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Knob and Tube Wiring

Please read this section if you don’t know if the electrical service to your home has been updated; it is a safety issue to install insulation over active knob and tube wiring.  Even if you see knob and tube wiring in your attic, it may not be active; the old wiring is not always removed when the electrical is updated.

Knob and Tube Wiring

K&T wiring is a type of electrical wiring popular from the 1880s through the 1930s among electricians in the US. Some professionals used this method through the early 1970s. This wiring method is easily identified by its design: a series of ceramic knobs holding black ceramic tubes, which encase the electrical wires. Since the 30s, K&T has gradually been replaced by wiring methods that can accommodate increased power demand in the home, and allow for effective home insulation.

The Basics: The K&T wiring method runs two wires (one hot, one neutral) through an encased insulator tube, which is then fastened along walls, floor joists, and ceilings by ceramic knobs. Unlike today’s electrical systems, which bundle all wires together, the hot and neutral wires are cased separately. K&T wiring also does not include the additional “grounded” wire that is standard practice today. Grounded wires help prevent shorts, electric shocks, system overloads by providing back up if the neutral wire fails. K&T homes will therefore likely have two pronged outlets instead of three-prong. Note: The presence of three prong outlets does not necessarily rule out the presence of K&T wiring in your home. As will be mentioned in more detail in the “Key Issues” section below, contractors can install Ground Fault Circuit Interrupters (GFCIs), which then allow for a three pronged outlet per GFCI.

Key issues with K&T: A number of issues have arisen with K&T as people in the field have learned more about it. All of the problems listed below can result in shorts, and potentially fires. That being said, if the K&T wiring is fully in tact there is no immediate risk presented to your home and family, so no need to go on red alert after reading this.

Here are the primary problems to be aware of:

--As with any system, K&T wiring deteriorates with age. Even in a home where the K&T wiring was installed toward the end of the method’s popularity, that means the wiring is – at its youngest - 45 years old. A common integrity issue that can occur during this time is wearing down of the casing, which is in K&T systems is rubber instead of the plastic wire casing used in electrical work today. The bare wires, exposed to air and moisture, present a significant risk to shorting and fires.  

--Adding circuits beyond the 12 per home that most K&T systems support is and has been a fairly frequent albeit unsafe practice. As the power needs of homes surpassed the 60 amp service size that typical K&T systems supply, homeowners have paid to add more ways to draw power to run appliances, electric heating, etc by means of cutting into an existing wire to add an outlet or adding new circuits directly at the panel. Standard service sizes today are around 100-200 amps. (Service size denotes the maximum current you can draw through your electrical system at one time. The more high-power appliances you have the larger your service size you will need so that you can, for example, run a dishwasher, laundry machine, and blender at once. Homes with lower service sizes will often short out when these appliances are simultaneously run. A home with electric heat needs a larger service size than a gas heated home). Adding more circuits that draw power from the supply without increasing the service size can overload the system, again potentially leading to shorts and fires.

--In addition to the homeowner’s desire to have more power supply, unscrupulous contractors have tampered with and overloaded K&T wiring to avoid the time, cost, and labor of rewiring a whole house. In these cases, the contractor might install a very small amount of new wiring that connects to K&T at the ceiling level so that it passes home inspection.

--Finally, K&T presents a liability issue. Some insurance companies will charge more for known K&T wiring, and others refuse to insure homes with it at all.

Insulation & Knob and Tube Wiring:The design of K&T wiring requires open space around the wire tube to properly cool. This directly conflicts with the design of home insulation: filling the wall cavities, sealing gaps and cracks, and controlling airflow to minimize heat loss (or heat gain during the hot months). If the two are combined, a home is at extreme risk for fire because the wires cannot cool. A knowledgeable and ethical installer, after observing K&T in your home, would not proceed with insulation until active K&T is replaced.

That said, only active K&T presents this risk in combination with insulation. Some homes that still contain K&T will have been completely rewired with a new system, but not removed the old K&T wires. In this situation, while the wires are still present they are inactive - no longer carrying power through them. Insulation MAY BE installed with inactive K&T because those wires do not generate heat anymore.

What do I do once I know my home has Knob & Tube? Call a qualified electrician (ask us for a recommendation if needed) to see if your K&T is active. After they establish if it is active K&T, they will be able to recommend next steps and give you a quote for rewiring your home.