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Spray foam and electrical wiring

Randy_Williams | Posted in General Questions on

I recently had a conversation with a couple members of my family who are both electricians. In their continuing education class the instructor mentioned that the National Electric Code is monitoring the use of field applied spray foam and electrical wiring that becomes imbedded in the process. There have been failures in the wiring because of the heat that can build up in the covered wires, usually the problem is with constant and motor loads that operate at a higher ampacity. It didn’t sound like there were any enforcement changes by inspectors at this time, but the instructor was making the problem aware to contractors. Anyone planning on using spray foam that may enclose electrical wiring may want to contact an electrician to verify the load on a circuit.

Has anyone experienced or heard of an electrical failure due to open or closed cell spray foam?

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Replies

  1. Expert Member
    Michael Maines | | #1

    Randy, thanks for the heads-up. I've never heard of a failure due to overheating in wires encased in spray foam, and with properly sized conductors I doubt it would be a problem, but I would also not be surprised if that could happen in constant-load situations. R-6/in. foam will slow heat loss 50% more than R-4/in. mineral wool, so on marginal circuits heat build-up could in theory be an issue.

    The problem I have heard about is the heat produced during the chemical reaction closed-cell foam undergoes while curing could damage the insulation around the conductors. I don't know if this has a basis in fact, but it's been discussed for years. One more reason to choose the new HFO-blown foam, other than its greatly reduced global warming potential, is that it gives off less heat as it cures.

    Open cell foam's R-value is similar to other common insulation materials, so I seriously doubt there would be any issues with that class of product.

  2. calum_wilde | | #2

    I've never heard of the issue but thank you for bringing it up.

    When air sealing the conduit that runs from the meter outside to the panel inside I used non hardening clay which I've seen electricians use thousands of times in marine applications for a similar purpose. The R valuve is likely very little, but its air sealed and for such a small area I'm fine with that.

  3. walta100 | | #3

    I would like to see photos and reports if this is a real world failure as opposed to a hypothetical possibility. Some instructors like to sound like they know more than they do.

    I found this lab test for the worst case code compliant install for 14 and 12 gauge wires after 10 hours, the max temp of the wires was less than 60% of the wires temp rating.

    I think we can call this a wife’s tail without further evidence.

    http://www.demilec.com/documents/Tech-Library/Building-Science/NM-Cable-encased-spray-foam-insulation.pdf

    Walta

  4. calum_wilde | | #4

    Walter Ahlgrim,

    If I'm reading that correctly those cables were derated for that test though. "Section 334.80 states that where more than two Type NM-B cables are run through a bored hole that is to be fire or draft stopped using thermal insulation or sealing foam, the ampacity must be adjusted. "

  5. walta100 | | #5

    The test conditions were “The conductors were subjected to their maximum capacity for a period of 10 hours.” I read this as no derating.

    334.8 only applies if you put 2 cables in the same hole and seal it in any way.

    Walta

  6. Randy_Williams | | #6

    I talked to one of the lead electrical inspectors in Minnesota. He indicated that the State Board of Electricity is looking into an instance where they were notified of a melted outer insulation in NM-B wire. Apparently a remodeling contractor who was removing closed cell foam from a wall cavity found the wire, but I think Walter is right, it was not caused by high temperatures within the wire, but by incorrectly installed closed cell spray foam. I believe either too much foam was installed in one lift or the foam rolled over on itself during the installation process causing a build up of heat which melted the wire insulation. Regardless, a red flag has gone up and the state of Minnesota is looking into the issue.

  7. jkstew | | #7

    Good topic. The AWG's rating is 700 circular mils per amp, which is very conservative, but the assumption is that wiring is in air and not in a bundle. The NEC has their own guidelines.

    AWG's guidelines are here: https://www.powerstream.com/Wire_Size.htm

    NEC's recommendations are here: http://www.usawire-cable.com/pdfs/nec%20ampacities.pdf
    Note the ambient temperature qualifications and bundling qualifications in notes 4 and 5.

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