What is Knob & Tube Wiring?

Knob & Tube Knob & Tube wiring was an early type of electrical wiring widely used until approximately 1950 when it was completely replaced by new wiring systems. It gets its name from its distinctive installation method:

• When running along flat surfaces, the wires are anchored by white ceramic pieces called "knobs" nailed to walls or ceilings.
• When passing through wooden beams, white ceramic tubes called "tubes" are used to protect the wires where they pass through drilled holes.

This installation method differs significantly from modern wiring systems where wires are stapled directly to surfaces with U-shaped clips, and pass through beam holes without any protective sleeves.

Another characteristic of Knob & Tube wiring is that its hot and neutral wires run separately without a ground wire. The wire has a copper core, insulated with rubber, and wrapped in fibric cloth. In contrast, a modern wire consists of a group of wires enclosed in a single PVC jacket containing a black hot wire, white neutral wire, and bare ground wire. The individual hot or neutral wire has a copper or aluminum core, insulated with heat-resistant plastic in black/white/red/yerrow/blue/green colour. These visual differences make it fairly easy to distinguish Knob & Tube and modern wiring.

Knob & Tube wiring was once used worldwide, not just in Canada. Similar wiring and installation methods can still be seen in many rural areas of China today. When originally installed properly and not modified unprofessionally, this wiring system can still be quite safe without major issues.

Disadvantages of Knob & Tube Wiring

First disadvantage: There is no ground wire. Knob & Tube wiring has only a hot and neutral wire to form a circuit. This makes it unsuitable for wiring three-socket grounded outlets, but poses no problem for wiring two-socket ungrounded outlets. Many modern appliances like computers and refrigerators require ground protection with three-prong plugs, while others like lamps and TVs don't need grounding with two-prong plugs. This disadvantage isn't unique to Knob & Tube wiring. For over a decade after Knob & Tube wiring was phased out until the early 1960s, wiring systems still lacked ground wires and used only two-socket outlets. This issue can be mitigated by adding ground wires or replacing some outlets with GFCI outlets.

Second disadvantage: The wire splices aren't contained in junction boxes. Instead, wires are twisted or soldered together and wrapped with electrical tape. These joints might be held by ceramic knobs or left hanging between support points. Over time, the tape can deteriorate and loosen. If the wires are disturbed, splices can easily separate - posing a safety risk. This connection method is clearly less reliable and difficult to modify. Modern wiring uses wire nuts to connect splices inside enclosed junction boxes, and the wire is clamped at the entry point to prevent damage and movement, making connections far more secure.

Main Problems with Knob & Tube Wiring

The primary issues with Knob & Tube wiring don't stem from the system itself, but from subsequent improper modifications. Older homes typically had very few circuits and outlets, inadequate for modern needs. Homeowners often added new circuits or outlets to the original system, making connections however they pleased. This can lead to loose or overheated joints, or even fires.

Additionally, overloaded circuits frequently blew fuses, prompting homeowners to replace original 15A or 20A fuses with 25A or 30A fuses. This can cause chronic overheating, which degrades insulation. When disturbed, the insulation may crack or break, exposing live wires - an extremely hazardous situation. During inspections, we frequently find exposed splices where tape has fallen off. Based on our experience, modified Knob & Tube systems do present significant fire and safety hazards.

Insurance Company Responses

Insurers tend to be very cautious about Knob & Tube wiring. Some will send inspectors before approving coverage, others require safety certification from electrical authorities, while some refuse coverage entirely or charge much higher premiums. If you're considering purchasing a home built before 1950, try to determine if it contains Knob & Tube wiring. If it does, be fully aware of potential insurance challenges.

Homeowner Strategies

During inspections, we often find Knob & Tube wiring in older homes. Since we only perform visual inspections, we usually can't determine the full extent of such wiring or the condition of hidden sections. If your home is over 60 years old, and you're unsure about its wiring - especially if you are planning a renovation - we recommend you hire a licensed electrician to inspect it thoroughly, and make necessary upgrades.

This article was originally published in "Real Estate Weekly", Issue #133, July 27, 2007.