Updated: Apr 6, 2022
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.
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 treated differently in different jurisdictions. In some areas, it must be removed at all accessible locations, while others merely require that it not be installed in new construction.
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 lifespan.
The original installation of knob-and-tube wiring is often superior to that of modern Romex® wiring. K&T wiring installation requires 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, the 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 contain 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.