Knob and tube electrical wiring questions & answers:

Frequently-asked questions (and answers) about knob and tube electrical wiring.

This article series answers questions about Knob and Tube electrical wiring. We define knob and tube wiring, we include photographs that aid in recognition of this generation of electrical wiring, and we describe both proper and improper K&T wiring installations, repairs, or circuit extensions.

This website provides information about a variety of electrical hazards in buildings, with articles focused on the inspection, detection, and reporting of electrical hazards and on proper electrical repair methods for unsafe electrical conditions. Our page top photo shows a home inspection client pointing out knob and tube electrical wiring in an older home.

Knob-and-tube wiring questions & answers

Knob and tube electrical of wiring has been installed in homes from the 1920s right up into the 1970's in some jurisdictions. The photo shows knob and tube electrical wires passing through a wall top plate in a 1920 New York home.

These questions & answers about knob and tube electrical wiring were posted originally at KNOB & TUBE WIRING - be sure to review the advice given there.

[Click to enlarge any image]

On 2019-06-14 by (mod) - using 2-wire circuits with no ground

Basically if the line cord on your appliances a three-pronged plug then it wants to be in a grounded receptacle. If the line port on your appliances a two prong plug them it's not looking for a ground and so you're not getting anything up by using it on a knob and tube circuit. Of course that is assuming that the circuit is safe and has not been damaged or is not unsafe for some other reason.

On 2019-06-13 by Joe

Is it still considered acceptable to plug an appliance with a 2 prong cord into an ungrounded 2 prong outlet with knob and tube wiring? If so, where is the line between what is ok and what becomes questionable?

On 2019-06-07 by (mod) - If the neutral wire in an electrical circuit is cut one

Christine

If the neutral wire in an electrical circuit is cut one thing that will certainly happen is that the circuit will cease to function. That's because there's no return path for current to the electrical panel and through the panel to ground or Earth.

What could also happen depending exactly on where the cut ends of wires why is a short to ground between the neutral wire and for example a pipe that's connected to Earth. And that case the circuit might appear to continue to function but it would be unsafe. And in fact someone touching that same pipe when the circuit is active could be shocked. In the ARTICLE INDEX you will find articles on Lost neutral wire and its effects.

On 2019-06-07 by Christine

What happens if the nuetral wire gets cut

On 2019-05-31 18:16:25.443361 by Anonymous - wants to insulate knob and tube wired wall cavities

I'm sorry, this is coming from the vantage point of the installer and manufacturer. My thoughts are as such...

Currently there is no theory, proven or dis-proven only the speculation of amp draw and thermal dissipation. I think we can all agree should a load be placed on a circuit beyond its capacity and that of the wire it will fail despite dissipation, its the casing of the wire be it plastic or fiber dipped that's the source of the initial flame.

In an open air cavity that flame then has the oxygen necessary to continue to burn until such time it reaches another ignition source to spread the flame.

So in theory until such point its tested, one would think, if that wire is deprived of oxygen being encased in fire rated foam should it fail, the flame would be quickly extinguished. In this scenario, if a valid theory then the cost associated with such testing would make sense?

On 2019-05-31 16:24:43.371465 by (mod) -

Interesting and reasonable plan. Unfortunately from the little I know about valid, defensible, authoritative electrical testing Vis a vis standards, the cost to set up a test condition and run it particularly one that included the varieties of condition in which old electrical wiring and knob and tube wiring may be found, might be greater than the cost of a house.

I would therefore consider a different approach. Unless we have the funding for such an interesting study, which might come from the foam manufacturers, we need to instead solve your immediate problem which is how to insulate the walls and have safe electrical wiring. That was the basis of my earlier suggestion of abandoning knob and tube circuits, routing new circuits to the same area through an accessible path, and then foaming the walls as you like.

On 2019-05-31 13:33:13.446490 by Anonymous

The product is USA Premium Foam. The product has been installed in these types of homes for over 30 years with no documented adverse effects ever reported certainly never associated with a fire. From a chemical composition it is essentially a non flammable plastic with Air injected.

I would like to explore an option for testing the product in an environment replicating K&T overloaded beyond its limits and record the results, open air overload to failure point and also foamed cavity overloaded to failure and record the results, then propose the test being performed by an independent laboratory do certify the results. The issue as it stands, there is no actual documentation outlining and interactions between insulation and K&T, seems just speculation at this point...

On 2019-05-31 03:05:26.490283 by (mod) -

I wanted to add that if it were my building I would consider adding insulation after disconnecting any circuits that I was not going to replace. You can certainly add replacement circuits external for the wall if necessary

On 2019-05-31 03:04:32.790449 by (mod) -

That's a very interesting question. I think there are st least two different investigations that are needed.

Does the manufacturer endorse the use of their product in the knob and Tube wall wired case that you describe?

And is the insulation and heat heat dissipation characteristic improved over k&t wires in open-air?

Also it occurs to me that an insulation contractor ought to be reluctant to use an off-label application of their foam given that they won't want to accept liability for what could happen in insulating of a wall cavity where the K&T wire condition is in fact unknown.

Have you found any research papers describing the situation of Interest? Let's both see what we can find. It will help if you can give a specific product name.

If we can find research supporting the claim that the wiring is actually made safer that would be a very important discovery.

On 2019-05-29 18:13:47.090272 by Rich White

Is there any reason why an amino-plastic fire rated low density foam could not be installed in a retro-fit fashion in an exterior wall containing K&T? The product absorbs and dissipates heat at a high rate, class A product with a 2 hr fire rating. There does seem to be a lot of speculation, lack of education and knee jerk reaction to K&T so as a result a lot of misinformation. Im looking for clarity, do I insulate and conserve energy, or forgo the insulation as rewiring is cost prohibitive?

Thanks in Advance

On 2019-01-09 03:44:15.851108 by (mod) -

Yes you can add GFCI protection to a circuit that has no ground. However some of the tests that would be used to test the GFCI may not work is some of those rely on creating a brief short to ground and there is no ground.

On 2019-01-08 22:48:41.437501 by Bob Whitehair

Can I add a GFCI breaker to a knob and tube circuit. The main service panel has been upgraded to 200 amps and the knob and tube circuits are connected to breakers. Fuses are gone

On 2018-12-26 05:52:41.443064 by (mod) -

Modern standards would require probably having a junction box and splicing in wire in better condition. What the old guys would have done if the copper was intact would have been to wrap the wire with cloth based electrical tape

On 2018-12-23 02:42:29.396547 by Robert in Oakland

Hi…someone had made some inline splices into my K&T, and as it was a bit sketchy, I removed the splices. Each K&T wire was left with around 1/2" of exposed bare wire where they had cut/removed the insulation in order to splice things...and my question is, how best to repair.

What I did was: Wrap the wire with 3M Super 33 tape (I went around 1" past the exposed wire on each side), cut a piece of loom length-wise and wrapped it around the wire for protection, and then taped the entire repair with friction tape. Yes, I'm pretty OCD ;)

Is this an appropriate repair? For some reason I was thinking it could be too substantial and heat could develop, but in truth, it's in a very cool crawl space...probably always 58 degrees down there.

Thanks for your advice-
RT

On 2018-12-05 13:59:33.377837 by (mod) -

Diego

Most-likely that cover plate holds two small light bulbs that lit to indicate that an electrical switch was in the "ON" position.

An example of use for such plates was an indoor switch that turns on a basement stair light that may not be visible when a basement stair door is shut, or an indoor switch that turns on an outdoor floodlight.

Pull the coverplate by using a utility knife to cut the paint along the plate perimeter where it meets the wall - that will let you remove the plate without extra cosmetic damage to the wall surface.

Then you may see that it carries two small lamps.

Photograph what you see and post it here if you will - we may both learn that I'm mistaken and it's some other more-unusual feature.

On 2018-12-04 by Diego

Hi,
I have the component in the attached image in a wall in a new house we just bought. I'm trying to figure out what that is? Any feedback will be appreciated.
Thank you
- image lost - apologies, please re-submit this image, an error in Comments Box caused this image to be lost.

On 2018-11-26 15:09:51.091501 by (mod) -

KT

If your outlet plate melted there is a dangerous overheating condition that needs to be found and fixed.

If you've done that and all that remains is adding insulation to repair a section of damaged wiring inside of an electrical box, using electrical tape or shrink tubing should be OK.

Often overheating is local to a connection - if that's the case then you might hope that there is no further wiring damage back along the wire length. If you can confirm that by finding un-damaged wire BEFORE it exits the electrical box that might be a reasonable surmise. If not, then further investigation is in order.

On 2018-11-26 13:48:00.693361 by knob and tube

I have Knob and Tube wiring that melted plastic outlet plate..on inspection the insulation had frayed about three(3) inches exposing bare wire ...I know i cannot join romex in the receptacle,and tape does't seem to work ..I thought about liquid tape or shrink tubing ..any idea ,advice ..appreciate it

On 2018-01-31 04:38:27.994538 by (mod) -

Correct so you have a 120 volt service and a 240-volt panel. you would need to speak to your electric company and use an electrician to provide a Service drop and service entry wiring for 240 volts. Or you would need to put in a simpler 120-volt panel. Be careful If you're not trained and familiar with electrical wiring there are great risks of shock or even death by electrocution.

On 2018-01-31 01:26:53.387773 by Jeff

In the service coming from the weather Head is only one neutral wire and one hot to the fuse box I'm trying to change to a breaker service and bypass the fuse box and install in the breaker panel but the terminals call for two hots and a neutral

On 2017-11-23 19:03:58.501702 by (mod) -

Freddie,

A quick summary of some points we make in the article KNOB & TUBE WIRING and in this article series on knob and tube wiring:

1. it is not, in most jurisdictions, required to remove K&T circuits provided the wiring is un-damaged

2. You are not permitted to extend an existing K&T circuit

3. You cannot safely "update" a portion of existing K&T circuit with new wiring if grounding is required, since the feeding K&T circuit would not provide an incoming ground.

4. Your local electrical inspector will probably be OK with you bringing the end of the existing circuit into a junction box where it enters the attic, then running new wire in the attic, presumably for attic lighting, but beware that any attic that also includes electrical receptacles as those won't be grounded (and should not mis-lead occupants by sporting receptacles with grounded-plug connector - that is a three-pronged plug receptacle)

5. I would be nervous about doing you are being asked to do. There is NO safe way that you can "test" the safety of existing K&T wiring where it passes through hidden spaces like walls and ceilings.
You can do continuity tests and tests for shorts but you cannot see the condition of the wire. For example, where K&T was run originally in an uninsulated wall cavity it might have performed well and safely but decades later after people added insulation by blowing it into the same wall cavity that same wire is now overheating. It is no longer running in open air.

So later when someone in an attic plugs in an electric heater and starts a fire in the building walls below where squirrels chewed the insulation off of the wire you and your employer are likely to be blamed.

On 2017-11-23 01:45:37.030671 by Freddie

Hi, I work for a company that wants me to replace old Knob and Tube wiring in attics, but I can't find a recommended procedure for safely doing so; tying into the existing K&T in the walls.
Do you know where I could find one, as well as a prescribed method of testing the wiring in the walls to make sure it's still safe?

...

Continue reading  at KNOB & TUBE WIRING - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

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• ASBESTOS ELECTRICAL WIRE INSULATION
• FALSE GROUND at RECEPTACLES
• FALSE GROUND, BOOTLEG & FLICKERING LIGHTS
• KNOB & TUBE WIRING of Proper and Improper Knob and Tube Wiring Details
  • KNOB & TUBE WIRING PROPERTIES
  • KNOB & TUBE WIRING ILLUSTRATIONS
  • DAMAGED KNOB & TUBE WIRING
  • EFFECTS of INSULATION on KNOB & TUBE CIRCUIT SAFETY
  • KNOB & TUBE WIRING EXTENSIONS ELECRICAL CODE CONCERNS
  • ADVICE on IMPROVING KNOB & TUBE WIRING SAFETY
• OLD ELECTRICAL WIRING TYPES
• OLD HOUSE ELECTRICAL SYSTEMS
• WET SUBMERGED K&T WIRING

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