The specs Dave gives in reason #1 include the 240 W-ish Sunpower modules. In their training they used to point out that three strings could be combined without fuses. You could use two Y connectors to parallel three strings, but no more combining after that, fuses or no fuses.
Scott On Mar 9, 2012, at 11:08 AM, Dave Click wrote: > My take: if you have no more than two strings feeding your inverter, this is > NEC compliant. If you have a ten-string array and you're planning to use a > 5-fuse combiner, you're probably out of luck. > > Reason 1: > For a typical 240W-ish module your Isc is around 8.5A. 8.5 x 1.56 = 13.3A. If > you have two strings in parallel you would need to put a 30A fuse on the > paralleled output-- according to the spreadsheet in SolarPro 4.6, the biggest > fuse allowed for any cSi module is 20A, so this would only work with modules > that have an Isc < 6.4A. > > Reason 2: > If you have a fault in a string, you could have (Isc x 1.25) current from > each parallel string sharing the fuse + (fuse value) 20A from the fuse > flowing into that fault. With just one string per fuse, your fuse protects > your module. With more than one string per fuse, the extra current > contribution from those strings bypasses the fuse protection. > > I'm not seeing how this product is useful for cSi systems that have more than > two strings. Maybe for some thin-films with low currents, a small fuse on > their output, and a big max series fuse rating. If a module had a 30A fuse > allowed and an Isc of 3.2A, you could do a 15A fuse on up to 3 strings. > > More info is in John Wiles's "PV and the NEC" guide, Appendix J. > > > On 2012/3/9 10:33, Richard L Ratico wrote: >> Can someone please explain why this is a bad practice? My understanding is >> the fuse will still prevent backfeeding a fault by more than one additional >> string, >> the one paired by the Y connector. All the other strings are downstream of >> the fuse. >> >> Dick Ratico >> >> --- You wrote: >>> Wrenchers, >>> Has anyone used MC4 Y Connectors to combine strings? This website claims >>> that they are TUV Certified but does not list the manufacturer. Seems >>> like best practice to fuse each string but wanted to see what others think >>> about this product and the idea of fusing every two strings instead of >>> each individual string. >>> >>> http://www.solarpenny.com/MC4-Y-Connectors-609-4431.htm >>> >>> Al Frishman >>> AeonSolar >> --- end of quote --- >> _______________________________________________ >> List sponsored by Home Power magazine >> >> List Address: [email protected] >> >> 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: [email protected] > > 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: [email protected] 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
