[RE-wrenches] Temperature Compensation calcs for wire

[Dave Click]

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
>
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>
> 805-438-5600
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