Duct Bank Cable Derating

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kenaslan

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Per NEC 2014 310.60(C)(77) for 310.60 Detail #3. 3-1/C 350 MV-105 cable

I Need 2000A. The question is, if I increase the duct bank conduit spacing, will the ampacity of the wire increase? Or no matter what I do 270A is the most I can get out of that cable and in turn will be forced to use 6 sets of 750.
 
It is possible that if you increase the spacing, the ampacity will increase.

Since it is not a standard configuration, you will have to do the Neher McGrath Calcs to see what it increases to.
 
At one time that section was not require by the code. It was put in as information. Several times I have seen
comments that said the standard method of calculations was adequate because it oversized the conductor.
However, the 2014 code may have changed this.
 
If you don't want to do the calcs, then best guess is all you can hope for. Cable ampacity is based on heating, and very dependent on what surrounds the conduits.

In the OP, there was no mention of whether this is concrete encased, how deep, what the type of material surrounding duct bank is, or surface material (asphalt is better than concrete) If you want to reduce cable quantity, I would also investigate direct bury.

Without details regarding duct bank, it is not possible to deduce better options.
 
The figures show the conduits spaced 7.5 inches apart. If you are asking whether you could instead space them 9 inches, or 12 inches, or some other distance apart, and thereby increase the ampacity to the value you need, I don't think you will succeed. You certainly won't get a usable answer without using a software package as others have already mentioned. Every time I used such a package, the results always came out lower than table ...(77).

How about using two parallel ductbanks, each using Detail 2 (3 circuits)? If you space them far enough apart (don't ask me for a specific minimum value, but I think 3 feet should do the trick), then you can treat them as being independent. That gives you an ampacity of 335 amps for 350 MCM conductor of the type you describe. Six of these give you an ampacity of 2010 amps. Can you do this and still make the connections at both ends?
 
If you could expand the distance between ducts-vertically and horizontally ?to 1000 mm [39?] in a concrete of 55 dgr.C.cm/w [RHO] and 90 of surrounding earth you would get 314 A ampacity [1880 A 6 cables].The concrete block dimensions 51*90?.
If you could set the block top depth to 1.5 ft. instead of 2.5 you?ll get 330 A per cable [1980 A 6 parallel cables].
All this calculated following Neher and McGrath.
 
Sorry guys. Did not mean to leave you hanging. I just got back. We decided to go with the 6 sets of 750 as time was of the essence. Will need to get our ETAP guy to look at it when he can fit it into his schedule.
 
Also going to use MC-HL in a direct pour as I cannot fit the cables into the bottom of the gear any other way. At least without a blow torch, but me thinks the client will have an issue with that.:thumbsup:
 
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