Neher-McGrath vs. 310-16 Values

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Well, I convinced the folks at work we should be using professional software to calculate all of our ductbanks using correct engineering analysis. Only problem is, the derating I get seems extreme.

Example: I am trying to design a ductbank rated 3600 amperes. The previous engineer who did the original installation, sized it for 3200 amperes with 12 sets of 500 kcmil. All is good per NEC Table 310-16 right? 12x380A=4560A.

Using the software, the worst duct position only has a caculated ampacity of only 211A. This means the original ductbank only has an ampacity of 2400A, far less than the 4560 of NEC Table 310-16. To get to 3600A, I actually have to go to 16 sets of 750 kcmil, adding additional sets of 500 kcmil I can't get to 3600 no matter what, each additional duct increases the derating more than the ampacity of the added wiring.

Does this seem right? This means that 750 kcmil derates 53%! Duct configuration is 8 wide by 2 high, 2" spacing between ducts and 2" cover on all sides.

[ February 21, 2005, 02:54 PM: Message edited by: line_noise ]
 

rbalex

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Location
Mission Viejo, CA
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Professional Electrical Engineer
Re: Neher-McGrath vs. 310-16 Values

That sounds about right.

It's that 2" spacing thats killing you. With them that close you're getting close to shoving them all in a common raceway. Look at what you would have with 48 conductors in a raceway.
 

bob

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Location
Alabama
Re: Neher-McGrath vs. 310-16 Values

Your approach is the correct method to use.
AS Bob says that 2" spacing is the problem. Why not use the 7.5" spacing per detail 3 Fig 310.60.
If you can install 2 banks 2 x 6 and seperate them
you could use the ampacity in Table B 310.6 and
use the 314 amps for 750 cu.

[ February 21, 2005, 04:23 PM: Message edited by: bob ]
 
Re: Neher-McGrath vs. 310-16 Values

To clarify, I meant 2" spacing between ducts, conduit size is 4", so my spacing is 6.5" center to center, a little worse than the examples in 310-60 but not much more so.

I'd increase spacing if it were new, but it is existing w/the 12 sets 4" w/500kcmil. Per the details on the drawings and most work I've seen over the years, 6.5" center to center spacing is quite common. The ductbank was constructed under an earlier project designed by another firm, now we are doing the next phase of expansion, adding significant load. The load is already 1700 amperes recorded, so I definitely need to use Engineering Supervision, I don't think the Code calculations and tables are conservative enough to keep me out of trouble forever on this one.

Seems like around here all the utilities use 2" spacing in between conduits, or less. PEPCO actually uses 5" fiberglass conduit with thinner wall than Sched 40 PVC, and only requires 1.5" spacing. As a result, most engineers have copied similar spacing, thinking if its good enough for the utilities, its good enough everywhere else.
 

charlie b

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Lockport, IL
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Retired Electrical Engineer
Re: Neher-McGrath vs. 310-16 Values

I have already expressed in this Forum, several times in fact, my opinion on this issue. Here is one previous post.

My only question for you would be to ask how you came up with the value of 3600 amps. Did you perform a service calculation, using Article 220, for an upgrade to, or replacement of, the existing facility? Was 3600 amps the result of that calculation? If the answers are both ?yes,? then you might be able to justify using Table 310.16. Otherwise, you may well have to resort to construction of a new duct bank.
 
Re: Neher-McGrath vs. 310-16 Values

We used recorded demand for the existing feeder load, and calculated for the new load. I don't dispute the need or the value of using the more conservative calculations, I came here more out of surprise at the results, wanted to make sure there wasn't a mistake.
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: Neher-McGrath vs. 310-16 Values

Every time I did a calculation of ampacity in an underground duct bank, the results came out far lower than those in 310.16. So your description did not surprise me. Good luck on this one.
 

charlie

Senior Member
Location
Indianapolis
Re: Neher-McGrath vs. 310-16 Values

Before we start running the electric utilities down for not doing things to match the Code, let me through my 2? into the mix.

Most electric utilities will not overload their conductors for an extended period of time. We are also not constrained by the Code rules for load calculations. Therefore, we will estimate the actual load and build accordingly. This is based on past experience and probabilities. We know that you will not use all of the service that you are installing.

We will sometimes guess incorrectly and have to rebuild something but most of the time we nail it. After all, we give a lifetime guarantee. :D
 
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