The serving transformer is also an issue. I've seen 800-amp service conductors on a 25kVA transformer. Just because you have wires large enough for a 400-amp service, does not mean you have a transformer capable of delivering 400-amps continuous. That is why you have to work with the serving utility with larger systems.
Bill. From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Scott McCalmont Sent: Thursday, March 04, 2010 3:30 PM To: RE-wrenches Subject: Re: [RE-wrenches] Supply Side Interconnections - Maximum Backfeed Going all the way up to the rating of the service might not be such a good idea. The utility may use smaller wires for the service drop than you would expect based on the ampacity tables in the NEC. Considering the example of a 400 A service: - PG&E (local utility) requires 350 kcmil Aluminum conductors. - Table 310.17 rates 350 kcmil Aluminum for 395 amps before temperature correction. With load diversity the smaller wires will be fine, but the output of a PV system is continuous. Running the output of a system sized at 400 amps through the wires on a hot summer day could overload them. So... it depends. Scott Scott McCalmont, P.E. McCalmont Engineering San Jose, CA (408) 204-0318 On Thu, Mar 4, 2010 at 3:02 PM, Michael Kelly <m...@solarflair.com> wrote: Kirpal, That was always my interpretation as well. So in the case of a 400A service, I can backfeed 400A when utilizing a supply side connection. Others have thoughts on this? - Mike -- Michael Kelly Project Manager / Project Engineer NABCEP Certified Solar PV Installer T SolarFlair Energy, Inc. 11 Mayhew Street Framingham, MA 01702 Direct Mobile: 617-899-9840 Main Phone: 508-293-4293 Main Fax: 508-293-4003 m...@solarflair.com On Thu, Mar 4, 2010 at 4:16 PM, Kirpal Khalsa <solarwo...@gmail.com> wrote: Mike.....i may be wrong....but I think i am right....you are limited by the wire size feeding the supply and/or the transformer size which supplies the service.....depending on the physical location of the tap you may be limited by the enclosure and the enclosure ratings as well would love to hear any other ideas... Cheers, -- Sunny Regards, Kirpal Khalsa NABCEP Certified Solar PV Installer Renewable Energy Systems www.oregonsolarworks.com <http://www.oregonsolarworks.com/> 541-218-0201 m 541-592-3958 o On Thu, Mar 4, 2010 at 12:50 PM, Michael Kelly <m...@solarflair.com> wrote: Hi All, Next to grounding, I find the calculation of the maximum amount of backfeed current allowed in a supply side connection to be the most vague issue in Article 690. Let's assume we have a 400A, 480/277 service in a commercial building, a 400A main service disconnect, and the jurisdiction is under NEC 2008. What is the maximum backfeed current allowed in a supply side connection? The only place the code points you to is 230.82(6), which is no help, or to Article 705, which is no help either. I know that in the draft version of NEC 2011, there is an expansion of 705.12(A) which states " Supply Side. An electric power production source shall be permitted to be connected to the supply side of the service disconnecting means as permitted in 230.82(6). The sum of the ratings of all overcurrent devices connected to power production sources shall not exceed the rating of the service." That is a helpful sentence from the Code, but as it is in draft form, what do we do under NEC 2008? What have you used as a maximum backfeed current for supply side connections? Thanks, Mike -- Michael Kelly Project Manager / Project Engineer NABCEP Certified Solar PV Installer T SolarFlair Energy, Inc. 11 Mayhew Street Framingham, MA 01702 Direct Mobile: 617-899-9840 Main Phone: 508-293-4293 Main Fax: 508-293-4003 m...@solarflair.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 <http://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 <http://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
_______________________________________________ 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
