NM Cable Installed Under Metal Roof

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Little Bill

Little Bill

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Tennessee NEC:2017
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Semi-Retired Electrician
If a person wanted to install NM cable between a metal roof and the decking under it, assuming firring strips between the two, is there a code article prohibiting it?

Other than a bad idea, IMPO, because of heat, I can't seem to find anything that says you can't.

If it is permitted, would you have to derate the conductors for heat?

This would be in an open raftered ceiling.
 
IMO, once you penetrate the metal roof deck, your outside, and nm cable is not suitable for wet locations
 
Yes IMO you would have to derate just as you would for raceways. We used to do that all the time on metal roofs so we didn't have to drill the rafters. Did that back when the nm was 60C inside the cable. None have burned down but I am curious what that wire looks like now.

Jusme- I don't think Bill was penetrating the roof.
 
Dennis Alwon said:
Yes IMO you would have to derate just as you would for raceways. We used to do that all the time on metal roofs so we didn't have to drill the rafters. Did that back when the nm was 60C inside the cable. None have burned down but I am curious what that wire looks like now.

Jusme- I don't think Bill was penetrating the roof.
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Nope, just running between the metal roofing and decking.
 
jusme123 said:
I was thinking commercial metal roof decking, how much space is between the metal roof and the nm?
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Usually with metal roofs on homes they frame the rough then run 1x4 perpendicular to the rafters and then install the tin roof on them. So there is only a 3/4" gap and if you staple to the side of the 3/4" board the nm just about touches the roof.
 
Dennis Alwon said:
Usually with metal roofs on homes they frame the rough then run 1x4 perpendicular to the rafters and then install the tin roof on them. So there is only a 3/4" gap and if you staple to the side of the 3/4" board the nm just about touches the roof.
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I'm not 100% sure on the strapping/furring size, but currently the roof has the exposed inside ceiling (tongue & groove I think) then the "green board" decking, then whatever is being used for the strips, then the metal roof top.

I WAG that the strips would be no more than 1", probably 3/4" as Dennis said.
I also assume the NM would be stapled to the "green board", making sure to stay 1-1/4" away from the strips.
 
Around here, south Texas, a standing seam roof, 1st the decking, then the tar (dried in) paper, then the metal roof is applied directly to the top of the roofing (tar) paper.
 
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