Derating for Hot Attic

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jetlag

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Eatonton, Georgia
I live in central Georgia where attic temp can easily reach 140 degrees or more. Even with attic fan can be very high. My question is why are conductors never derated for this around here? I wont run the figures but NMB is cut to less than half capacity at this heat. And yet no one seems to worry. If they did most branch circuit would go up at least one wire size. I think about south FLA. a #12 cu would probably drop to 7 or 8 amps
 
jetlag said:
If they did most branch circuit would go up at least one wire size.
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Perhaps not. At an ambient of 140F, the derating factor for THHN is 71%. A #12 THHN would then have an ampacity of 71% of 30, or 21 amps. Therefore, you could still use #12 THHN for 20 amp branch circuits.
 
charlie b said:

Perhaps not. At an ambient of 140F, the derating factor for THHN is 71%. A #12 THHN would then have an ampacity of 71% of 30, or 21 amps. Therefore, you could still use #12 THHN for 20 amp branch circuits.
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After reading the bundling thread this hit me. I quickly thought about it and did the same math and came to the same conclusion. Even in the north east we get hot temps. Yesterday we hit 102 with a heat index of 112. Even south FLA doesn't get much hotter than that, but they may get more days like that.
 
mikeames said:
Yesterday we hit 102 with a heat index of 112.
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Mike, we beat you by two degrees, setting a new all-time record for the date, and set another record last night: it only went down to 80 degrees. :mad: There outta be a law!
 
nm 60 degree wire

nm 60 degree wire

charlie b said:

Perhaps not. At an ambient of 140F, the derating factor for THHN is 71%. A #12 THHN would then have an ampacity of 71% of 30, or 21 amps. Therefore, you could still use #12 THHN for 20 amp branch circuits.
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I knew someone would give this reply. I was under impression that nm cable can only be counted as 60 degree wire even though it contains 75 degree conductors because they are bundled in a sheath. Ive always accepted this to be true , have heard it for long time from inspectors and other sources. Does any one know if this is stated in the code. ? If you derate the nm at 140 degrees as 60 degree wire , the capacity is less than half
 
One could argue tha nm maintains a space and that thhn in emt is more likely to be called bundled.Personally having lived in fl for 35 years i have never seen romex damaged by attic heat.Many even survive heat from major fires
 
The conductors within the cable are rated for 90 degrees. You can use the 90 degree rating for derating purposes. Is this cable on top of the attic insulation?
 
jetlag said:
I was under impression that nm cable can only be counted as 60 degree wire even though it contains 75 degree conductors because they are bundled in a sheath. Ive always accepted this to be true , have heard it for long time from inspectors and other sources. Does any one know if this is stated in the code. ? If you derate the nm at 140 degrees as 60 degree wire , the capacity is less than half
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334.80 Ampacity.
The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60?C (140?F) conductor temperature rating. The 90?C (194?F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60?C (140?F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.11.
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There you have it..straight outta the NEC.
 
334.80 Ampacity.
The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60?C (140?F) conductor temperature rating. The 90?C (194?F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60?C (140?F) rated conductor.
 
LarryFine said:
Mike, we beat you by two degrees, setting a new all-time record for the date, and set another record last night: it only went down to 80 degrees. :mad: There outta be a law!
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Yea I heard about that. I feel for you. Walking the dog was sweaty work.

For the heck of it I monitor my atic temp with one of those wireless thermometers it peaked at 127.9
 
Attic temp

Attic temp

I used a Fluke thermometer and measured the attic temperature when the outside air temperature was 88 degrees F...on day number three at 88 degrees [attics continue to absorb radiant heat and get hotter each day]...on the 3rd day the attic temp was 143 degrees up close to the rafters and around 133 at top of the ceiling joists....thats at 88 F ambient outdoors....I did not check it when the outside got to the 95-100 range which is quite normal in the summer [San Francisco Bay mid to southern peninsula].

This too [derating NM in attics] is something I've wondered why no one questions. At 143 F, the derating factor is .58, that makes a #12 NM only worth 17.4A. I really think this falls under, "no one wants to go there."
 
dana1028 said:
At 143 F, the derating factor is .58, that makes a #12 NM only worth 17.4A. I really think this falls under, "no one wants to go there."
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NM can be derated using the 90? C (194?F) col. ...why would you derate a cable in a 143?F area when the cable is rated at 194?F?
 
celtic said:
NM can be derated using the 90? C (194?F) col. ...why would you derate a cable in a 143?F area when the cable is rated at 194?F?
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It does seem odd but that is what you are suppose to do. Look at the correction factors at the bottom of Table 310.16. You look up the ambient temp and match it to the conductor insulation and use that correction factor.

My question is "What is ambient temp?" Is it the temperature in the air at any point for any period of time or is it based on average temp for the year, month, etc. If the wire is in the attic for 5" must it still be derated with the correction factor????
 
Dennis Alwon said:
If the wire is in the attic for 5" must it still be derated with the correction factor????
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Most likely no correction is needed for 5" of wire. Look at 310.15(2)Ex.
 
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