William-
Happy to help. You need to use 310.15(B)(2)(a) in conjunction with
T310.15(B)(16) and (17), where ampacity is given for 30C ambient.
For T310.15(B)(18-20), where ampacity is given for 40C ambient, you
would use 310.15(B)(2)(b). The THWN in 310.15(B)(20) is only for
messenger-supported wiring, and I assume that it's only #8 and larger
because of some fine print note in the UL Chartreuse Book or something.
I've never used 310.15(C), and I'd guess that its resulting cost savings
would exceed the cost of your time in arguing with the AHJ about not
using 310.15(B)(16)! I don't recall ever seeing the formula used and
looking at these factors:
Yc = component ac resistance resulting from skin
effect and proximity effect
Rca = effective thermal resistance between conductor
and surrounding ambient
I'm sure that calculating these out for the various environments where
our conductors are going is an incredibly interesting process.
310.15(B)(DKC)
On 2014/1/24, 14:42, William Miller wrote:
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-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Dave Click
Sent: Wednesday, January 22, 2014 5:32 PM
To: re-wrenches@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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