310.15(B)(2)(a)

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Minuteman

Senior Member
A while back, I was hired to upgrade a outdoor 1200 amp, 120/240 Single phase MDP to a 1600 amp, 120/240 Delta 3 phase.

They had six 200 amp, single phase subpanel mains in the old MDP and added three 125 amp, 3 phase A/C units.

We re-used the two 5" Schedule 80 PVC underground (and under slab)conduits that ran from the CT can to the new MDP, and pulled paralleled 800 kcmil. That would be 8 total conductors per conduit. No problem... I thought.

I had to get an EE to stamp a drawing before I could pull the permit (only new construction requires a permit before the work begins). The facility has a Engineering Group that they use, who says that I have to derate the current carrying conductors per 310.15(B)(2)(a) because there are 8 current carrying conductors. No stamp!

I sited 310.4 and 310.15(B)(4)(c) as my support for not derating. We are still using the 2002 NEC

What saith thou?

edited to correct code reference

[ December 29, 2005, 12:26 AM: Message edited by: Minuteman ]
 

pierre

Senior Member
Re: 310.15(B)(2)(a)

Without considering for harmonics, you will have a minimum of 6 current carrying conductors per conduit. 310.15(B)(2(a) shows you will need to adjust for 80%.


800kcmil = 555 amps as per the 90 degree column.

555 amps X .8 = 444 amps after adjustment for the 6 ccc.

444 amps x 4 sets = 1776 amps


If the PE is considering that the neutral is part of the ampacity adjustment as per 310.15(B)(4)(c), you will need to perform a 70% adjustment

555 x .7 = 389 amps

389 amps times your 4 sets = 1556 amps
 

Minuteman

Senior Member
Re: 310.15(B)(2)(a)

Thanks Pierre,

But the paralleled conductors are considered ONE each per 310.4 right?


edited to correct grammar

[ December 29, 2005, 01:20 AM: Message edited by: Minuteman ]
 

pierre

Senior Member
Re: 310.15(B)(2)(a)

They are considered "one" as far as the ampacity rating is concerned. They are considered as multiple conductors for ampacity adjustment as per the very last sentence of:
310.4
"Conductors installed in parallel shall comply with the provisions of 310.15(B)(2)(a)."
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: 310.15(B)(2)(a)

One more correction. I used 600 kcmil
If that is the case, you can't supply a single 1600 amp main as your total ampacity is only 1520 amps, assuming that the grounded conductor is not counted as a current carrying conductor. If the calculated load is 1520 amps or less and if you have more than one service overcurrent protective device, you are code compliant.
Don
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: 310.15(B)(2)(a)

Originally posted by Minuteman: I had to get an EE to stamp a drawing before I could pull the permit. . . . The facility has a Engineering Group that they use. . . .
I am concerned, and on the borderline of being alarmed, at this. I will cling to hope that this is not what it sounds like. If I were interested in investigating the circumstances, I might start by asking who did the drawing.

If you tell them that you need a 1600 amp service and wish to reuse two existing conduits, then leave them to choose the conductor sizes, after which they create a drawing and apply a seal and signature, then I will settle down, concluding that I misinterpreted the situation.

But if you were to send them a completed drawing, and they sent it back with a seal and signature, then they will have violated professional ethics, at a minimum, and quite likely will have violated state law as well. If I were to become possessed of factual, verifiable information that proves this is what happened, I would be required to report it to the State's licensing department. It would probably result in a fine being imposed, and it might even result in someone's PE license being suspended.

I am not asking for details, and I don't feel obliged to investigate the circumstances. I have no facts in my possession, and am not required to report rumors. But I will advise caution in dealing with engineering companies. The PE seal means one thing, and one thing only. It means, "This work was done by me, or under my supervision." You have to let them in on the project early enough for them to be able to truthfully make that statement.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: 310.15(B)(2)(a)

Who did the drawing (or who would have done it, if the design did not have a technical issue to resolve)? If the process is that you hand the PE a finished drawing, he (or she) reviews it, finds it acceptable, seals and signs it, and gives it back to you (along with an invoice for professional services), then the PE would be violating the law.

The PE can certainly accept any information, suggestions, design sketches, and other preliminary work that you have done, and can take it all into consideration. What PEs cannot do (legally) is to seal a document that was not prepared by them or under their supervision.
 

Minuteman

Senior Member
Re: 310.15(B)(2)(a)

Sorry Charlie,

They are doing the drawing, not me. I just did the work first. The AHJ informed me that I needed a drawing for their records. We just dialed down the main breaker, to match the derated amperage. I just wanted some explanation as to WHY I had to derate, since I read the code as saying that my parallel conductors count as one.
 
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