Commonly used in North America from about 1880 into the 1940s, Knob-and-Tube (K&T) wiring was an early method of electrical wiring in many buildings. Nowadays, this system is not only obsolete, but even considered dangerous. Some of the trepidation concerning knob-and-tube wiring, however, is unwarranted.
In and of itself, K&T is not dangerous. The hazards stem from its age, incorrect modifications, and situations in which wires are enveloped by building insulation. Also, K&T has no ground wire, and therefore cannot function for any three-pronged appliances.
Although K&T wiring is considered obsolete, there is no existing code requiring its complete removal. Different jurisdictions have different requirements regarding this system. Some mandate removal at any accessible locations. Others are more lenient and simply require that it not be used in any new construction. It is important that inspectors are aware of the codes in the areas in which they work.
How Does Knob-and-Tube Wiring Work?
In this system of wiring, insulated copper conductors protected by porcelain insulating tubes move through drilled holes in lumber framing and are supported by porcelain knobs that are nailed down. In areas in which wires enter a device such as a switch or a lamp or are pulled into a wall, they are shielded by rubber insulation called "loom" or by flexible cloth.
What Are The Advantages of Knob-and-Tube Wiring?
- K&T wires are not as likely to be punctured by nails as Romex cables because K&T wires are held back from the framing.
- K&T wiring has an ampacity that is higher than that of other wiring systems of the same gauge. This is because the hot and neutral wires are separated by about 4 to 6 inches from one another, allowing the wires to easily dissipate heat into the free air.
- The porcelain elements have a lifespan that is practically unlimited.
- The installation of modern Romex wiring is often inferior to the original installation of K&T wiring. This is because knob-and-tube installation demands more expertise to install than Romex, hence, people who are unskilled in the process rarely ever install it.
Are There Problems With K&T Wiring?
- Due to the fact that K&T is so old, more opportunities have existed for improperly performed modifications; they are far more commonly found in K&T wiring than in other modern wiring systems such as Romex.
- K&T has a tendency to sag and stretch over time.
- The insulation that surrounds the wiring is a fire hazard.
- Grounding conductors are not present in knob-and-tube wiring and are necessary to reduce the hazards of electrical fire and equipment damage.
- In older K&T systems, the wiring is insulated with fiber materials that are susceptible to deterioration and contains insulation with additives that can oxidize copper wire, resulting in cracking and peeling.
K&T wiring is less resistant to damage when compared to modern wiring insulation. K&T wiring that is insulated with asbestos and cambric has no moisture exposure rating.
Because K&T wiring is easily accessed, amateurs will often splice K&T wiring incorrectly with modern wiring.
Knob-and-tube wiring is designed to disperse heat into free air. Insulation around the wires disturbs the process and causes a buildup of heat, resulting in a fire hazard. It is a requirement in the 2008 National Electrical Code (NEC) that this system of wiring not be enveloped by insulation, and that it should not be in "...hollow spaces of ceilings, walls, and attics where such spaces are insulated by rolled, loose, or foamed-in-place insulating materials that envelop the conductors."
Localities may or may not adhere to the NEC requirements. For example, the California Electrical Code permits insulation to remain in contact with knob-and-tube wiring, as long as specific conditions are met, which include, but are not limited to, the following:
- The certification of the system's safety must be provided by a licensed electrical contractor and filed with the local building department.
- The insulation must be non-combustible and non-conductive.
- Accessible areas in which wiring covers the insulation must display a warning sign. Some localities require the posting to be in Spanish and English.
- Insulation must meet all regular requirements.
In the time in which K&T wiring was first introduced, common electrical appliances in a home were mainly teakettles, toasters, clothing irons, and coffee percolators. The vast electrical requirements of homes in the mid-to-late 20th century could hardly have been envisioned during the late 18th century, a time in which many merely considered electricity to be a passing fad. Existing K&T systems are infamous for adaptations made by poorly trained handymen (rather than qualified electricians) in an attempt to meet the increasing amperage requirements for refrigerators, televisions, and an abundance of other electric appliances. The result was wiring systems that were left susceptible to overloading. The response of many homeowners was to install fuses that had resistances too high for the wiring. The result was heat damage to the wiring because the fuses would blow less frequently.
Knob-and-Tube Wiring and Insurance
The potential risk of fire causes many insurance companies to refuse insurance to houses with K&T wiring, although in cases in which an electrical contractor declares the system safe, exceptions are sometimes made.
Advice about Homes with Knob-and-Tube Wiring
- Do not run an excessive number of appliances, as this can result in a fire.
- Have a qualified electrician evaluate your system in order to confirm that the system was installed correctly and modified properly.
- Replace any brittle or cracked wiring.
- Properly maintain the system.
- K&T wiring is not to be used in bathrooms, laundry rooms, kitchens, or outdoors. Wiring must always be grounded in these locations.
- Consider all options before deciding to rewire your house.
- Any insulation surrounding K&T wires should be carefully removed by the homeowner or by an electrician.
- Prospective homebuyers should get an estimate or a quote for the cost of replacing existing K&T wiring, as this amount can be used in price negotiations.
Knob-and-tube wiring is a probable safety hazard because of improper modifications to the system and due to the use of building insulation. Inspectors need to be knowledgeable about this antiquated system and be able to educate their clients about the possible dangers.