Vertically Stacked Direct Burial Duct Bank

[smoothops10]

I've seen several utility and campus style standard specifications prohibit vertically stacking ducts in a direct burial duct bank. Sometimes an exception is given for communications ducts where they are allowed to be on the top row in a 2x2. I am trying to understand the reasoning behind the prohibition of vertically stacked ducts in direct burial install. I realize the weight of a set of 500kCMIL may be 6+ lbs/ft and maybe has something to do with not wanting the weight of the heavy duct on top of another duct below it whereas a comms duct would weigh little and be subject to much lower pulling forces and minimal weight in lbs/ft but really what does it matter if the first row of ducts is properly compacted? Any thoughts appreciated - thanks.

 

[ron]

My guess would be the deeper ducts would not dissipate heat at the same rate as the upper.

Look at the Table B.310.15(B)(2)(7) Detail 1 (single row) versus the other Details (multi-row) compared to it sister table 310.15(B)(16), deeper conduit with mutual heating gets derated

 

[Julius Right]

In our facilities- Power Stations-there are duct banks of 30-50 ducts each and the ducts are arranged in horizontal and vertical rows and columns according to location possibilities. The cable run order is dictated -in my opinion-by two rules:
-At first the time of the cable will run-the earlier will be at lower position and second factor is the specific weight: the heavier will be on lower position.
Typically, medium voltage cables are in the lowest position, the heavy low voltage power cables above and the control in the uppermost position.

 

[Julius Right]

By the way, I don't think NESC prohibits superposed vertically duct rows[I mean NESC 2007 sections 33,34,35].

 

[kingpb]

its all based on the operating temperature of the cables. In a large duct bank the cable is modeled and to even out temperatures it may be moved from inside to outside tube, or lower in the arrangement. It is sometimes an iterative process.

Soil does not dissipate heat as well as concrete, unless your using utilizing special back fill material. Going deeper is a disadvantage.