Ampacity Calculations for Lower Depths
B310.60.C.(2)b states that for deeper portions an ampacity derating factor of 6% per foot shall be permitted. Based on this, one can calculate a derating factor of 2.1 which indeed doesn't sound right. Put in another way, if a cable is installed in a duct 16.6-ft deep it will have no rating whatsoever therefore not even a 1 Amp load would be permitted; not realistic either.
I'm certainly not sure what the NEC means but it could be a "compound interest" type of calculation where the figure (1-6%)or 0.94 is raised to the power of the depth. For example, the 35' case would be 0.94^35 = 11.5% deratng; at least something realistic.
I interestingly, certain European cable companies supply tables for derating due to depth and typically the factor becomes larger as depth increases up to 10 meters at which point the factor becomes constant, about 10-15%. This too seems realistic
Finally, what is the meaning of the NEC wording "shall be permitted"? why isn't it stated "shall be required"? Is it due to the fact that an earlier paragraph allows an ampacity determination based upon "engineering supervision"? In this case the rather simple formula stated in the NEC just before Para. 310.60 could be used which has no parameter for depth - which doesn't seem right either.
I would appreciate any comments whatsoever on this issue and a way to present a calculation to an inspector who could understand and accept a reasonable derating factor.
THanks GUys!
B310.60.C.(2)b states that for deeper portions an ampacity derating factor of 6% per foot shall be permitted. Based on this, one can calculate a derating factor of 2.1 which indeed doesn't sound right. Put in another way, if a cable is installed in a duct 16.6-ft deep it will have no rating whatsoever therefore not even a 1 Amp load would be permitted; not realistic either.
I'm certainly not sure what the NEC means but it could be a "compound interest" type of calculation where the figure (1-6%)or 0.94 is raised to the power of the depth. For example, the 35' case would be 0.94^35 = 11.5% deratng; at least something realistic.
I interestingly, certain European cable companies supply tables for derating due to depth and typically the factor becomes larger as depth increases up to 10 meters at which point the factor becomes constant, about 10-15%. This too seems realistic
Finally, what is the meaning of the NEC wording "shall be permitted"? why isn't it stated "shall be required"? Is it due to the fact that an earlier paragraph allows an ampacity determination based upon "engineering supervision"? In this case the rather simple formula stated in the NEC just before Para. 310.60 could be used which has no parameter for depth - which doesn't seem right either.
I would appreciate any comments whatsoever on this issue and a way to present a calculation to an inspector who could understand and accept a reasonable derating factor.
THanks GUys!
