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  • Writer's pictureJohn@LiteHouse

What is Knob-and-Tube Wiring?

Knob-and-tube (K&T) wiring was an early standardized method of electrical wiring in buildings, commonly used in North America from about 1880 to the 1940s. The system is considered outdated and may pose a security risk, although some of the concerns associated with it are unwarranted.

Knobs and Tubes Wiring Facts: Knobs and Tubes Wiring

  • It is not inherently dangerous. The danger of this system results from its age, improper adjustments and situations where the building insulation wraps around the conductors.

  • It does not have a ground wire and therefore cannot serve any three-prong appliances.

  • Although it is considered deprecated, there is no code that requires its complete removal.

  • It is treated differently in different jurisdictions. In some areas it must be removed in all available places, while in others it is not, but check it for safety reasons.

  • Not allowed in any new construction.

How knob and tube wiring works:

K&T wiring consists of insulated copper conductors passing through openings in wooden frames through protective porcelain insulating tubes. They are supported along the entire length with nailed porcelain buttons. Where wires enter an electrical fixture such as a lamp or switch, or have been pulled into a wall, they are protected by a flexible fabric or rubber insulation called a "loom".

Advantages of knob and tube wiring:

  • K&T lines have a higher capacity than wiring systems of the same gauge. This is because the hot and neutral conductors are separated from each other, usually by 4 to 6 inches, which allows the conductors to easily dissipate heat into the open air.

  • K&T wires are less likely to puncture nails than Romex cables because the K&T wires are held outside the frame.

  • Porcelain components have an almost unlimited lifespan.

  • The original knob and tube wiring installation is often better than modern Romex wiring. Installing K&T wiring requires more skill than installing Romex, and for this reason it was rarely installed by unskilled people.

Problems associated with K&T Wiring:

  • Unsafe modifications are much more common with K&T wiring than with Romex and other modern wiring systems. Part of this is because the K&T is so old that there was more opportunity for inappropriate adjustments.

  • The insulation that surrounds the wiring is a fire hazard.

  • It tends to stretch and sag over time.

  • It is missing a ground wire. Grounding conductors reduce the risk of electrical fire and damage to sensitive equipment.

  • In older systems, the wiring is insulated with varnish and fibrous materials that are prone to damage.

Compared to modern wiring insulation, K&T wiring is less resistant to damage. K&T cabling insulated with cambric and asbestos is not intended for exposure to moisture. Older systems contained insulation with additives that can oxidize the copper wire. Bending the wires can cause cracking and peeling of the insulation.

K&T cabling is often mistakenly combined by amateurs with modern wiring. This is possible due to the easy access to the K&T wiring.

Building insulation:

K&T wiring is designed to dissipate heat to the open air and insulation will interfere with this process. Insulation around K&T wires will cause heat to build up, creating a fire hazard. The 2008 National Electrical Code (NEC) requires that this wiring system not be covered with insulation. It specifically states that this wiring system should not be in…

hollow spaces of walls, ceilings and attics where these spaces are insulated with loose, rolled or foamed-in-place insulating material that wraps the conductors.

Local jurisdictions may or may not adopt the NEC requirement. For example, the California Electrical Code permits insulation to be in contact with knob-and-tube cables provided certain conditions are met, such as, but not limited to, the following:

  • A certified electrical contractor must confirm that the system is safe.

  • The certificate must be submitted to the local building authority.

  • Access points where insulation covers wiring must be marked with a warning label. In some areas, this sign must be in Spanish and English.

  • The insulation must be non-flammable and non-conductive.

  • Normal insulation requirements must be met.


When K&T wiring was first introduced, common household electrical appliances were limited to little more than toasters, kettles, coffee machines and coffee machines.

clothes irons. Electrical requirements of mid to late 20th century homes

could not have been foreseen during the late 18th century, when electricity was seen by many as a passing fad. Existing K&T systems are notorious for modifications made to accommodate the increasing current loads required by televisions, refrigerators and a host of other electrical appliances. Many of these attempts were made by poorly trained DIYers rather than experienced electricians, whose work made the wiring system susceptible to overload.

  • Many homeowners accommodated the insufficient current of K&T wiring by installing fuses with resistances that were too high for the wiring. The result of this modification is that the fuses would not blow as often and the wiring would suffer thermal damage due to excessive current loading.

  • It is not uncommon for inspectors to find joints wrapped in masking tape or duct tape instead of electrical tape.

K&T electrical installation and insurance:

Many insurance companies refuse to insure houses that have push-button wiring due to the risk of fire. Exceptions are sometimes made for homes where the electrical supplier deems the system safe.

Tips for those with K&T wiring:

  • Have the system evaluated by a qualified electrician. Only an expert can confirm that the system has been installed and adjusted correctly.

  • Do not operate an excessive number of appliances in the home, it could cause a fire.

  • If 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 to be used safely in these locations.

  • Changing a home's wiring can take weeks and cost thousands of dollars, but unsafe wiring can cause fires, complicate real estate transactions, and make insurance companies nervous.

  • Homeowners should carefully consider their options before deciding whether to remodel their home.

  • The homeowner or electrician should carefully remove any insulation that is around the K&T wires.

  • Prospective homebuyers should get a K&T wiring replacement cost estimate. From this amount, they can negotiate a cheaper price for the house.

In summary, knob and tube wiring is likely to pose a safety hazard due to improper modifications and addition of building insulation. Inspectors must be wary of this antiquated system and be prepared to inform their clients of its potential dangers.

The inspectors at LiteHouse Services Group LLC look for Knob-and-Tube wiring in every older home we inspect. When found, we will refer you to a licensed electrician for safety evaluation and estimates for upgrading.

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