Knob and tube wiring (also known and K&T in some circles) is a method of electrical wiring that was standard practice through most of the United States during the period of the 1880s thru the 1940s. This method of electrical wiring gets its name from two of its most identifiable components, the knob and tube.
The knob refers to the ceramic cylinders that you’ll often see nailed directly into floor joists or wall studs to act as anchors for the wiring. Typically, a small groove is found in the circumference of the knob, acting as means to more securely fasten the wire to the cylinder.
The tube refers to the hollowed-out porcelain tubes that were used as insulation for wires passing through floor joists and other potentially flammable materials. Sometimes these cylinders were also stacked on top of one another to keep wires from touching one another.
Much of the wiring itself was wrapped in a natural rubber insulation, differing from advanced insulating plastics that are used today.
Knob and tube wiring practices became largely obsolete as electrical practices advanced and efficiency and safety standards grew. There are still instances where knob and tube wiring is used, it’s rare and only used in unusual agriculture or industrial situations.
The overly simplistic answer is, “It depends.” Are all the components — the knobs, the tubes, the wires and their insulation — still intact? If that’s the case, then there are not necessarily critical threats posed by your knob and tube wiring. However, there are important details to consider. Knob and tube wiring is a technique that has not been employed as a standard by electricians for seventy years.
The ceramic and porcelain knobs and tubes can degrade and become cracked, diminishing their ability to properly and safely support and route the wires. More commonly, however, is the degradation of the wiring’s rubber insulated coating. Over time, rubber can dry out, crack, and disintegrate, exposing bare wires and increasing your risk of an electrical shortage or even fire! Exposure to air and moisture increases this risk, and with Portland’s high-precipitation climate, it’s something we should all be aware of.
Additionally, there are hazards associated with having knob and tube electrical work near fiberglass insulation. There are also risks in attaching new electrical work to old knob and tube systems.
The risks associated with knob and tube wiring aren’t limited to safety hazards. There are also financial and insurance-based implications to consider, as well. Some insurers may (sometimes dramatically) increase premiums for homeowners looking for coverage for their knob and tube-equipped homes. There have even been instances where insurance companies have refused to provide coverage altogether until homes have been rewired to present-day codes! And while this may not affect every homeowner…it could if an inspection reveals issues.
The solution to K and T remediation can involve deactivating and/or physically removing the specific circuits that have K and T and rewiring with code compliant wiring. Depending on the situation, K and T wiring can be left in place and new wiring run to the fixtures and/or outlets where K and T is currently connected. If walls or ceilings are being opened for a remodel, the old K and T can be removed while new wiring installed. New wiring can also be “fished” behind existing walls without removal of the wall.