View Full Version : Identifying Knob & Tube Wiring

10-24-2007, 07:26 AM
We're thinking of buying a two-story house built in 1940 which has a little knob and tube wiring in the basement, but most of the visible wiring is romex-type cable (there have been a lot of additions and modifications to the house after it was built). It has a 100 amp main breaker panel with breakers, not fuses, and there's only romex-type cable coming from the panel. The current owners (not the original owners) don't know how much knob and tube wiring is in the oldest part of the house, or whether it was ever replaced. The insurance company we want to use, USAA, says it won't insure the house if there is any "active" knob and tube wiring anywhere in it.

Is there an easy way to tell how much active knob and tube wiring is behind the walls, in the ceiling, etc., without cutting holes in the plaster? Unfortunately the attic above the original part of the house is floored, so we can't easily examine the second story ceiling.

I was wondering whether one could take the cover plates off switches and outlets and examine the wiring coming into the boxes and reliably tell whether it was supplied by knob and tube wiring.


Ian Gills
10-24-2007, 07:56 AM
If you are allowed to do it then your suggestion will probably be the best way.

I think your concern is justified. There is chance that the more modern wiring has been installed in the more accessible areas and then just tied up to old wiring in the inaccessible ones. Definitely worth a check if you or a qualified electrician are allowed to do so.

I wonder if it might also be worth checking for ground on the receptacles? If they are all two-prong then this might be an indication that there is old wire servicing them. An open ground on a three-prong receptacle would also raise my suspicions. Similarly if you see three-prong GFCI receptacles everywhere (not just around water areas) then this might be another give-away.

What do the pros say?

10-24-2007, 08:04 AM
Can someone describe what I'd look for in the boxes to identify Knob & Tube?

10-24-2007, 09:00 AM
Do the smart thing before buying a home.

Hire a Home Inspector to evaluate the home for hidded flaws in all the aspects of home building.

If the house is over 60 years old there could be more to worry about than knob and tube wiring.

10-24-2007, 11:28 AM

I kind of disagree with you. While I think most non-knowledgeable people are wise to get an inspector I don't think most are worth thier cost.

My buddy recently bought a house and the inspector didn't note or say anything about the electrics just dropping down from the joist with no conduit and the lack of a wiring clamp going into the water heater. In addition the inspector missed the fact that front of the tank was rusting indicating a leak. Also didn't make any notes about the lack of sealing on the duct work. The inspector also failed to confirm that all electrical was working etc..


10-24-2007, 12:21 PM
And knob and tube in a 60 year old house? Are you sure this is right? I thought K&T was not used much after the 1930s?

A Knob and Tube installation was addressed in the 1971 cycle as circuit extensions only.
This rule remains in effect through the 2005 cycle and is unchanged for the 2008 cycle page 236 of the 2008 NEC

The 1968 cycle allowed the installation of Knob and Tube wiring in a new installation, Article 324 section 324-2 use page 116 of that NEC.

When seeking a good Home Inspector I recommend using this site (http://www.nachi.org/forum/) and clicking on the information found in the top left corner of the web page.

10-24-2007, 01:47 PM
Great link. Thanks


10-24-2007, 04:50 PM
Can someone describe what I'd look for in the boxes to identify Knob & Tube?

First clue: No ground.
Second clue: All wires are black.
Third clue: Wires enter box with a thick fiber-like sheath, called loom, around it.

10-25-2007, 01:04 AM
Sounds like an unreasonable burden being placed on you by the insurance company. You will have a damn hard time verifying that there is no knob and tube active anywhere in the house. I did not realize it was still in use in the 40's. A knowledeable inspector is necessary here.

some external indicators could be:
knob and tube visibly coming out of the fusebox. (:rolleyes:)
bakelite surface mount baseboard receptacles
bakelite surface mount switches
ceiling lights that turn on only by pullchain

all indicators of a very old electrical retrofit.

the invasive approach means looking at the wires entering boxes. Individual, black cotton covered wires means knob and tube is used.