# Derating question



## wildleg (Apr 12, 2009)

where do you get 70% from ? you have 14 current carrying conductors

you are allowed to use the 90 degree ampacity for derating, providing the result is not greater than the rules allow for the ampacity of that conductor, had it not been derated.


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## Dennis Alwon (May 9, 2009)

you would use the 90C rating if your wire is rated 90C-- btw it is 50%


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## amptech (Sep 21, 2007)

Thank you Dennis. I was so preoccupied with the temp column issue I flubbed the % for 10-20 conductors. The guy still insists you derate from the 60 deg column always.


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## local134gt (Dec 24, 2008)

amptech said:


> The guy still insists you derate from the 60 deg column always.


Sure, when your using 60 deg wire.


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## wildleg (Apr 12, 2009)

amptech said:


> Thank you Dennis. I was so preoccupied with the temp column issue I flubbed the % for 10-20 conductors. The guy still insists you derate from the 60 deg column always.


well, that is flat wrong. I guess I'll go grab my codebook and find the section

- 110.14 C the last sentence: Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, (temperature) correction, or both.


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## Dennis Alwon (May 9, 2009)

wildleg said:


> well, that is flat wrong. I guess i'll go grab my codebook and find the section


110.14(c)


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## Dennis Alwon (May 9, 2009)

Also if you look at nm cable-- I realize that this is not an nm install you see art. 334.80 which states you can use 90C for NM cable for de-rating as long as the final amp. is not greater than the temp. amp of the cable (60)


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## P-Electrician (Aug 2, 2012)

For derating you go by the insulation temp rating. So if its 90 c insulation you use the 90 deg column, etc. Conductor ampacity is determined based on the temp rating of your terminations. If the lugs are stamped 75 deg then you use the 75 deg coulumn. If not labled you use 60 deg column, Even if you have 90 deg insulation. Thats what i was taught in class anyway.


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## wildleg (Apr 12, 2009)

P-Electrician said:


> For derating you go by the insulation temp rating. So if its 90 c insulation you use the 90 deg column, etc. Conductor ampacity is determined based on the temp rating of your terminations. If the lugs are stamped 75 deg then you use the 75 deg coulumn. If not labled you use 60 deg column, Even if you have 90 deg insulation. Thats what i was taught in class anyway.


sorry, but you are incorrect.

the maximum ampacity you can ultimately use, after derating and temp correction are what you said (the weakest link in the chain), but for purposes of calculating the adjustments, article 110.14 C says you can start out with the higher insulation factor to begin your calculation.


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## Dennis Alwon (May 9, 2009)

wildleg said:


> sorry, but you are incorrect.
> 
> the maximum ampacity you can ultimately use, after derating and temp correction are what you said (the weakest link in the chain), but for purposes of calculating the adjustments, article 110.14 C says you can start out with the higher insulation factor to begin your calculation.


I agree. And depending on the size of the conductors or OCPD the rating of the unknown terminations may be 75C Not 60C



> (C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
> (1) Equipment Provisions. The determination of termination provisions of equipment shall be based on 110.14(C)(1)(a) or
> (C)(1)(b). Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination
> provisions shall be based on Table 310.15(B)(16) as appropriately modified by 310.15(B)(7).
> ...


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## erics37 (May 7, 2009)

I wrote up this little cheat sheet for my apprentices in apprentice class. Pretty sure it works :laughing:

*Conductor Derating Procedure*

1) Look up base ampacity for insulation type in appropriate column (ex. 90 degree column for THHN) of Table 310.15(B)(16).

2) Apply correction factor for ambient temperature (if applicable) from Table 310.15(B)(2)(a).
_– If installed on a rooftop exposed to sunlight, factor in appropriate “temperature adder” per Table 310.15(B)(3)(c) –_

3) Apply adjustment factor for more than 3 current-carrying conductors per Table 310.15(B)(3)(a). Note this value.

4) Look up the conductor ampacity in the 75°C column (or 60°C where limited by NM sheath or 60°C insulation of other conductors in the same raceway, or other limitations). Note this value.

5) Use the lower of your results from Step 3 or Step 4. This is your final allowable ampacity.

*Notes*


Small conductors are limited to OCPD sizes based on 240.4(D) for general applications. See 240.4(G) for exceptions based on specific conductor applications.
If conductors serve a continuous load, remember that your final ampacity must be 125% of load (or, inversely, load can not exceed 80% of circuit ampacity).


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## MollyHatchet29 (Jan 24, 2012)

erics37 said:


> I wrote up this little cheat sheet for my apprentices in apprentice class. Pretty sure it works :laughing:
> 
> Conductor Derating Procedure
> 
> ...


Damn lucky apprentices...


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## P-Electrician (Aug 2, 2012)

wildleg said:


> sorry, but you are incorrect.
> 
> the maximum ampacity you can ultimately use, after derating and temp correction are what you said (the weakest link in the chain), but for purposes of calculating the adjustments, article 110.14 C says you can start out with the higher insulation factor to begin your calculation.


how so? say you are using thwn copper which has an insulation rating of 75 deg c, you have to derate from the 75 deg column, you cant use the 90 deg column? 110.14 c supports this


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## Dennis Alwon (May 9, 2009)

P-Electrician said:


> how so? say you are using thwn copper which has an insulation rating of 75 deg c, you have to derate from the 75 deg column, you cant use the 90 deg column? 110.14 c supports this


It is thhn/thwn dual rated. Read opening post


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## P-Electrician (Aug 2, 2012)

i know i was just using that as an example. his is dual rated so he can use the 90 column but i was saying that if it wasnt you have to use whatever the temp limitations of the conductor are to derate from.


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## Mulder (Sep 11, 2010)

P-Electrician said:


> i know i was just using that as an example. his is dual rated so he can use the 90 column but i was saying that if it wasnt you have to use whatever the temp limitations of the conductor are to derate from.


Right, but a 75C termination does not figure into derating conductors.


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## Dennis Alwon (May 9, 2009)

P-Electrician said:


> i know i was just using that as an example. his is dual rated so he can use the 90 column but i was saying that if it wasnt you have to use whatever the temp limitations of the conductor are to derate from.


 But you quoted the poster who was responding to the OP who had 90C wire and he can derate from there as long as the final amp. is not greater than the 75C


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## P-Electrician (Aug 2, 2012)

Mulder said:


> Right, but a 75C termination does not figure into derating conductors.


correct, i didnt mean for it to sound like it did, that was my bad in explaining it.


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## Mshea (Jan 17, 2011)

This is a huge issue with the new ampacity tables in Canada. We are arguing what the NEC says and the answers are opposing. Certain groups insist that you can only use the ampacity column associated with the terminal limitation. IE if the breaker is listed at 75C then you can only use the 75c ampacities. IE no advantage to 90c insulation. The exception in the NEC is argued too. Our part 1 chairman insists this is the NEC requirement but an NEC expert says that you can use the 90c ampacity and derate but the final answer cannot be higher than the 75 degree ampacity. He stated that the chief inspector in Arizona won't allow it but other jurisdiction do?
While the Canadian question is not answered to my satisfaction. I would say It makes perfect sense to use the 90c column for derating and at the end, if the wire is loaded equal or less than the ampacity from the 75c then you are good to go.


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## morg123452000 (Nov 4, 2013)

wildleg said:


> well, that is flat wrong. I guess I'll go grab my codebook and find the section
> 
> - 110.14 C the last sentence: Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, (temperature) correction, or both.


 So this is the confusing part from 110. 14C.. I can use the 90ﾟ column if my conductor is 90ﾟ rated and ignore the 75ﾟ termination rating?


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## splatz (May 23, 2015)

morg123452000 said:


> So this is the confusing part from 110. 14C.. I can use the 90ﾟ column if my conductor is 90ﾟ rated and ignore the 75ﾟ termination rating?


I agree that the way it's worded is not very clear, but I think that's just giving you permission to use wire types from the 90* column for 75* terminals (and of course, you must apply the maxiumum ampacity in the 75* column).


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## Quickservice (Apr 23, 2020)

MollyHatchet29 said:


> *Damn lucky apprentices...*


Amen!... My class was from the "School of hard knocks."


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## Dennis Alwon (May 9, 2009)

morg123452000 said:


> So this is the confusing part from 110. 14C.. I can use the 90ﾟ column if my conductor is 90ﾟ rated and ignore the 75ﾟ termination rating?



If your wire is rated 90C then you use that ampacity for de-rating purpose but in no case can the ocpd be larger than the 75C rating if the panel and all the equipment is rated 75C. If, for example, the panel is not marked then if the ocpd is 100 amps or less or if the conductor is 1 awg or less then we must use the 60C rating. If the wire is larger than #1 or the ocpd is rated more than 100 amps than we can use the 75C rating


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