[RE-wrenches] Temperature Compensation calcs for wire

[William Miller]

Dave:

Thanks for this post, although in order to completely understand, it I
sacrificed a significant amount of time working up a spreadsheet and running
various scenarios.  My philosophy is that this will pay off in the long run.
I always create a spreadsheet so I can figure something out once and use it
always.

When analyzing these codes, I was wondering which table to use:  table
310.15(B)(2)(a) or table 310.15(B)(2)(b), and why?  The values are
significantly different.  For example at 62°C the values are 0.65 and 0.71,
respectively.  I deduced that they must be used in conjunction with ampacity
values from the correct 310.15(16) through (20) tables in regards to ambient
temperature.  I noted that for THWN the later tables (40°) do not go below
8AWG, so I figure I need to stick with 30° tables.  Is this correct?

Lastly, has anyone  used the 310.15(C) formula for calculating the values.
This should not be that hard.  What is considered "Engineering Supervision"?

Thanks again.

William

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Dave Click
Sent: Wednesday, January 22, 2014 5:32 PM
To: re-wrenches at lists.re-wrenches.org
Subject: Re: [RE-wrenches] Temperature Compensation calcs for wire

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-con
ductor-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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