<html><body><div style="color:#000; background-color:#fff; font-family:HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:10pt"><div><span>William,</span></div><div style="color: rgb(0, 0, 0); font-size: 13px; font-family: HelveticaNeue, 'Helvetica Neue', Helvetica, Arial, 'Lucida Grande', sans-serif; background-color: transparent; font-style: normal; "><span><br></span></div><div style="color: rgb(0, 0, 0); font-size: 13px; font-family: HelveticaNeue, 'Helvetica Neue', Helvetica, Arial, 'Lucida Grande', sans-serif; background-color: transparent; font-style: normal; "><span>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.</span></div><div style="color: rgb(0, 0, 0); font-size: 13px; font-family: HelveticaNeue, 'Helvetica Neue', Helvetica, Arial, 'Lucida Grande', sans-serif; background-color: transparent; font-style: normal; "><span><br></span></div><div style="color: rgb(0, 0, 0); font-size: 13px; font-family: HelveticaNeue, 'Helvetica Neue', Helvetica, Arial, 'Lucida Grande', sans-serif; background-color: transparent; font-style: normal; ">Jeffrey Quackenbush</div><div class="yahoo_quoted" style="display: block; "> <br> <br> <div style="font-size: 10pt; font-family: HelveticaNeue, 'Helvetica Neue', Helvetica, Arial, 'Lucida Grande', sans-serif; "> <div style="font-size: 12pt; font-family: HelveticaNeue, 'Helvetica Neue',
Helvetica, Arial, 'Lucida Grande', sans-serif; "> <div dir="ltr"> <font size="2" face="Arial"> On Friday, January 24, 2014 9:43 PM, William Miller <william@millersolar.com> wrote:<br> </font> </div> <blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; margin-top: 5px; padding-left: 5px;"> <div class="y_msg_container">Dave:<br><br>Thanks for this post, although in order to completely understand, it I<br>sacrificed a significant amount of time working up a spreadsheet and running<br>various scenarios. My philosophy is that this will pay off in the long run.<br>I always create a spreadsheet so I can figure something out once and use it<br>always.<br><br>When analyzing these codes, I was wondering which table to use: table<br>310.15(B)(2)(a) or table 310.15(B)(2)(b), and why? The values are<br>significantly different. For example at 62°C the values are 0.65 and 0.71,<br>respectively. I deduced
that they must be used in conjunction with ampacity<br>values from the correct 310.15(16) through (20) tables in regards to ambient<br>temperature. I noted that for THWN the later tables (40°) do not go below<br>8AWG, so I figure I need to stick with 30° tables. Is this correct?<br><br>Lastly, has anyone used the 310.15(C) formula for calculating the values.<br>This should not be that hard. What is considered "Engineering Supervision"?<br><br>Thanks again.<br><br>William<br><br>-----Original Message-----<br>From: <a ymailto="mailto:re-wrenches-bounces@lists.re-wrenches.org" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">re-wrenches-bounces@lists.re-wrenches.org</a><br>[mailto:<a ymailto="mailto:re-wrenches-bounces@lists.re-wrenches.org" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">re-wrenches-bounces@lists.re-wrenches.org</a>] On Behalf Of Dave Click<br>Sent: Wednesday, January 22, 2014 5:32 PM<br>To: <a
ymailto="mailto:re-wrenches@lists.re-wrenches.org" href="mailto:re-wrenches@lists.re-wrenches.org">re-wrenches@lists.re-wrenches.org</a><br>Subject: Re: [RE-wrenches] Temperature Compensation calcs for wire<br><br>William-<br><br>Yes, you can use the 90C column for ampacity adjustment when using 90C wire,<br>per the awkward 110.14(C)- "Conductors with temperature ratings higher than<br>specified for terminations shall be permitted to be used for ampacity<br>adjustment, correction, or both."<br><br>Assuming copper wiring and the NEC '11 T310.15(B)(16):<br>Let's say you have a 40A breaker with a 75C terminal rating and you're<br>looking to land THWN-2 on it. Since the terminal has a 75C rating, you need<br>to make sure that this terminal rating is rated for at least 40A in the 75C<br>column. Since a 75C terminal with a #10 can only handle 35A, you'd have to<br>go to a #8 to get a rating of at least 40A-- in this case, a #8/75C can<br>handle 50A.<br><br>So
why install 90C-rated conductors at all? When using 90C conductors you<br>can use the 90C ampacity column to apply your derate factors. Let's say your<br>#8 conductor (selected above) is in 50C ambient with 4 conductors in the<br>raceway.<br><br>If using #8 THWN, use 75C columns:<br>50A x 0.75 (T310.15(B)(2)(a)) x 0.8 = 30A That's too small for a 40A<br>breaker, so you're stuck with a #6:<br>65A x 0.75 x 0.8 = 39A (OK)<br><br>IF using #8 THWN-2, can use 90C columns:<br>55A x 0.82 x 0.8 = 36A (OK per 240.4(B))<br><br>And I'll beat Mr. Brearley to posting a relevant SolarPro article:<br><a href="http://solarprofessional.com/articles/design-installation/code-compliant-con" target="_blank">http://solarprofessional.com/articles/design-installation/code-compliant-con</a><br>ductor-sizing?v=disable_pagination<br><br>DKC<br><br>On 2014/1/22, 19:32, William Miller wrote:<br>> Friends:<br>><br>> I try to be rigorous in application of NEC codes to
everything I do, <br>> including wire sizing. I understand that even though I am using <br>> conductors rated at 90°C, the breakers I use have terminals rated at <br>> 75°C so when deriving the values for ampacity for a given gauge from <br>> the tables, I have to use the 75°C column.<br>><br>> What is not clear, however, is which column I use when applying <br>> temperature derating. Table 315(B)(2)(b) has a column for 75 and a <br>> column for 90. I am using 90° wire. The values for 90° are much more <br>> generous than the 75° values and I would like to use them. Which is <br>> correct?<br>><br>> As always, thanks to everyone on this list for all of the help and advice.<br>><br>> William<br>><br>> Gradient Cap<br>><br>> Lic 773985<br>><br>> millersolar.com <<a href="http://www.millersolar.com/"
target="_blank">http://www.millersolar.com/</a>><br>><br>> 805-438-5600<br>><br>><br>><br>> _______________________________________________<br>> List sponsored by Home Power magazine<br>><br>> List Address: <a ymailto="mailto:RE-wrenches@lists.re-wrenches.org" href="mailto:RE-wrenches@lists.re-wrenches.org">RE-wrenches@lists.re-wrenches.org</a><br>><br>> Change email address & settings:<br>> <a href="http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org" target="_blank">http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org</a><br>><br>> List-Archive: <br>> <a href="http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org" target="_blank">http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org</a><br>><br>> List rules & etiquette:<br>> www.re-wrenches.org/etiquette.htm<br>><br>> Check out participant bios:<br>>
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bios:<br>www.members.re-wrenches.org<br><br><br></div> </blockquote> </div> </div> </div> </div></body></html>