Old Electrical Wiring
Knob-and-tube (K&T) wiring was an early standardized method of electrical wiring in buildings, in common use in North America from about 1880 to the 1940s. The system is considered obsolete and can be a safety hazard, although some of the fear associated with it is undeserved.
Facts about Knob-and-Tube Wiring:
- It is not inherently dangerous. The dangers from this system arise from its age, improper modifications, and situations where building insulation envelops the wires.
- It has no ground wire and thus cannot service any three-pronged appliances.
- While it is considered obsolete, there is no code that requires its complete removal.
- It is not permitted in any new construction.
How Knob-and-Tube Wiring Works:
K&T wiring consists of insulated copper conductors passing through lumber framing drill-holes via protective porcelain insulating tubes. They are supported along their length by nailed-down porcelain knobs. Where wires enter a wiring device, such as a lamp or switch, or were pulled into a wall, they are protected by flexible cloth or rubber insulation called “loom.”
Advantages of Knob-and-Tube Wiring:
- K&T wiring has a higher ampacity than wiring systems of the same gauge. The reason for this is that the hot and neutral wires are separated from one another, usually by 4 to 6 inches, which allows the wires to readily dissipate heat into free air.
- K&T wires are less likely than Romex cables to be punctured by nails because K&T wires are held away from the framing.
- The porcelain components have an almost unlimited life span.
- The original installation of knob-and-tube wiring is often superior to that of modern Romex wiring. K&T wiring installation required more skill to install than Romex and, for this reason; unskilled people rarely ever installed it.
Problems Associated with K&T Wiring:
- Unsafe modifications are far more common with K&T wiring than they are with Romex and other modern wiring systems. Part of the reason for this is that K&T is so old that more opportunity has existed for improper modifications.
- The insulation that envelopes the wiring is a fire hazard.
- It tends to stretch and sag over time.
- It lacks a grounding conductor. Grounding conductors reduce the chance of electrical fire and damage to sensitive equipment.
- In older systems, wiring is insulated with varnish and fiber materials that are susceptible to deterioration.
- Compared with modern wiring insulation, K&T wiring is less resistant to damage. K&T wiring insulated with cambric and asbestos is not rated for moisture exposure. Older systems contained insulation with additives that may oxidize copper wire. Bending the wires may cause insulation to crack and peel away.
- 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.
K&T wiring is designed to dissipate heat into free air, and insulation will disturb this process. Insulation around K&T wires will cause heat to build up, and this creates a fire hazard. The 2008 National Electrical Code (NEC) requires that this wiring system not be covered by insulation. Specifically, it states that this wiring system 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 may or may not adopt the NEC’s requirement.
When K&T wiring was first introduced, common household electrical appliances were limited to little more than toasters, tea kettles, coffee percolators and clothes irons. The electrical requirements of mid- to late-20th century homes could not have been foreseen during the late 18th century, a time during which electricity, to many, was seen as a passing fad. Existing K&T systems are notorious for modifications made in an attempt to match the increasing amperage loads required by televisions, refrigerators, and a plethora of other electric appliances. Many of these attempts were made by insufficiently trained handymen, rather than experienced electricians, whose work made the wiring system vulnerable to overloading.
Many homeowners adapted to the inadequate amperage 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 heat damage due to excessive amperage loads. It is not uncommon for inspectors to find connections wrapped with masking tape or Scotch tape instead of electrical tape.
K&T Wiring and Insurance:
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 days or 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.
324-4. Uses Not Permitted. Concealed knob-and-tube wiring shall not be used, in commercial garages,
theaters and similar locations, motion picture studios, hazardous (classified) locations or in the hollow
spaces or walls, ceilings and attics when such spaces are insulated by loose, rolled, or foamed in place
insulating material that envelops the conductors.
Exception: This article is not intended to prohibit the installation of insulation where knob-and-tube wiring is present, provided the following are complied with:
The wiring shall be surveyed by an electrical contractor licensed by the State of California. Certification shall be provided by the electrical contractor that the existing wiring is in good condition with no evidence of deterioration or improper over-current protection, and no improper connections or splices.
- Repairs, alterations or extensions to the electrical system will require permits and inspections by the authority having jurisdiction for the enforcement of this code.
- The certification form shall be filed with the authority having jurisdiction for the enforcement of this code and a copy furnished to the property owner.
- All accessible areas in the building where insulation has been installed around knob-and-tube wiring shall be posted with a notice, clearly visible, stating that caution is required when entering these areas.
- The insulation shall be noncombustible as defined by Section 415(b) of Part 2 of this Title.
- The insulation shall not have any electrical conductive material as part of or supporting the insulation material.
- Nothing in this exception will prohibit the authority having jurisdiction for the enforcement of this code from requiring permits and inspections for the installation of thermal insulation.