Adjustment Factors, Correction Factors for Conductors

[Delcobob]

When calculating conductor ampacity, and both the adjustment factors of 310.15(B)(2)(a) and correction factors due to ambient temperature per 310.16 apply, do you figure in both derating factors, or the worst case of the two? If both are applied, which is figured first? Two different answers are possible, depending on which factor is performed first.

 

[charlie b]

You use both. More to the point, you multiply the tabulated value of ampacity (using the 90C column, if you have 90C rated wire) by each of the two factors, to get the final ampacity, noting that you can?t use a ?final ampacity? value that is higher than the ampacity in the 75C column.

If both are applied, which is figured first? Two different answers are possible, depending on which factor is performed first.

Not true at all. It?s the same regardless of which you apply first. Reference: ?The Commutative Property of Multiplication.?