THHW listed twice on Table 310.16

[vylia1990]

I'm studying for my Administrators exam in Washington State and I realized that Table 310.16 has THHW listed both in the 75C and 90C temperature rating categories.

Which temperature rating I should use if asked to size THHW?

I apologize if this question has already been asked somewhere, I did a search a couldn't find anything.

 

[augie47]

If you look at 310.104 you will see the reason it is ibn two columns is due to a wet & dry location rating.
The column used (75 or 90) depends on wet vs dry. Any questions should provide that input

 

[Dennis Alwon]

Look at the table Thhw is not the only insulation listed in both columns. Usually wires with insulations that has a "-2" are rated 90C in both wet and dry locations.

 

[Dennis Alwon]

The reason we know that answer is because Table 310.105(A) explains it.

 

[infinity]

If a test question says THHW then you use the 75° C ampacity when in wet locations and the 90 ° C ampacity when dry.

 

[vylia1990]

Okay thanks. That's very helpful!

 

[ron]

Section 110.14(C) - will tell you what column to use. 100A or less, use 60 deg (unless you know the termination rating is higher). Greater than 100A, use 75 deg. Derate from the 90 deg if the wire rating is 90 deg, but use the lower of the resulting between the derate and the 60 or 75 value..