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 > > > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: RE-wrenches@lists.re-wrenches.org > > Change email address & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org > _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Change email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org ----- No virus found in this message. Checked by AVG - www.avg.com Version: 2013.0.3462 / Virus Database: 3681/7024 - Release Date: 01/22/14 ----- No virus found in this message. Checked by AVG - www.avg.com Version: 2013.0.3462 / Virus Database: 3681/7024 - Release Date: 01/22/14 _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Change email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
