Why Knob & Tube Wiring Is Sometimes Referred To As Open Wiring And Other Wiring Anecdotes

Knob & tube wiring was originally installed in homes commencing around the turn of the 20th century and was used right up until World War II. It is also referred to as open wiring because it needs a certain amount of open space around the conductors to dissipate heat generated during its use. This led to a huge problem during the energy crisis of the 1970’s when everyone jumped on the over-insulate bandwagon. Wires that were designed to have circulating free air around them to keep them cool and safe were suddenly encapsulated in spray foam or layered insulation causing an instant fire hazard.

In our local trading area we often find a strange wire type during whole home inspections or rewiring projects. It’s mostly found in homes built right after World War II up to 1949. The wire has a tough plastic outer jacket that has no markings on it. We’ve seen it in two wire and three wire versions. Inside the two wire jacket is a black and a brown wire and inside the three wire jacket is black, brown and blue. Turns out that during that war there was an aircraft manufacturing plant in our community. After the war was over all the “surplus” wiring “found” its way into local folks’ home wiring projects. It’s usually pretty crispy when we find it because it has been running at a voltage 20 times its rating for 60 or 70 years.

Most people have heard of putting pennies behind the fuses in old electrical systems so the fuse won’t blow. One time we found the main fuses in a 60 amp service replaced with cut-to-fit pieces of 1/2″ copper water pipe. Creative, yet deadly.