large duct bank and derating

[mshields]

I'm putting in an outdoor switchboard in and bringing just over a dozen 480V feeders. I plan to run these to the building in a duct bank 3 - 4" conduits high by 5 - 4" conduits wide.

I seem to recall that I have to worry about derating my conductors when the duct bank exceeds a certain size. I can't find anything about that in the NEC. Am I worrying for nothing or is there in fact a derating requirement for this application?

Thanks,

Mike

 

[charlie b]

I think Table 310.60(C)(77) would be the starting point for reviewing the ampacity. You will quickly note that it does not apply to your installation, since the 3x5 array is not one of the figures to which the table refers. So if you want to calculate an ampacity, you need to use a software program that can handle the conduit configuration, the burial depth, the thermal resistance of the soil, the conductor type(s) in use, and a few other factors. My favorite program that only does this type of calculation is AmpCalc, offered by CalcWare. Last time I asked my company to buy a copy, it ran around $2,000. Other programs can also do this calculation. So perhaps your company already has something that would answer your question.

 

[Julius Right]

As charlie b. already said, the NEC cable in duct bank ampacity tables do not fit to most of applications:weeping:. I did my self ?hand made" software in Visual Basic and I checked it with NEC tables. You can prepare such software in Excel, for instance, following Neher&McGrath or IEC 60287-1-1 and 2-1.
The notion "derating" is connected with a "base" ampacity. If you mean to compare with NEC Table 310.15(B)(16) for direct buried cable it could be a big derating factor-even 0.5 in the same conditions [earth RHO, temperature, depth, distance between ducts and other].

 

[charlie b]

I seem to recall that I have to worry about derating my conductors when the duct bank exceeds a certain size.

The notion "derating" is connected with a "base" ampacity.

Julius is right (the pun is not my fault :happyno: ). What you would be doing is determining the "ampacity," a term whose definition includes consideration of the conditions of use. When you look at ductbank runs, you don't start with a tabulated value of ampacity, and then discover what needs to be done to reduce that value. You just calculate what the value is.