line_noise
Member
Well, I convinced the folks at work we should be using professional software to calculate all of our ductbanks using correct engineering analysis. Only problem is, the derating I get seems extreme.
Example: I am trying to design a ductbank rated 3600 amperes. The previous engineer who did the original installation, sized it for 3200 amperes with 12 sets of 500 kcmil. All is good per NEC Table 310-16 right? 12x380A=4560A.
Using the software, the worst duct position only has a caculated ampacity of only 211A. This means the original ductbank only has an ampacity of 2400A, far less than the 4560 of NEC Table 310-16. To get to 3600A, I actually have to go to 16 sets of 750 kcmil, adding additional sets of 500 kcmil I can't get to 3600 no matter what, each additional duct increases the derating more than the ampacity of the added wiring.
Does this seem right? This means that 750 kcmil derates 53%! Duct configuration is 8 wide by 2 high, 2" spacing between ducts and 2" cover on all sides.
[ February 21, 2005, 02:54 PM: Message edited by: line_noise ]
Example: I am trying to design a ductbank rated 3600 amperes. The previous engineer who did the original installation, sized it for 3200 amperes with 12 sets of 500 kcmil. All is good per NEC Table 310-16 right? 12x380A=4560A.
Using the software, the worst duct position only has a caculated ampacity of only 211A. This means the original ductbank only has an ampacity of 2400A, far less than the 4560 of NEC Table 310-16. To get to 3600A, I actually have to go to 16 sets of 750 kcmil, adding additional sets of 500 kcmil I can't get to 3600 no matter what, each additional duct increases the derating more than the ampacity of the added wiring.
Does this seem right? This means that 750 kcmil derates 53%! Duct configuration is 8 wide by 2 high, 2" spacing between ducts and 2" cover on all sides.
[ February 21, 2005, 02:54 PM: Message edited by: line_noise ]