THHN Derating in PVC underground

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Chessnut67

Member
Location
Debary Florida
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Electrical Manager
New to the forums and posting threads. I have an issue I am trying to resolve and hope I can get some help.
I did a little project in a small town where they sub-out the inspections to a private company. On this project, I ran 3/4" sch 40 PVC underground with 4 hots, 4, neutrals, and one ground all #12 THHN. So having between 7 and 9 current carrying conductors in conduit, i calculated the derating as 21a and put on a 20 amp breaker.
The private inspection company failed my inspection saying I had wrong size wire in conduit. He asked me to show him how I arrived at this size wire and after i showed him, he said he didn't believe i was correct and still failed the job.
Can anyone tell me how to prove to him it is correct and/ or what do you do with a private inspection company when they tell you you are wrong, he couldn't prove I was wrong just stated I was wrong, now they won't pay for the job even though they are already using the electric I installed.
 
That private Inspection agency need to hire better inspectors. He really could not calculate 8 CCC's in a raceway? That's like Inspector 101.
 
This is the key section below.

I also assume you are using Thwn-2 not thhn which is not allowed in wet locations

(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.
Click to expand...
 
Yes you are correct, it's THWN-2/ THHN actually marked on the wire.
And I was pretty sure doing calculations that I was correct using the 90C rating only for derating purpose, but the big issue is telling these back wood bumpkins that it is correct, he took it to his supervisor and neither one of them could figure it out.
Thanks for help and comments
 
infinity said:
That private Inspection agency need to hire better inspectors. He really could not calculate 8 CCC's in a raceway? That's like Inspector 101.
Click to expand...
I think his issue was using 60C vs 90C for de-rating or maybe 75C but it is clear 90C is allowed.
 
Dennis Alwon said:
I think his issue was using 60C vs 90C for de-rating or maybe 75C but it is clear 90C is allowed.
Click to expand...
Could be but that calculation is equal to first grade inspection knowledge. I would have a hard time calling someone an electrical inspector if they can't perform one of the simplest calculations.
 
Chessnut67 said:
Yes you are correct, it's THWN-2/ THHN actually marked on the wire.
And I was pretty sure doing calculations that I was correct using the 90C rating only for derating purpose, but the big issue is telling these back wood bumpkins that it is correct, he took it to his supervisor and neither one of them could figure it out.
Thanks for help and comments
Click to expand...
You were correct. Tell them to show you how they come up with whatever they did, and cite the code sections that back them up. Or even tell them to go to any trade school/apprenticeship program and see how they are teaching to do this sort of calculations. It should be exactly what you did.

Maybe they don't understand when they can use each temperature rating but otherwise have the rest right?
 
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