# ampacity temp. correction for THHN



## cavecom (May 22, 2011)

Trying to wrap my head around ampacity corrections. My question:

If you used copper 4/0 THHN at 75 deg.C would the ampacity be 106.6 ?

90 deg. C rating = 260
correction factor= .41
260 x .41 = 106.6

copper 4/0 THHW at 75 deg. C has an ampacity rating of 230
Is there an advantage to using a lower temp rated wire in this case?

If THHW copper 4/0 were used at 70 deg. C would it be derated by .33 to 75.9 amps?

I thought the whole reason for using the higher temp rated wire was to be able to use smaller wire after correction for temp. Or is there something fundementally wrong with my approach? This is a major stumbling block to my understanding of the ampacity tables, as it seems counterintuitive to me.


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## Dennis Alwon (May 9, 2009)

The purpose of the higher temp is to help with derating. Think of the same question and you had to use the 60C or 75C rating.


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## HARRY304E (Sep 15, 2010)

cavecom said:


> Trying to wrap my head around ampacity corrections. My question:
> 
> If you used copper 4/0 THHN at 75 deg.C would the ampacity be 106.6 ?
> 
> ...


Also look at 110.14(C).

Welcome to the forum..:thumbup::thumbup:


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## cavecom (May 22, 2011)

Dennis Alwon said:


> The purpose of the higher temp is to help with derating. Think of the same question and you had to use the 60C or 75C rating.


Ok, let me ask the question differently....
What is the ampacity of copper 4/0 THHN at 75 deg. C?
If it isn't 106.6, what is it and how do I get the right answer?

BTW- I'm using the 2002 NEC table 310.16, as this is the local standard I'm being tested on. I realize the answer may be slightly different on the 2011 NEC where the correction factor at 75 deg. C is .5 instead of .41


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## don_resqcapt19 (Jul 18, 2010)

All of the ampacities in Table 310.16 are based on a 30°C ambient. If the temperature is higher than that you have to apply the temperature correction factors. The ampacity of conductor from the 75°C column in a 75° ambient is zero. You are already at the maximum permitted temperature so any current would raise the temperature of the conductor above its rating.


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## Dennis Alwon (May 9, 2009)

cavecom said:


> Ok, let me ask the question differently....
> What is the ampacity of copper 4/0 THHN at 75 deg. C?
> If it isn't 106.6, what is it and how do I get the right answer?


If you use 4/0 copper THHN then it is rated 90C then you must start there-- 260 amps. If there is derating for pipe fill or ambient temp, etc then you must apply the derating from there. So 260* .41 = 106.6 amps. So your answer is correct. You can see at 75C ambient temp you would not even be allowed to use 60C or 75C conductors because the temp is already too high for them.

Don's explanation above is right on.


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## hardworkingstiff (Jan 22, 2007)

If the ambient temperature got to 75C (that's like 167F), I think most of us would be dead.


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## cavecom (May 22, 2011)

Dennis Alwon said:


> If you use 4/0 copper THHN then it is rated 90C then you must start there-- 260 amps. If there is derating for pipe fill or ambient temp, etc then you must apply the derating from there. So 260* .41 = 106.6 amps. So your answer is correct. You can see at 75C ambient temp you would not even be allowed to use 60C or 75C conductors because the temp is already too high for them.
> 
> Don's explanation above is right on.


I think I've discovered the flaw in my reasoning- just because a higher temp rating is given for a given conductor doesn't mean it can be used at that table(uncorrected) value up to it's max temp value. So that means that at 70 deg. C:
4/0 THHW has almost peaked it's temp rating and is derated to 80 amps
4/0 THHN has some headroom temp-wise and is rated at 151 amps

My incorrect assumption was the ability to use the 30 deg. rating at it's max temp rating. Got it! :clap: Thanks for help making this sense to me.


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