Ampacity of wiring in foam insulation

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Rearic

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Medina, NY, usa
In my area I am inspecting more homes insulated with Isynene foam. I cannot find any information about ampacity requirements in the NEC relevant to this situation, only a mention in the Handbook. I know in my gut that this is an important issue. The ampacity of wiring totally imbedded in foam cannot be the same as in a normal installation . Is anyone looking at this?
 
Currently the ampacity is the same unless multiple cables run through the same hole.

See the last paragraph of 334.80 of the 2008 NEC.
 
It might be worth noting that while #12 NM-B is normally not allowed to handle current near it's thermal rating (18 amp continuous vs 30 amp thermal rating), the same would not be true for larger conductors or conductors and loads found in an industrial environment.
 
weressl said:
So it is not a fire hazard, just shortens the insulation life. (Although it still probably remains well within the house's life expectancy.)
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For the smaller wire, that appears to be so. Mostly I expect because they are loaded below their thermal limits.

Years ago, I ran a calculation for wire sandwiched between layers of R-20 to R-30 insulation and I seem to recall it can became a problem.

I can't find my copy of the Toronto study so I don't remember the R-values they used.

The NEMA bulletin did note that NM-B has been installed in heavily insulated walls, etc with no reported detrimental effects. You might consider that these are not usually encased wires. They run next to a wall or joist, which helps remove the heat from the cable.

I might suggest the installer use some wisdom: If you have a cable loaded close to the thermal limit, try to route it so it will be on the edge of heavy insulation, not encased within.
 
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