William-
Yes, you can use the 90C column for ampacity adjustment when using 90C
wire, per the awkward 110.14(C)- "Conductors with temperature ratings
higher than specified for terminations shall be permitted to be used for
ampacity adjustment, correction, or both."
Assuming copper wiring and the NEC '11 T310.15(B)(16):
Let's say you have a 40A breaker with a 75C terminal rating and you're
looking to land THWN-2 on it. Since the terminal has a 75C rating, you
need to make sure that this terminal rating is rated for at least 40A in
the 75C column. Since a 75C terminal with a #10 can only handle 35A,
you'd have to go to a #8 to get a rating of at least 40A-- in this case,
a #8/75C can handle 50A.
So why install 90C-rated conductors at all? When using 90C conductors
you can use the 90C ampacity column to apply your derate factors. Let's
say your #8 conductor (selected above) is in 50C ambient with 4
conductors in the raceway.
If using #8 THWN, use 75C columns:
50A x 0.75 (T310.15(B)(2)(a)) x 0.8 = 30A
That's too small for a 40A breaker, so you're stuck with a #6:
65A x 0.75 x 0.8 = 39A (OK)
IF using #8 THWN-2, can use 90C columns:
55A x 0.82 x 0.8 = 36A (OK per 240.4(B))
And I'll beat Mr. Brearley to posting a relevant SolarPro article:
http://solarprofessional.com/articles/design-installation/code-compliant-conductor-sizing?v=disable_pagination
DKC
On 2014/1/22, 19:32, William Miller wrote:
Friends:
I try to be rigorous in application of NEC codes to everything I do,
including wire sizing. I understand that even though I am using
conductors rated at 90°C, the breakers I use have terminals rated at
75°C so when deriving the values for ampacity for a given gauge from the
tables, I have to use the 75°C column.
What is not clear, however, is which column I use when applying
temperature derating. Table 315(B)(2)(b) has a column for 75 and a
column for 90. I am using 90° wire. The values for 90° are much more
generous than the 75° values and I would like to use them. Which is
correct?
As always, thanks to everyone on this list for all of the help and advice.
William
Gradient Cap
Lic 773985
millersolar.com <http://www.millersolar.com/>
805-438-5600
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