Background: Knob and tube (K & T) wiring is a hot topic in the real estate industry, and a common concern uncovered during a home inspection. K & T wiring derives its name from the use of the porcelain knobs and tubing that were used to secure the wiring and protect it as it passes through building materials. It was commonly installed in U.S. homes through the 1940’s and is still found in many older homes today. Knob & Tube wiring is obsolete, but not inherently dangerous. However, there are concerns and considerations for a prospective buyer of a home that has K & T.
K & T is different than modern Romex wiring in that it does not have an equipment grounding conductor (ground wire). The ground wire is used to reduce instances of electrical fire and damage to appliances. This means that receptacles fed by K & T cannot safely serve appliances that require grounding (three-prong plug), such as most refrigerators, window air conditioning units and many others. This means that it is no longer suitable for use in places like kitchens, bathrooms, laundry rooms, exterior, etc..
Age is a concern. As stated, most of the K & T found in homes today is 80+ years in age. The wiring is often brittle, sagging or stretched. Oxidation can accumulate on wiring.
K & T is often found to be improperly altered. Over time, homeowners tend to modify the wiring to suit the increasing need for electricity. This often leads to problems due to un-workmanlike alterations to the wiring such as poor splicing, improper terminations at junction boxes, receptacles and appliances.
As homeowners add insulation to increase energy efficiency, dangerous situations may arise with K & T wiring. K & T wiring dissipates heat to the surrounding air. If insulation is added to wall cavities, the attic floor, or other areas, it often encapsulates the wiring. This diminishes the wiring’s ability to dissipate heat and can result in over-heating.
Insurance companies don’t like it. Many companies will not insure the home. Other will offer insurance, but with an increased cost.
Replacing the wiring is expensive. Re-wiring can run into the 10’s of thousands of dollars. It is an invasive process, and can be time-consuming.
Summary: While it is no longer used in residential construction, there is no building or electrical code that requires it to be replaced (in most areas). If properly installed, maintained and utilized, the risks are low. However, there are several reasons to consider replacing it, as stated above. My recommendation is to have it reviewed by an electrician who is able to evaluate the wiring and connections more thoroughly than what is performed during a general home inspection. They should assess the condition of the wiring, connections, and any alterations to ensure that it is safe, or recommend replacement.
Author: Ernest Homer, Homer Inspection Services