I wouldn't. Speaking as an engineer, if I did not know what value to insert into a formula, I would use a value that I knew to be either higher or lower than what the actual value would be, and I would choose the higher or the lower based on which of those gave me the more conservative result. In this instance, a "conservative value" of ampacity would be low. That is, you do not want your calculated ampacity to be higher than it should be. So you don't use an assumed value for an unknown variable, if that assumed value gives you a higher value for ampacity. In the formula under consideration, if you use zero for deltaTD (i.e., the lowest possible value), that would tend to give you a higher answer for calculated ampacity than you would get by using a large number for deltaTD. So whatever else might be the best approach to this situation, I would not pick zero as the value for deltaTD.
Originally Posted by Smart $
Actually, I would not use that formula at all. I think it is useless. If I had to calculate the ampacity for an underground ductbank, I would use a packaged software program.
Charles E. Beck, P.E., Seattle
Comments based on 2008 NEC unless otherwise noted.