What is Knob & Tube Wiring: Is It Safe?
Knob and tube wiring was the first residential electrical wiring system. It was introduced in the 1880’s after Thomas Edison’s 1879 invention of the long lasting electric light bulb.
Knob & tube remained a common method of wiring homes until the 1940’s in most of the U.S.
Spoiler Alert: It is obsolete and unsafe by modern electrical standards.
Even though knob & tube hasn’t been widely used in 80 years, it is still found in many homes.
Wondering if you have knob and tube in your home?
Knob and tube is easily identified by it’s white ceramic ‘knobs’ and white ceramic ‘tubes’ (that penetrate walls) and hold insulated copper wire.
Safety Concerns with Knob & Tube Wiring
I receive some form of this comment on a regular basis…
“This house has had knob and tube for over 100 years and is still standing. So it’s fine”
Here’s the problem.
- Past results do not predict future performance.
- Homeowners all use their house differently. The family that owned the house for the last 20 years may never have used a particular room or outlet. A new homeowner might use that room or outlet on day one, and put it under a heavy electrical demand.
Here is a partial list of the reasons knob & tube wiring is dangerous:
- No ground wire for protection
- Wires can overheat if in contact with insulation (they usually are)
- Splices/modifications must be performed in a specific manner (and often aren’t)
- Insulation can become brittle, exposing live wires
- It can’t get wet without becoming a fire hazard (are you 100% in control of leaks as a homeowner?)
- It’s old and everything has a lifespan
Due to these concerns the best course of action is to replace this wiring. However, an electrician may decide the system is acceptable if their evaluation determines it’s in good working condition.
What is the cost to replace knob and tube wiring?
Knob & tube is still lurking in many homes in the city of Syracuse built before 1940. It’s common for knob and tube to be visible in the attic only. When this is the case, some of the time the knob and tube has been replaced every else in the home except for a single circuit(wire) providing light to the attic.
The expense to correct a single circuit will be low. But the electrical system should still be evaluated by an electrician to verify that there is only one circuit.
The cost to replace knob and tube wiring varies widely based on:
- Square footage of the home
- The amount of knob and tube remaining in the house
- Method of replacement – Do walls need to be dismantled? Is surface mounted wiring an option?
Depending on these factors the cost could range from $500 to $20,000. The only way to get an accurate price is to consult with an electrician for the specific home.
One last reason why replacement is the way to go. Many insurance companies will not insure a home that has knob & tube wiring. Those that will typically charge a premium.
Learn more: https://inspectapedia.com/electric/Knob_and_Tube_Wiring.php
Legal Stuff: Any references to code are to IRC. Check with your jurisdiction to see local regulations which may differ. Homeowners should never perform repairs, maintenance or inspections that they are unqualified to perform. It’s always a good idea to seek out a qualified professional for help.
Scott Brown is a New York state licensed home inspector. He is an alumnus of Syracuse University and has worked in the construction/inspection industry for more than 10 years. He grew up working on remodeling projects around the house with his dad, much to his displeasure at the time. Scott is the owner of Brightside Home Inspections in Syracuse, NY.