Peter and Larry,
Brian's post was correct and consistent with the 2011 NEC. It is not correct to apply both 1.25 factors on the dc side. This has been done for years, but it is inconsistent with the rest of the code. Conditions of use are only applied to maximum current-defined by 690.8(A) as rated Isc x 1.25. It is confusing and that is why the NEC got so specific in the 2011 version. Bill. From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of wire...@gmail.com Sent: Friday, February 11, 2011 12:32 PM To: RE-wrenches Subject: Re: [RE-wrenches] Calculation of Maximum Circuit Current Very good. Thank you Peter. On Fri, Feb 11, 2011 at 2:44 PM, Peter Parrish <peter.parr...@calsolareng.com> wrote: Short answer: DC/PV side two factors of 1.25; AC side one factor. Reasons: the common factor of 1.25 is due to the treatment of PV systems as continuous sources of power, the second factor of 1.25 is for the "cloud effect". It is entirely possible to get more than 1,000 W/m^2 on a PV module if there are large cumulus clouds that don't block the sunlight but act as pretty good reflectors of sunlight. As a result you need to increase Isc (times the number of combined strings if that is the case) by 1.56, and all computed AC currents by 1.25. Same goes for and switches and OCPDs. Then of course you need to compute the ampacity of the wire being used, taking care to apply the temperature correction and conduit fill correction before comparing to the "maximum current" calculation. - Peter Peter T. Parrish, Ph.D., President California Solar Engineering, Inc. 820 Cynthia Ave., Los Angeles, CA 90065 CA Lic. 854779, NABCEP Cert. 031806-26 peter.parr...@calsolareng.com Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 -----Original Message----- From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of wire...@gmail.com Sent: Friday, February 11, 2011 11:03 AM To: RE-wrenches@lists.re-wrenches.org Subject: [RE-wrenches] Calculation of Maximum Circuit Current In studying for NABCEP certification I am a little confused about whether you multiply the short circuit current by 1.25 once or twice (1.56) in order to calculate maximum circuit current in determining wire size. Same for overcurrent protection. I have always done it twice (1.56) for both PV source circuit and PV output circuit. Once for over irradiance and once for continuous use. But a handout at a recent seminar that I went to totally confused me. Can anyone spell it out very clearly? Thank you. Larry Liesner Wirewiz Westport, CT Phone: 203-644-2404 Fax: 203-557-0556 wire...@gmail.com www.wire-wiz.com _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & 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 Options & 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 -- Larry Liesner System Design Elektron Solar, LLC 16 Ketchum St. Westport, CT 06880 203-557-3127 (office) 203-644-2404 (cell) 203-549-0977 (fax) wire...@gmail.com
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