[RE-wrenches] Temperature Compensation calcs for wire

[Jeffrey Quackenbush]

William,

I was confused by this too until I stared at it for awhile. In the bolded heading to all the tables 310.15 (16) – (20), it says either "Based on Ambient Air Temperature of 40C (104F)" or "Based on Ambient Air Temperature of 30C (86F)". Basically conductors with a temperature rating of 150C+ use the 40C de-rate factors and conductors rated 60C to 90C (which is what a PV installer will use in almost every scenario) use the 30C column. These temp de-rate values used to be posted directly under the ampacity tables, so you wouldn't have noticed the difference between the 40C and 30C values before 2011.

Jeffrey Quackenbush



On Friday, January 24, 2014 9:43 PM, William Miller <william at millersolar.com> 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-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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