I maybe kicking a dead dog. However, it is my understanding derating wire, you starts in the 90 column and then compare to 75 column to see if ampacity rating is possible. However, I am attending a class where they are saying start in the 75 Column (terminal rating) and derate. But using that thought. if the ampacity is under 100 amp why not start in 60 column?
Derating wire for terminal Connections
- Thread starter dixson
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- Lockport, IL
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The answer lies in 110.14(C). Actually, I should say that the answer lies hidden in that article. :wink: It is not easy to follow.
As I read the article and your question, you can use the 75C column to obtain ampacity, even for a circuit rated under 100 amps, if the terminations are rated for 75C.
As I read the article and your question, you can use the 75C column to obtain ampacity, even for a circuit rated under 100 amps, if the terminations are rated for 75C.
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- Wisconsin
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- PE (Retired) - Power Systems
Yes, absolutely.
just to clarify... this school is saying that a 75c rated terminal using use-2 wire for a PV system has to be rated according to the 75c terminal and any derating starts with the terminal rating and not the wire rating. This is incorrect, wire temp rating trumps terminal rating for derating ONLY!!
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- New Jersey
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Charlie provided the answer 110.14(C).
Twoskinsoneman
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- West Virginia, USA NEC: 2020
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This is a tough concept to wrap you head around when you first start learning how the NEC does conductor ampacity. To be honest I wish the wording of 110.14(C) was more direct but it is there and everything these guys have said is dead on.
Remember when your done with your 90 degree derating you are still restricted by the terminal. That is to say is if you do a derating calculation and the ampacity is greater than that of the 60 - 70 degree amapcity (respectively) you still cannot exceed the terminal ampacity.
Man that does read as clear as I thought it out in my head....
Remember when your done with your 90 degree derating you are still restricted by the terminal. That is to say is if you do a derating calculation and the ampacity is greater than that of the 60 - 70 degree amapcity (respectively) you still cannot exceed the terminal ampacity.
Man that does read as clear as I thought it out in my head....
charlie
Senior Member
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- Indianapolis
I hate to say this but the school, or more exactly, the instructor is wrong. I used to think that everything the instructor, teacher, professor, etc. was absolute. I have since changed my mind.
If this is just a PV class, the instructor may not be an electrician in the same manner that most HVAC guys are not. In other words, he can do the work and most of it is great but he may not know all of the code rules that apply to the work that he is doing.
Lastly, you may have misunderstood the instructor. You might want to print out this thread and ask him to verify, with code references, what he is telling you.
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- Chapel Hill, NC
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- Retired Electrical Contractor
Yes you can but you cannot use any ampacity that is above the 75C. For instance if you have a #6 copper wire that has 90C insulation then you can start at 75 amps.
If your terminals were 60 C and you had 90C wire then you would still derate the wires from the 90c column, however if the ampacity after derating is higher than the 60C column you cannot protect the circuit with that derated ampacity. The terminals must be protected at the 60 C rating. Is that clear as mud?
Hmmm... my internet connection must be foggy
The most confusing aspect of how most people describe this process is that they fail to use descriptive modifiers for the term ampacity. For example it becomes much clearer using the following terms: circuit ampacity, conductor ampacity, terminal ampacity, etc. Simply giving an ampacity rating is ambiguous without such modifiers.
Here's how I see it...
For 75?C-rated circuits (which means the lowest temperature rating in the circuit is 75?C) using 90?C-rated conductors:
- Determine circuit ampacity.
- Determine ambient correction using 90?C column and derating factor.
- Divide circuit ampacity by temperature-correction factors from previous step. Result is minimum 90?C conductor ampacity.
- Determine conductor size based on result of previous step.
- Compare circuit ampacity to the 75?C-rated ampacity of same size conductor. If less than, the chosen size is compliant. If greater than, compare to next larger size and use this size if circuit ampacity is now less than the 75?C-rated ampacity of same size conductor.
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- Location
- New Jersey
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- Journeyman Electrician
That's correct. Start with the temperature rating of the conductor not the termination.
Pierre C Belarge
Senior Member
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- Westchester County, New York
If this is just a PV class, the instructor may not be an electrician in the same manner that most HVAC guys are not. In other words, he can do the work and most of it is great but he may not know all of the code rules that apply to the work that he is doing.
Lastly, you may have misunderstood the instructor. You might want to print out this thread and ask him to verify, with code references, what he is telling you.
Both of the points that Charlie is making here are valid and common. The PV instructor may be very knowledgeable in PV and not too sure of electrical.
Then again, because of the way some listen and the way some speak, it is not inconceiveable that wires got crossed up somewhere (pun intended :grin
It looks like you have a good understanding of the procedures and requirements necessary to move forward.
...yet one method is to start with the required circuit ampacity and apply inverse adjustment and derating factors. This gives you a solid number to look for in the conductor rating column, rather than taking a shot in the dark at the size and hoping it works after applying the adjustment and derating factors.
Anyway you go about it, the circuit ampacity has to be equal or less than the terminal temperature rating of the conductor size used AND the corrected conductor ampacity must be equal or greater than the circuit ampacity.
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