Knob and Tube Wiring

Knob and Tube wiring is a method of electrical wiring where the conductors that provide energy to the electrical outlet or light fixture is run on two insulated conductors that are kept separate as opposed to two insulated conductors that are enclosed in the same cable sheath as is the current standard practice.

I recently saw a movie from the 1960’s depicting a scene from the 1930’s where the energized conductors were run on the surface of the wall, but that is the only instance I have seen them run in that manner. Knob and tube wiring is always (or nearly always?) concealed in the walls or in the attic spaces.

Where the conductors go through a stud, or a piece of wood, they are insulated with a short tube of ceramic, and where the wires are run along a board, or in the attic they are supported by ceramic knobs that keep the wires from touching the material that they are adjacent to. This is where the name Knob and Tube is derived.

Another name for Knob and Tube wiring is Open wiring. Open wiring is an acceptable wiring method in the current Canadian Electrical Code with some modifications on the original installation practices, but it is not used in contemporary installations.

In buildings that were originally wired with this Knob and Tube wiring, there are some risky characteristics.

One is that splices may be made in the wiring without a junction box within the wall or attic space. The wire being branched off at the junction is wrapped around the first wire like a vine around a tree branch, and was then soldered and covered with an adhesive fabric tape.

Another is that where the conductors terminate in a device, original installations didn’t make these connections in a fire resistant box, but rather simply ended at the device and had the conductors supported by a close adjacent board with insulated tubes inserted in the holes.

These two characteristics are risky because in the case of an overload on the circuit the increased current causes heat at the points of higher resistance, such as at splices in the circuit conductors and at the device that the conductors are terminated at.

In today’s wiring practices these points are contained within boxes that are resistant to heat to prevent fires.

A combination of a century of electrical upgrades and home owner jury rigging and the inherent risks of the original Knob and Tube installations, this wiring in homes today doesn’t meet modern safety standards. This is reflected in the insurance premiums on houses with Knob and Tube wiring and the growing reluctance of insurance companies to insure homes that still have this wiring in them.

The fix for Knob and Tube wiring is to decommission it, and replace it with new wiring done to current electrical standards. The size of this job is dependent on the amount of wiring in your house that needs to be replaced and the layout of your house. As the approaches to replacing it are numerous, and the job can be complex, it is to your advantage to have a smart electrician on hand for this work. Of course, I would like to think that I might be included in that statement.