What is knob and tube wiring?
You will not see any k&t wiring in newer homes. However, if your home was built pre-1950, take a look in the basement as you likely had k&t. Look for wires running through porcelain cylinders or “tubes” inserted in holes in the wooden floor joists. There are also porcelain “knobs,”. These keep the wires secure preventing them from touching the wood along which the wires run. The wires are usually insulated with a rubberized cloth fabric.
Knob-and-tube (K&T) wiring was an early standardized method of electrical wiring in buildings. It saw common use in North America from about 1880 to the 1940s. The system is considered obsolete and can be a safety hazard. However, some of the fear associated with it is undeserved as you’ll read below.
One of the main differences between knob and tube wiring versus modern wiring is the lack of ground wire. Because of this, k&t wiring cannot accommodate any electrical items with three pronged plugs. Additionally, the risk of shocks and fire is much greater. Secondly, the material used to insulate the wire is different. Modern wire uses plastic where k&t uses rubber. The rubber breaks down over time and becomes brittle which is frequently the reason for overheating or mechanical abuse.
Issues with knob and tube wiring
Insulation over the wiring:
With the wiring being coated in a rubber/cloth insulation, it needs lots of space to dissipate the heat that builds up when an electrical current is flowing through. If these wires are surrounded by housing insulation, it is a fire waiting to happen.
When knob and tube wiring was first used there were very few electrical appliances in the average house. Nowadays, with people having more appliances on a circuit, the system can easily become over heated. These k&t systems where never designed to handle the demand for electricity in the modern world.
As it’s such an old system, proper replacement parts are not always available. This could be the reason a lot of makeshift handyman fixes are so dangerous. Knob and tube wiring is typically easily accessible in the basement. This is perhaps the reason why this wiring is often spliced unsafely with modern wiring by home handymen. We always recommend you get a certified electricians.
Serious problems can occur if this type of wiring is damaged. This damage could be from either due to wear and tear, handyman fixes, or other types of damage. Porcelain knobs and tubes can crack, and the wires tend to sag and fray over time exposing live wires.
As mentioned above, the rubberized cloth insulation on k&t wiring becomes brittle over time, and can flake off. This then leaves the bare wire exposed.
Possible issues obtaining home insurance
When shopping for home insurance, the insurance company will always want to know what time of wiring your home has. Some companies will refuse to insure you if your house has k&t wiring. This is because they consider the risk to be too high. However, there are companies that will insure you home but at a higher premium or deductible. Additionally, they may require an electrical inspection done by a professional before your insurance company will offer coverage.