Knob-and-tube (K&T) wiring was an early standardized method ofelectrical wiring in buildings, in common use in North America from about 1880to the 1940s. The system is considered obsolete and can be a safety hazard,although some of the fear associated with it is undeserved.
Facts AboutKnob-and-Tube Wiring:
It has no ground wire and thus cannot service any three-pronged appliances.
While it is considered obsolete, there is no code that requires itscomplete removal.
It is treated differently in different jurisdictions. In some areas, itmust be removed at all accessible locations, while others merely require thatit not be installed in new construction. Inspectors should be sure to know thecodes in the jurisdictions in which they operate.
It is not permitted in any new construction.
How Knob-and-TubeWiring Works:
K&T wiringconsists of insulated copper conductors passing through lumber framingdrill-holes via protective porcelain insulating tubes. They are supported alongtheir 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 byflexible cloth or rubber insulation called "loom."
Advantages ofKnob-and-Tube Wiring:
K&T wiring has a higher ampacity than wiring systems of the samegauge. The reason for this is that the hot and neutral wires are separated fromone another, usually by 4 to 6 inches, which allows the wires to readilydissipate heat into free air.
K&T wires are less likely than Romex cables to be punctured bynails because K&T wires are held away from the framing.The porcelain components have an almost unlimited lifespan.
The original installation of knob-and-tube wiring is often superior tothat of modern Romex wiring. K&T wiring installation requires more skill toinstall than Romex and, for this reason, unskilled people rarely ever installedit.
Problems Associatedwith K&T Wiring:
Unsafe modifications are far more common with K&T wiring than theyare with Romex and other modern wiring systems. Part of the reason for this isthat 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 chanceof electrical fire and damage to sensitive equipment.
In older systems, wiring is insulated with varnish and fiber materialsthat are susceptible to deterioration.
Compared with modernwiring insulation, K&T wiring is less resistant to damage. K&Twiring 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 isoften spliced with modern wiring incorrectly by amateurs. This is perhaps due tothe ease by which K&T wiring is accessed.
K&T wiring isdesigned to dissipate heat into free air, and insulation will disturb thisprocess. Insulation around K&T wires will cause heat to build up, and thiscreates a fire hazard. The 2008 National Electrical Code (NEC) requires thatthis wiring system not be covered by insulation. Specifically, it states thatthis wiring system should not be in…
hollow spaces ofwalls, ceilings and attics where such spaces are insulated by loose, rolled orfoamed-in-place insulating material that envelops the conductors.
Local jurisdictionsmay or may not adopt the NEC’s requirement. The California Electrical Code, forinstance, 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 witha 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.
When K&T wiring was first introduced, common household electricalappliances were limited to little more than toasters, tea kettles, coffeepercolators and
clothes irons. The electrical requirements of mid- to late-20 century homes
could not have been foreseen during the late 18 century, a time during which electricity, to many, was seen as apassing fad. Existing K&T systems are notorious for modifications made inan attempt to match the increasing amperage loads required by televisions,refrigerators, and a plethora of other electric appliances. Many of theseattempts were made by insufficiently trained handymen, rather than experiencedelectricians, whose work made the wiring system vulnerable to overloading.
Many homeowners adapted to the inadequate amperage of K&T wiring byinstalling fuses with resistances that were too high for the wiring. The resultof this modification is that the fuses would not blow as often and the wiringwould suffer heat damage due to excessive amperage loads.
It is not uncommon for inspectors to find connections wrapped withmasking tape or Scotch tape instead of electrical tape.
K&T Wiring andInsurance:
Many insurancecompanies refuse to insure houses that have knob-and-tube wiring due to therisk of fire. Exceptions are sometimes made for houses where an electricalcontractor has deemed the system to be safe.
Advice for those withK&T wiring:
Have the system evaluated by a qualified electrician. Only an expert canconfirm that the system was installed and modified correctly.
Do not run an excessive amount of appliances in the home, as this cancause a fire.
Where the wiring is brittle or cracked, it should be replaced. Propermaintenance is crucial.
K&T wiring should not be used in kitchens, bathrooms, laundryrooms or outdoors. Wiring must be grounded in order to be used safely inthese locations.
Rewiring a house can take weeks and cost thousands of dollars, butunsafe wiring can cause fires, complicate estate transactions, and makeinsurers skittish.
Homeowners should carefully consider their options before deciding whether to rewire their house.
The homeowner or an electrician should carefully remove any insulationthat is found surrounding K&T wires.
Prospective home buyers should get an estimate of the cost of replacingK&T wiring. They can use this amount to negotiate a cheaper price for the house.
In summary, knob-and-tubewiring is likely to be a safety hazard due to improper modifications and theaddition of building insulation.