Thank you to all who responded to this thread - particularly Jason/Dave/Jay for pointing out the maximum OCPD requirements.
Getting back to my original question.. I would like to use Dave's "extremely small subset" example of two SB5s operating at 277V. In this case, if we were to parallel the output of each inverter onto a 50A OCPD (#6 wire), could we downsize the wire to #10 from the split going back to each individual inverter? or would we be better served (required?) to run #6 all the way back to each unit? Since we are dealing with current-limited devices I would tend to think we can downsize but wanted to get Wrenches' feedback on this. -- Corey Shalanski Joule Energy New Orleans, LA ᐧ On Fri, Jun 27, 2014 at 12:41 PM, <[email protected] > wrote: > > > Message: 5 > Date: Fri, 27 Jun 2014 11:32:02 -0400 > From: Dave Click <[email protected]> > To: [email protected] > Subject: Re: [RE-wrenches] Combining Multiple Inverters > Message-ID: <[email protected]> > Content-Type: text/plain; format=flowed; charset=ISO-8859-1 > > This is an interesting conversation, but I'm not sure it's going to > really impact anything. > > When an inverter is listed, it is tested with some "maximum output > overcurrent protection" as part of its rating per UL 1741. Typically > inverter manufacturers set this to be just above the inverter's max > current + 25%. As you know, many inverters can work at more than one > voltage, so for the SB5s their max current ratings at 277/240/208V are > 18/20.8/24A, respectively. The max overcurrent protection is 50A. This > figure often appears in the manual but not in the data sheet. For SMA, > it's in the manuals. For some manufacturers, it's only available from > tech support. > > For the Sunny Boy line, the only way you could have more than one > inverter on a shared OCPD is if you have two SB5s operating at 277V > (18.1A rated current each, so 2x18.1x1.25 < 50A). The SB6, SB7, and SB8 > all have currents too high to make this work, as they share the 50A max > OCPD. The SB3/3.8/4 have a max OCPD of 30A, which is too low to > double-up since each SB3 requires an OCPD of at least 20A. The TLs don't > work either, as the max for any unit is too low. I don't see it in the > SMA manual, but I know that in other manuals I've seen a manufacturer > requirement for a dedicated OCPD. The only inverters I know of that have > a max OCPD rating far beyond their rated current are micro-inverters. > > As for the shared disconnect, since at least the 2005, 690.15 has noted, > "A single disconnecting means in accordance with 690.17 shall be > permitted for the combined ac output of one or more inverters or ac > modules in an interactive system." That being said, the NEC requires ac > modules and micro-inverters to have ac disconnects (connectors, > typically) per 690.6 and 690.15(A). 690.15 notes that an inverter be > able to be disconnected from all sources, but if doubling up these SB5s > you could argue that this shared disconnect accomplishes that via the > anti-islanding protection... so I agree now that this is a grey area. > > Anyway, it comes down to whether it's worth worrying about this for what > is an extremely small subset of compatible inverter configurations, when > you can just do something like install a $50 Eaton BR24L70RP (70A bus, > 240V, 4 spaces) with two 30A breakers and be on your way. > > DKC
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