310.16

[Sparky32297]

So, I agree that you can use the 90 degree column for derating, but I was wondering if it is spelled out in writing anyplace in the code ?

 

[charlie b]

So, I agree that you can use the 90 degree column for derating, but I was wondering if it is spelled out in writing anyplace in the code ?

Take a look at the last sentence of 110.14(C) - before the start of sub-paragraph (1). That is the basis we all use for derating from the 90C table (presuming, of course, that you are using wire with 90C insulation). I could wish it were written more clearly, however.

 

[infinity]

So, I agree that you can use the 90 degree column for derating, but I was wondering if it is spelled out in writing anyplace in the code ?

From the 2008:

110.14 Electrical Connections.
(C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

 

[Dave58er]

I always figured it was the wording of 310.15 (B)(2) -"the allowable ampacity of each conductor"- that told me to use the column that related to whichever insulation type I was using.

My take on that wording is if you use a different column you're not using the ampacity of that conductor.