De-rating wire

[frankr]

I have always used the 90 degree column for my derating calculations, I have recently been told that this is incorrect by an engineer. I have been taught to do this by instructors as well as inspectors. Is there a code section that directs you to do so or is this just "industry standard" practice that is incorrect?

 

[480sparky]

You use whatever temperature column that corresponds to the conductor you are working with. If you are using a 90?C conductor, you use the 90?C column. If you are using a 75?C conductor, you derate using the 75?C column. If you have a 60?C conductor, you start in the 60?C column.

See Table 310.13(A).

 

[roger]

I have always used the 90 degree column for my derating calculations, I have recently been told that this is incorrect by an engineer. I have been taught to do this by instructors as well as inspectors. Is there a code section that directs you to do so or is this just "industry standard" practice that is incorrect?

You have been taught correctly, see 110.14(C)

Roger

 

[Dennis Alwon]

Article 110.14(C)

(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.

 

[raider1]

I agree with what the others have said.

The last sentence of the quote Dennis provided from 110.14(C) make it pretty clear that you can use the 90 degree column to start your ampacity adjustments for 90 degree rated conductors.

Chris