derating factor

[kimrichi]

what is the derating factor for THWN conductor when placed in ambient temprature of 124 F

 

[Dennis Alwon]

.67 based on Table 310.15(B)(2)(a)

 

[kimrichi]

this is what i know but in a book i am reading[stallcups calcualation} it says 76

 

[Smart $]

this is what i know but in a book i am reading[stallcups calcualation} it says 76

Perhaps an error. That is the ambient derating for a 90?C rated THWN-2 conductor.... or THWN correction based on 40?C [Table 310.15(B)(2)(b)]. For the latter, the uncorrected ampacity has to be based on 40?C... and there are no such ampacity tables in the NEC.

 

[don_resqcapt19]

Perhaps an error. That is the ambient derating for a 90?C rated THWN-2 conductor.... or THWN correction based on 40?C [Table 310.15(B)(2)(b)]. For the latter, the uncorrected ampacity has to be based on 40?C... and there are no such ampacity tables in the NEC.

Actually there are...just not ones that we normally use. 310.15(B)18 through (B)21 are based on 40?C.

 

[Smart $]

Actually there are...just not ones that we normally use. 310.15(B)18 through (B)21 are based on 40?C.

Actually there are more... the 310.60 tables.

I used Acrobat's search for 40?C before I made that statement... but after your post I remember copying "40?C" from Table 310.15(B)(2)(b)'s header. Analyzing what went wrong, I discovered the degree characters are not the same (? in ambient tables vs. ? in ampacity tables).

 

[Dennis Alwon]

NEC 2011 has T. 310.15(B)(2)(b) that has the correction factors for 40C. That would be .76 at 75C column but I would expect that we would be using the 30C table