3000 amp feeder sizing.

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captnconduit

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Masury, OH
I am working with an engineer on a project that has a 3000 amp main switchboard. We are feeding this underground from a pad mount transformer. He is insisting on derating the primary cable due to duct bank configeration and wants us to install (10) 750 mcm THHN copper per phase to get 3000 amp. I feel this is overkill. What do you guys think?
 
captnconduit said:
I am working with an engineer on a project that has a 3000 amp main switchboard. We are feeding this underground from a pad mount transformer. He is insisting on derating the primary cable due to duct bank configeration and wants us to install (10) 750 mcm THHN copper per phase to get 3000 amp. I feel this is overkill. What do you guys think?
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Without knowing complete details on the duct bank it's impossible to say. It would also be good to know something about the loads and the calculated load for the service -- there may be a reason the engineer is being conservative (ie., an industrial switchboard that's 80% loaded with large motors could require a more conservative approach than a service for a strip mall).

Martin
 
captnconduit said:
This is feeding office and warehouse space. The biggest load is HVAC. I anticipate the load to be about 20%.
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OK, but what about the duct bank configuration. Is it 1 layer of 10 pipes? 2 layers of 5? 3 layers of 4 with spares? What is the spacing on the conduits?
 
You might want to look at the post titled "Conductor Ampacity In a duct bank?" inElectrical Calculations/Engineering started 2/11/2008. My opinion is that what you propose is not adequate, but there's not enough information to know for sure. I'm obviously on the PE side of things, but I would suggest that the PE who designed this did what he did for a reason. I would strongly suggest not changing things unless the PE agrees to the change.

Martin
 
hmspe said:
You might want to look at the post titled "Conductor Ampacity In a duct bank?" inElectrical Calculations/Engineering started 2/11/2008. My opinion is that what you propose is not adequate, but there's not enough information to know for sure. I'm obviously on the PE side of things, but I would suggest that the PE who designed this did what he did for a reason. I would strongly suggest not changing things unless the PE agrees to the change.

Martin
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I think you have a problem with the link. Try this one.
http://forums.mikeholt.com/showthread.php?t=95663&highlight=Conductor+Ampacity+duct+bank

captnconduit said:
This is feeding office and warehouse space. The biggest load is HVAC. I anticipate the load to be about 20%.
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How did you get main size up to 3000 amps?
 
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bob said:
I think you have a problem with the link....
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Actually that was just cut and paste from the thread. Links are probably easy but are not something I've done here. My experience is that links frequently die or fail to work. Just saying "look here" doesn't help much if the link gets broken, so I chose to post the search terms instead. The underline is because the text I cut and pasted is underlined in the forum. Sorry for the confusion.

Martin
 
captnconduit said:
I am working with an engineer on a project that has a 3000 amp main switchboard. We are feeding this underground from a pad mount transformer. He is insisting on derating the primary cable due to duct bank configeration and wants us to install (10) 750 mcm THHN copper per phase to get 3000 amp. I feel this is overkill. What do you guys think?
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I'm with you on this one, how do you determine the derating without a known load calc.

I'd be more concerned about keeping the parallel conductor lenghts the same in such a

short run, adding more and larger conductors will not help the effort. jmo.
 
captnconduit said:
If we go his way, 4, 4, 3, (1 spare). I want to go 5, 4 (1 spare) all 500mcm.
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Neither of those matches the description at the top of Table 310.16 (i.e., "not more than three current-carrying conductors in raceway, cable, or earth"). Neither matches any of the figures of underground ductbanks in article 310 or in Annex B. So I could, if I were to have an influence on the installation, take the stance that you do not know the ampacity of any conductor, unless you perform a calculation that specifically addresses the installed configuration. And unless you were to show me that the 3000 amps was based on an Article 220 load calculation, that is the stance I would take.

On the other hand, if the results of a load calculation show a total load under 3000 amps, I would be willing to use the ampacities from Table 310.16 (i.e., 380 amps for a 500 MCM copper, with a total ampacity of 3420 amps for nine such conductors). I have seen a declaration from the state inspectors in Washington to this effect. The difference is that that load calculation process is known to be very conservative.
 
charlie b said:

Neither of those matches the description at the top of Table 310.16 (i.e., "not more than three current-carrying conductors in raceway, cable, or earth").
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Each of the conduits carry 3 conductors + neutral.
Is it your opinion that the individual conduits in the duct bank are not each a raceway or are you considering the duct bank a raceway? or none of the above.
 
bob said:
Is it your opinion that the individual conduits in the duct bank are not each a raceway or are you considering the duct bank a raceway? or none of the above.
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None of the above.
bob said:
Each of the conduits carry 3 conductors + neutral.
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Let us look at just one of the conduits. Take the one at the top-left of the array, and call it "Conduit #1." It contains three current-carrying conductors. Thus, looking at it alone, it meets the description at the top of Table 310.16.

However, that conduit is not alone. There are three more current-carrying conductors just a little bit to the right, and they are contributing heat to the surrounding concrete and dirt. In addition, there are three more current-carrying conductors just below Conduit #1, and three more below the one to the right, and three more somewhere else near by, and three more below them, and I don't think that covers them all yet.

In my view, the notion of "not more than three current-carrying conductors in raceway" presumes that every other wire in the entire world is so far away from these three that there are no mutual heating effects, that neither will raise the temperature of the dirt that surrounds the other, and that neither impedes the rejection of heat that the other is generating via I2R losses. How far apart must that be? The NEC does not, as some seem to suppose, give us an answer to that question. Some have told me that the fact that several of the figures (for example, in Annex B) show a 7.5 inch spacing is enough to infer that that is far enough to preclude mutual heating effects. I disagree. For one thing, the tabulated ampacities in Annex B are far below those shown (for the same conductors) in Table 310.16. If 7.5 inches were enough, then why doesn't Annex B give us the same ampacities as Table 310.16?

So if I were in a position to "make the call," and if I wanted to take a hard line stance, I could choose to disallow the use of Table 310.16 for underground ductbanks, and only allow two options. One option would be for you to build an installation that exactly matches one of the figures in the NEC (and I would accept anything shown in Annex B). The other option would be for you to show me an ampacity calculation that was performed under engineering supervision, and that employed the Neher-McGrath methodology. I could, if I so chose, base that hard line stance on the fact, for fact it is, that the NEC does not provide any other alternatives.

My state inspectors have elected to allow the use of Table 310.16 for underground ductbanks, when the load is based on an article 220 calculation. I believe that is because they are offsetting the additional heat generated by nearby conduits against some of the conservatisms that are built into the article 220 calculation process. But that is just a guess on my part.
 
charlie b said:
My state inspectors have elected to allow the use of Table 310.16 for underground ductbanks, when the load is based on an article 220 calculation. I believe that is because they are offsetting the additional heat generated by nearby conduits against some of the conservatisms that are built into the article 220 calculation process. But that is just a guess on my part.
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I tend to agree with this statement. When you said
Neither of those matches the description at the top of Table 310.16 (i.e., "not more than three current-carrying conductors in raceway, cable or Annex B
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it sounded contradictory to your previous statement and 310.16 would not apply. I wanted clear this up.
 
Did I clear it up, or does it still sound contradictory? :confused:


Just in case it's still not clear, let me say that my "neither of those matches" statement was actually referring to two things:
  • The information at the top of 310.16 limits us to 3 conductors, and I explain above how some might think that a ductbank with several runs of 3 conductors does not fit that description.
  • The ductbank figures in the NEC include a 2x2 array, a couple 2x3 arrays, and several others. There is no 5 x 2 array, and no 4 x 3 array, in any of the NEC figures, so neither of those matches an NEC figure.
Better, worse, or no change? :wink:
 
charlie b said:
The information at the top of 310.16 limits us to 3 conductors, and I explain above how some might think that a ductbank with several runs of 3 conductors does not fit that description.
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I thought your were proposing this as your belief. You cleared it up.

The ductbank figures in the NEC include a 2x2 array, a couple 2x3 arrays, and several others. There is no 5 x 2 array, and no 4 x 3 array, in any of the NEC figures, so neither of those matches an NEC figure.
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No problem with this statement.
 
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