Underground Ductbank Ampacity Calc - Very Limiting

[designer82]

I ran an underground ductbank ampacity calculation in ETAP.

Although the ductbank is very large with about 46 conduits, the calculation results are crippling.
It shows a 1200 Amp base ampacity cable derated to only about 400 Amps. So that's like a 67% derate.
We're already running 4 sets of 500Kcmil for 1200A feeders.

I'm thinking these underground ductbank ampacity calculations are too conservative and may just ignore it.


Any experience with this? Do these results seem reasonable?

 

[GoldDigger]

I can't give you practical experience, but from the view of theory, the problem is that the center ducts in that large a bank do not have a direct thermal path to the surrounding earth. There will be a significant temperature gradient from the inner ducts to the ducts on the edge of the bank, and the required derating will be much higher for the innner ducts IF all of the ducts will be carrying wiring which is loaded to the same fraction of its open air ampacity.

Does ETAP allow you to calculate based on the individual loading of the wiring in each duct considering the position of each duct within the bank? Are all of the ducts within the bank going to be loaded to the same extent?
Have you looked at the feasabiity of building two or more duct banks with separation between them?

 

[Julius Right]

If RHO=90 and 20oC-as per NEC- four 500 MCM copper EPR/XLPE insulated in 46 filled ducts duct bank 12" apart 100% loaded 7*7 ducts[46 ducts 3 lives+1 neutral] 3 ducts in the middle empty] 170.6 A [each live conductor] calculated according to IEEE 835/1994 [following Neher and McGrath ].

If you may use only 75% LF [according to Neher and McGrath] you'll get 211 A in same conditions.