Ampacity and De-rating

[Trey4U]

Isn't it true that the 90 C column of Table 310.16 is for de-rating and most installations will use the 75 C column to figure allowable ampacities?

 

[ActionDave]

That is correct.

 

[infinity]

Isn't it true that the 90 C column of Table 310.16 is for de-rating and most installations will use the 75 C column to figure allowable ampacities?

Depends on whether or not you're using a 90? C conductor and a wiring method that allows 75? C ampacity. For example, THHN/THWN is only rated for 75? C in wet locations and NM cable can only be used at it's 60? C ampacity so your original statement is true in many cases but not all cases.