Do I have to derate for burial depth on conductors rated less that 2001 Volts

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amamic33431

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We are currently doing a large project that has required us to directional bore a (12 way 4" PVC duct bank and 6 way 4" PVC duct bank). In order to miss other utilities we have had to bore at an average depth of 14'. The conductors being installed will be carrying 480/277 Volts. Do we have to derate based on the burial depth of the conduits or does this only apply to conductors rated at more that 2001 Volts?
 
amamic33431 said:
We are currently doing a large project that has required us to directional bore a (12 way 4" PVC duct bank and 6 way 4" PVC duct bank). In order to miss other utilities we have had to bore at an average depth of 14'. The conductors being installed will be carrying 480/277 Volts. Do we have to derate based on the burial depth of the conduits or does this only apply to conductors rated at more that 2001 Volts?
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It appears you do not have to derate. I will say at 14ft you have met table 300.50;)
 
Thanks for the reply.

I am not really worried about minimum cover requirements but more concerned with derating of the conductors. An engineer is telling that we must derate the conductors as per 310.60 (C) (2) (b). But doesn't this section apply to conductors rated at between 2001 and 35000 Volts only? Not conductors rated at less than 600V? If we have to derate the rated ampacity of the conductors will drop substantially. These are parallel conductors of 600MCM @ 1200' we are trying to avoid having install additional conduit and an additional set of conductors.
 
amamic33431 said:
But doesn't this section apply to conductors rated at between 2001 and 35000 Volts only? Not conductors rated at less than 600V?
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Yes. 310.60 would not be the applicable section for your installation.

310.15(B)(2)(B) may have an effect on your installation if you didn't rack the raceways in the duct bank.

Pete
 
pete m. said:
310.15(B)(2)(B) may have an effect on your installation if you didn't rack the raceways in the duct bank.
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petem -
Is there any where in the code where this spacing is specified? I don't see it in 310, 344, or 300.5.

It appears one could select a spacing of zero and meet code.

ice
 
amamic33431 said:
An engineer is telling that we must derate the conductors as per 310.60 (C) (2) (b). But doesn't this section apply to conductors rated at between 2001 and 35000 Volts only? Not conductors rated at less than 600V?
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As other have already said: You are correct. There is no code requirement for you to install this other than to code minimums.

Anything else is a design issue - and the NEC is really a poor design manual.

However, I'm curious about this engineer giving you direction. So, me just guessing: The engineer is telling you that the thermal resistance of the earth does not go away just because the cables are rated less than 2001V. And since these cables are being burried 14 feet, there is going to be a heat transfer problem when the cables are loaded up. It appears the engineer picked a code section that he considered applicable, without measuring the soil thermal resistance.

amamic33431 said:
we are trying to avoid having install additional conduit and an additional set of conductors.
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Yes, it was readily apparent that was the reason.

If I were the owner, I'd be asking:
1. What is this going to cost me if the installation is code minimum and runs hot and fails in 5 - 10 years?

2. If the extra copper is put in now, will the installation run cooler and maybe last 25+ years?

If I were the contractor, working to a fixed price - maybe I'd be asking a diferent set of questions.

Another curiousity question: What do the drawings/specs say?

ice
 
Our plant has hundreds of duct banks throughout, and all of our engineers use Neher-McGrath Method to get the ampacity of the conductors. We have locations where we have installed 3 sets of 750kcmil for an 800A breaker because of depth and soil conditions.

Have your engineer look into what is needed on your site.
 
amamic33431 said:
Thanks for the reply.

I am not really worried about minimum cover requirements but more concerned with derating of the conductors. An engineer is telling that we must derate the conductors as per 310.60 (C) (2) (b). But doesn't this section apply to conductors rated at between 2001 and 35000 Volts only? Not conductors rated at less than 600V? If we have to derate the rated ampacity of the conductors will drop substantially. These are parallel conductors of 600MCM @ 1200' we are trying to avoid having install additional conduit and an additional set of conductors.
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The engineer is mistaken and you are correct.

KevinVost said:
Our plant has hundreds of duct banks throughout, and all of our engineers use Neher-McGrath Method to get the ampacity of the conductors. We have locations where we have installed 3 sets of 750kcmil for an 800A breaker because of depth and soil conditions.

Have your engineer look into what is needed on your site.
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Now if their engineer determines that derating would be needed and the job specks. did no call for it then all you can do is do it and charge accordingly
 
Would Annex "B" come in to play here ? I have not had to deal with this size duct banks yet, but it looks as if; B.310.15(B)(5) Tables B.310.6 and B.310.7., may have some useful info.

Add Figure B.310.2, to that.
 
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amamic33431 said:
.... An engineer is telling that we must derate the conductors...
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The question is what is this engineer to the project?

The project engineer has the option to determine ampacities under a 310.15(C) calculation or the Tables per 310.15(A)(1).

If the engineer noted is not the project enginner, he has no say... at least not directly to the contractor, unless he/she represents the other party to the contract... but even that can be a thin line if there are stamped, approved prints.

Are there prints? PE stamped? AHJ approved for construction? etc....
 
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