Cable derating

[inspector 102]

Given 3-insulated conductors on a messenger, 250 kcmil, spanning overhead to a pole then into Sch 80 conduit down the pole for 12 feet then underground for 8 feet into a generator transfer switch, what would be the ampacity capabilities of the conductor. Would Table 310.20 apply all the way or would the cable have to be derated due to the fact it runs through Sch 80?

 

[charlie b]

Re: Cable derating

I wouldn?t use the term ?derated.? Rather, I would say that you need to determine the rating based on the overall route. Going a total of 20 feet in a conduit tells me that the ampacity of that portion of the run has to be determined from Table 310.16. The ampacity of the rest of the run can be determined from Table 310.20. NEC 310.15(A)(2) then requires that you take the lowest applicable ampacity. In other words, the ampacity of the run must be based on 310.16.

 

[xoxota]

Re: Cable derating

I agree with Charlie B. Choose the lowest ampacity which will be the value given in 310-16. Then you can apply temperature correction factors (derating) to the ampacity value from 310-16.