grounded in order to be used safely in these locations.
- Rewiring a house can take weeks and cost thousands of dollars, but unsafe wiring can cause fires, complicate estate transactions, and make insurers skittish.
- Homeowners should carefully consider their options before deciding whether to rewire their house.
- The homeowner, or an electrician, should carefully remove any insulation that is found surrounding K&T wires.
- Prospective home buyers should get an estimate of the cost of replacing K&T wiring. They can use this amount to negotiate a cheaper price for the house.
In summary, knob-and-tube wiring is likely to be a safety hazard due to improper modifications and the addition of building insulation.
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