Knob and Tube Wiring

Knob and Tube wiring was one of the earliest techniques used to distribute electricity in our homes. It’s referred to as Knob and Tube because of the ceramic insulators that were used to pass the wire through and around obstacles. The Tube lined a hole through a stud or joist and the wire fed through it to protect the structure from contact. Knobs were used to secure the wire and at points with changes in direction. The wire itself was a product of the times; copper with a cloth woven covering impregnated with a waxy substance to protect the wires from moisture. Unlike modern wiring, each conductor was a separate wire and there was no ground wire.

Aluminum wires in electrical panel

The first installations were all retrofits and the wiring was mounted on wall surfaces. As new homes were built knob and tube wiring was incorporated into the wall structure like today’s wiring is. Since using electricity was new, there were very limited uses for it and the amount of current supplied was small, typically 30 amps. Today’s homes use at least 100 amps and in those of any size 200 amps is common. As new electric appliances, televisions, computers and other devices came along electrical services were upgraded in older houses.

It wasn’t overly difficult to upgrade the electrical service and distribution panel, but the existing distribution wiring hidden in the walls was a greater challenge. The effort and disruption of opening walls to replace the wiring was more than many homeowners were prepared to undergo. Often older houses developed into hybrid systems. This left sections of Knob and Tube wiring in some of the more difficult to access original sections of the home and updated wiring was used in remodelled sections and new additions.

This is part of the reality that home buyers face today when purchasing homes that predate about 1940. Somewhere in that building one or many runs of old Knob and Tube wire may still be hidden in the wall. It’s not that the wiring doesn’t work, but it doesn’t meet modern electrical and safety standards. Amateur connections between old and new wiring can be problematic and because the presence of Knob and Tube is considered undesirable, old wiring is sometimes left hidden and goes undisclosed. To be fair, the current owner may have no idea that it’s there.

Ultimately this falls into the risk that a home buyer accepts when they purchase an older home. Along with the charm comes the workmanship and technology of the day. As Home Inspectors, we look for signs of Knob and Tube wiring, but it’s a visual inspection and we can’t open up the walls to satisfy our curiosity. Another reason that Knob and Tube wiring becomes relevant at the time of purchase is that insurance companies are none too fond of it. You may have fewer companies to choose from, and those that do cover it, may do so at a higher cost.

The purpose of this article was to help you understand Knob and Tube wiring and the issues it raises in buying older homes. Wiring issues can be always be addressed with enough effort. It is just one of many issues to be considered and a licenced electrical contractor should be consulted when it is discovered. Hopefully the remaining use of Knob and Tube is limited and the associated costs to upgrade will be manageable.


Author: Rob Cornish is a Home Inspector in Ottawa, Canada. © 2013 HomeXam Inc.
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