NEC temperature correction factors for conductor ampacity

[PE blues]

NEC has two temperature correction tables 310.15(B)(2)(a) and 310.15(B)(2)(b), table-a for 30C ambient and table-b for 40 C ambient. Under what conditions do we use each of these tables?

 

[Wael]

you use these tables based on the foot (Base temperature defined in each Ampacity tables' titles.

For easiness, just follow the foot notes on the bottom of each each of Ampaciity tables.

For example: T.310.15 (B) (16) is based on 30 C Ambient T. so you go to 310.15(B)(2)(a)

While T.310.15 (B) (18) is based on 40 C Ambient T, so you go to 310.15(B)(2)(b)

All the best.

 

[PE blues]

Here's a question from spin up

What temperature correction factor is used for 75C conductors at 39C ambient temperature

a)1.11

b)1.0

C)0.82

D)0.88

The correct answer is 0.88.

According to 310.15(B)(2)(a) this is 0.88, and 310.15(B)(2)(b) is 1., why did they pick option D and not B

 

[Wael]

you need to be familiar a little bit with the ampacity tables. the tables with 75C are based on 30C Ambient Temperature.

Therefore, only answer D is applicable for this question.