Is Knob and Tube Wiring Safe?

History of Knob and Tube Wiring

Knob and tube wiring is the electrical system that was generally used in residential structures built before 1930.  The system contains cloth wrapped copper conductors routed through ceramic tubes when the wire passes through the wood framing and around ceramic knobs to support the cables at regular intervals. The safety of old knob and tube type wiring is a common question that comes up during a home inspection.  

There are some serious safety issues to consider.

The first is it has only one insulating layer around each conductor. Newer wiring methods will typically have a layer of insulation around each conductor in addition to an outer sheathing of plastic or metal to protect all the wires.  This results in at least two layers of protection with newer wiring as opposed to only one with knob and tube wiring. 

In addition to the lack of protection around the conductors with a knob and tube setup, the wiring is often more than 100 years old and is routed in very hot locations.  The passage of time and exposure to heat breaks down the single layer of insulation. It is common to find exposed bare metal conductors within this type of system. This, of course, is a very dangerous situation as the bare metal can shock or electrocute people.  Additionally, the bare metal can spark result in a fire. 

Other Safety Concerns.

Other safety concerns with this type of wiring are the lack of a grounding system. Without a proper ground you have a greater risk of being shocked and damage to your electronics from a power surge. The outdated layout of the circuits throughout the house and the possibility that improper modifications have taken place to the system over the years can create issues that might be unknown when buying a house.   

As a final downside to this wiring, homeowner’s insurance companies are becoming increasingly less tolerant of this old system due to these safety concerns.  Most companies ask if this system is present in a house they are insuring, and some charge an additional fee. Some insurance companies flat out refuse to offer coverage on older homes with this type of old wiring.  In the years ahead, the problems obtaining insurance with this type of wiring will likely increase and, at some point, it may become impossible to get coverage. 

Given the potential fire and safety risk, plus the potential hassles obtaining insurance, it is my opinion that knob and tube wiring is unsafe. I encourage people to hire a qualified electrician and get it replaced.

Contact Crawford Inspections to check to see if your house has this issue.