Table 310.15 75/90 Degree rating/derating

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kwired said:
Unless I misunderstood something though the A&E is asking for larger conductors then NEC would require as minimum, so any liability on the wire manufacturer part is not effected anyway as the manufacturer already claims it has a higher ampacity then it is being used for.
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Yep. It would be a different matter if the A/E specced say 6 sets of 750MCM for the switchgear.
 
kwired said:
Unless I misunderstood something though the A&E is asking for larger conductors then NEC would require as minimum, so any liability on the wire manufacturer part is not effected anyway as the manufacturer already claims it has a higher ampacity then it is being used for.
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I might have overlooked that as my statements are only to the statement referenced. The additional info was simply educational at best.

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Just to be clear i did say " As an antidotal statement," prior to the lecture on wire manufacturer's general position.

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MasterTheNEC said:
Just to be clear i did say " As an antidotal statement," prior to the lecture on wire manufacturer's general position.

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Just to be clear, I suspect that your statement was meant as a not necessarily representative personal experience (anecdote) rather than something to counter the effect of a poison (antidote).

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Thanks Grammar Monitor....sorry my education is limited as you all know. I rely heavily on auto correct.

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Yes I meant Anecdotal ....my experience based on fellow manufacturers which ended in me stating it was of my opinion. Sorry.
 
sssss said:
6. The A/E said 90 degree C column cannot be used, because if the conductor have a temperature of 90 degree C, the 75 degree C rated terminal maybe damaged when connecting to the conductor.
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It's not that it "cannot be used" period, it is that it cannot be your final answer. The 75-C underated terminal ampacity must also be sufficient for the load and OCPD.


Out of curiosity, what is an A/E? I take it E means engineer, but what does A mean?
 
Carultch said:
It's not that it "cannot be used" period, it is that it cannot be your final answer. The 75-C underated terminal ampacity must also be sufficient for the load and OCPD.


Out of curiosity, what is an A/E? I take it E means engineer, but what does A mean?
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I took it to mean Architect but that was only a guess.
 
Carultch said:
It's not that it "cannot be used" period, it is that it cannot be your final answer. The 75-C underated terminal ampacity must also be sufficient for the load and OCPD.


Out of curiosity, what is an A/E? I take it E means engineer, but what does A mean?
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MasterTheNEC said:
I took it to mean Architect but that was only a guess.
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I took it to mean Architect as well.

But maybe it was A&E, the television network?:D
 
kwired said:
I took it to mean Architect as well.

But maybe it was A&E, the television network?:D
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You are right, A means Architect.
In my organization, A/E is considered as one type of contract. Even though in this particular job, there is no architect involvement, I still call them A/E contractor.

Anyways, I think I will just talk to the building code people to ask for their opinions.

Again, thank you for everyone's input.
I'll update once I hear anything from the code people.
 
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After two months, the A/E came back with SKM calculation. They are setting the maximum temperature of the wire at 75 Degree instead of 90 Degree. A/E said that using 90 Degree as maximum is NEC violation. Does anyone think they are correct?
 
Yes, they are correct. But I think some clarification is needed here. There are two entirely different things being discussed in this thread, although that fact might not be obvious.

The discussion began with a question of derating from the 75C column as opposed to the 90C column. I think that has been adequately addressed.

The latest information from the A/E is addressing the issue from a different perspective: that of how high the temperature of a conductor is allowed to reach. A conductor with THHN insulation (and others that have been mentioned in other posts above) can sustain a temperature of 90C without suffering excessive damage. But if (as is very likely) it is connected using a termination that is only rated for 75C, then the design and installation is not allowed to cause the temperature to exceed 75C. That is what I mean when I say the A/E is right. Their computer model was used to run various levels of current through the conductors, and calculated the temperature that would be reached at that current level. When the temperature hit 75C, the maximum allowable current had been reached.
 
sssss said:
After two months, the A/E came back with SKM calculation. They are setting the maximum temperature of the wire at 75 Degree instead of 90 Degree. A/E said that using 90 Degree as maximum is NEC violation. Does anyone think they are correct?
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The wire can only get to 75*C since the terminations are 75*C, however one is still allowed to use the 90*C table for derating. As long as the amperage does not exceed the 75*C table for ampacity after conduit fill and ambient temperature corrections are applied, you are gtg.
 
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