the purpose of Table 310.15(B)(2)(a) and it's sister on the next page

[mshields]

Table 310.15B2a shows derating factors for ambient temperatures lower than 30deg C. it's sister on the next page, i.e. Table 310.15 B 2 b shows derating factors for ambient temperatures other than 40 deg C.

Obviously the first table is for when it is colder than the acceptable range 30 to 40 deg C. One would expect that the second table is for hotter than 40 degrees C hence covering the whole range. But whereas the first table only covers the lower than 30deg C, the second table covers the entire range of temperatures starting with 10 or less and going all the way up to 225deg C.

So for the colder temperatures, which table does one use to determine the derating factor? I would think that the answer is that you always use the first table for cold area's but what then is the purpose of having the cold temperatures covered in the second table as well?

 

[infinity]

I believe it's because if you used one of the ampacity tables starting with Table 310.15(B)(18) and so on they're based on a 40? C ambient temperature. For tables Table 310.15(B)(16) and (17) they're based on the 30? C ambient temperature.

 

[texie]

Table 310.15B2a shows derating factors for ambient temperatures lower than 30deg C. it's sister on the next page, i.e. Table 310.15 B 2 b shows derating factors for ambient temperatures other than 40 deg C.

Obviously the first table is for when it is colder than the acceptable range 30 to 40 deg C. One would expect that the second table is for hotter than 40 degrees C hence covering the whole range. But whereas the first table only covers the lower than 30deg C, the second table covers the entire range of temperatures starting with 10 or less and going all the way up to 225deg C.

So for the colder temperatures, which table does one use to determine the derating factor? I would think that the answer is that you always use the first table for cold area's but what then is the purpose of having the cold temperatures covered in the second table as well?

Mike,
There are 2 tables. 310.15(B)(2)(a) is based on 30 degree ambient and 310.15(B)(2)(a) is based on 40 degree ambient. If you look at the ampacity tables you will note that in the heading for each it is noted what the ambient temp. the table is based on. You then use the appropriate ambient correction table.