The best way to tell if your home or home you are interested in has knob and tube wiring is to thoroughly inspect the basement and attic wiring. Usually, you can spot the ceramic knobs nailed to the joists and wires snaking through the tubes. K & T wiring (knob and tube wiring) was commonly used in North America from 1880 to 1940. It is the oldest wiring method found in American homes. When left in its original state it has proven to be reliable and safe. As a wiring method in uninsulated joist and stud cavities it is protected from damage and allows for proper air circulation. When first installed K & T wiring maintained a 1-inch clearance of the wood framing and the tubes isolate conductors when passing through wood. Over the years though many K & T wired homes have become compromised by home modifications, such as the installation of insulation.
When many homeowners or prospective homeowners find out that they have K & T wiring, they often have many questions and concerns. When dealing with this type of wiring it is imperative to maintain distance with the knobs, tubes and wires. Do not envelope the area with a type of thermal insulation. You should maintain 3 inches minimum between wires and 1 inch minimum to all surfaces. You should provide protection when K & T wiring is exposed less than 7 feet above the floor. You should protect with running boards up to 7 feet high in attics with stairs or a permanent ladder. It is okay to extend K & T wiring to other wire methods with proper splices. Splices need to be in a box. Bushing is okay only from the raceway to open equipment.
When I come across knob and tube wiring in a home inspection, I always recommend an evaluation of the electrical system by a licensed electrical contractor to ensure safety. It is also recommended that you contact your homeowner insurance company immediately to determine the company’s underwriting standards for K & T wiring. Your insurance company may require home rewiring before issuing a policy. Rewiring a home can cost anywhere from $10,000 to $50,000 depending on the size of the home.