334.80 derating

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There is no reason they should not be enforcing this section. These rules have been there for a long time.

The portion that says where passing through framing members that are to be fire/draft stopped I thought was new in 2008 but I see it was also in 2005 so it is not all that new either.

The rules of 310.15 apply to all wiring methods so even if 334 did not mention this, so you should still have to derate if there are more than 3 current carrying conductors bundled together.

Other than the fire/draft stop wording the 334.80 section is mostly redundant. It just tells you to go to 310.15 which already covers the situation with its own wording.
 
Whether or not the AHJ enforces something doesn't mean it’s not required to be done. An inspector may overlook or even allow a code violation to occur, but YOU are ultimately still responsible to be in compliance with the NEC, regardless of AHJ policy and behavior.

That being said, I don't see strong enforcement of these rules when it comes to attic spaces and roof-tops which is particularly worrisome to me being in SW Florida.
 
bphgravity said:
That being said, I don't see strong enforcement of these rules when it comes to attic spaces and roof-tops which is particularly worrisome to me being in SW Florida.
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NC gets pretty hot but I have never seen evidence of overheating in a nm cable due to attic heat. Have you?
 
No, but that is not the point. The slight overloading of a conductor mainly due to high ambient temperature doesn;t show up instaneously like a ground-fault does. In fact, it may never degrade the insualtion enough to cause a failure. But the code NEVER allows a conductor to exceed its temperature rating, even for one second.
 
bphgravity said:
But the code NEVER allows a conductor to exceed its temperature rating, even for one second.
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That would be impossible to enforce. That would mean the highest temp in an attic would have to used and there is no telling what that would be for a given home. House may be in the shade as opposed to the sun. The amount of insulation, ventilation, etc can effect the temp. What temp would one use?
 
But Dennis, does it matter? Think about the damage that they are saying comes from a few cables collected in a hole with insulation, and imagine comparing that to a long length of cables together in an attic. In my mind, it's amazing that the hole got attention before the attic.

On the flip side, generally the cables are on the living side of the insulation, so perhaps that's why it's not been made an issue.
 
Dennis Alwon said:
NC gets pretty hot but I have never seen evidence of overheating in a nm cable due to attic heat. Have you?
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What does that have to do with the rules?

bphgravity said:
No, but that is not the point.
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I agree.


But the code NEVER allows a conductor to exceed its temperature rating, even for one second.
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I agree and have 'fought' with Charlie B over this issue.

Dennis Alwon said:
That would be impossible to enforce.
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How so, simply enforce it like any rule.


That would mean the highest temp in an attic would have to used and there is no telling what that would be for a given home.
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I think using years of weather data one could come a lot closer to the temp then simply deciding to ignore it.


What temp would one use?
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Well we know it will be at least the highest recorded ambient which will be much more than 86F.
 
Thanks Bob but you totally missed the point of my post. :) There is no way to use the highest recorded in a particular attic esp. since the house isn't insulated or anything at the time of wiring.
 
Dennis Alwon said:
NC gets pretty hot but I have never seen evidence of overheating in a nm cable due to attic heat. Have you?
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That is because it rarely gets over 120 degrees in a properly ventilated attic. If you stick to 240.4(D) and load the circuit accordingly it will NEVER exceed the 140 degree F. Self enforcing.:D The 120 degree temp will most likely not be seen by any of the wires because most if not all will be in or under the insulation. Bob,how would you calculate the wire temp in the insulation? Insulation does not generate heat it only restricts the exchange.
 
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One-eyed Jack said:
That is because it rarely gets over 120 degrees in a properly ventilated attic. If you stick to 240.4(D) and load the circuit accordingly it will NEVER exceed the 140 degree F. Self enforcing.:D
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Jack-- you have to use the correction factor when the temps get over 86 degrees. At 120 degrees you would need a correction factor of .82 for a 90C wire. Not a problem in most cases unless there are other derating factors to consider. Once you NM cables get larger then #10 wire this can make a big difference.
 
Dennis Alwon said:
Thanks Bob but you totally missed the point of my post. :)
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I don't think so.:)

You were pointing out it is difficult to predict the futer temps in someones attic.:)

I agree, you will not be able to say it will never be more than 137.3425 F but you can certainly assume that if the areas highest temps are typically 105F that any attic in that area will reach that temp and likely 15, 20, 30 F more.
 
Dennis Alwon said:
Jack-- you have to use the correction factor when the temps get over 86 degrees. At 120 degrees you would need a correction factor of .82 for a 90C wire. Not a problem in most cases unless there are other derating factors to consider. Once you NM cables get larger then #10 wire this can make a big difference.
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I think that is exactly what Jack was saying. :)
 
Smart $ said:
As for the reason behind the 'excepting' of AC/MC I have no credible idea.
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For that you would need to go back a few code cycles and read the ROP for the reason why Exception #5 was added.

For non-multi-receptacle circuits the 60% derating has no affect on #12 MC cables when bundled in any quantity.
 
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