Adjustment factors

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hhsting

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I have #10awg 90C cable temp but terminals are rated at 60C.

Breaker feeding the cables is 30A. The cables in conduit have more than 4 current carrying conductors.

I have plans which state that at 90C its 40A and 40x0.8=32A. This is acceptable

However as I read 310.15(B) i think its incorrect because corrected ampacity 32A is greater than ampacity of 60C temp rating termination which is 30A.

So am I correct or incorrect in this case temperature rating of the conductor cannot be used and they need to increase the size?
 
You are incorrect, based on the information provided #10 is sufficient in size. There are two separate checks, which I like to think of as terminations versus middle of run:

Terminations: no temperature or bundling derating, limited to terminal temperature, #10 @ 60C = 30A.
Middle of run: ambient temperature and bundling derating, can use full insulation temperature ampacity. #10 @ 90C = 40A; 40A * 0.8 = 32A.

The lesser of these two computations is 30A, so the 30A OCPD is fine.

Cheers, Wayne
 
Way I understand it your good after adjustment factors if it keeps your ampacity of the corrected conductor at 30amperes or greater. But if it’s less than 30 amp than that’s when conductor size would need to be upgraded. I know the language makes it sound like if it’s greater than 30+ amperes it’s a no go, but I believe it means if your corrected ampacity is below 30amps.

if I am wrong someone feel free to correct.
 
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Ah, so the sentence under consideration from 310.15(B) is "The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with the provisions of 110.14(C)."

The part "if the corrected and adjusted ampacity does not exceed" just means use the lesser of the two numbers. I.e. you can use the first number if it does not exceed the second number; otherwise you have to use the second number.

Cheers, Wayne
 
Take post #2 and generalize it to a chain.

At each link you have a separate ampacity calculation: At the origin terminal where the terminal temperature rating controls, then in the conduit where you use the full temperature rating of the insulation but you have derating, perhaps in a different section of conduit where you have to deal with elevated ambient temperature, etc. down to the final terminal with its temperature rating.

Each link in the chain will have its own ampacity. The chain is as strong as the weakest link.

Jon
 
The way I understand 310.15)B) second paragraph is:


I have at 90C #10 awg 40A. With 4 current carrying conductors adjusted ampacity is 32A.

32A > 30A ( temperature termination amapcity)

So then I would have to use 60C not 90C. Which means 30A @60C with 4 current carrying conductor is 24A. However 24A is not next standard size for 30A breaker so I would have to increase my size to #8.

Correct or incorrect?
 
hhsting said:
The way I understand 310.15)B) second paragraph is:


I have at 90C #10 awg 40A. With 4 current carrying conductors adjusted ampacity is 32A.

32A > 30A ( temperature termination amapcity)

So then I would have to use 60C not 90C. Which means 30A @60C with 4 current carrying conductor is 24A. However 24A is not next standard size for 30A breaker so I would have to increase my size to #8.

Correct or incorrect?
Click to expand...

Incorrect.

Roger
 
totally incorrect
you really need some of Mike's videos.
You can use the conductor ampacity based on insulation type for derating (90° for THHN)
You have to take 110.14 into account in termination.
Then you need to tale 240.4 into account for any overurent limitation (#10 limited to 30 amp, gererally)
 
The wording is slightly confusing, in that the alternative in the "unless" clause isn't immediately obvious. The alternative is not "redo the adjustment and correction computation using the termination temperature ampacity" as the OP suggests. The alternative is "just use the termination temperature ampacity, without any adjustment or correction factors."

Cheers, Wayne
 
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You have two temperatures to consider. Termination temp of 60C - in no case can you use less than 10 AWG or you will be undersized for termination temp rating.

second temp is insulation temp. You have 90C insulation. With four current carrying conductors your adjustment factor is .8 and you came up with 32 which your 30 amp load (you didn't exactly specify the load but kind of don't matter right now) is still under that value.

Now add say four more conductors to get you an adjustment factor of .7 and your adjusted ampacity is 28. If the actual load is 29 or 30 you would need a larger conductor. 10 AWG is large enough for 60C terminals but is not large enough for adjusted ampacity for insulation capacity.
 
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