[RE-wrenches] Combining Multiple Inverters

[Chris Mason]

You are not dealing with current limited devices, you are dealing with the
grid which is infinite. This is why you put the OCPD at the grid end of the
wiring, not the inverter end.


On Mon, Jun 30, 2014 at 2:38 PM, Corey Shalanski <
cshalanski at joule-energy.com> wrote:

> 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, <
> re-wrenches-request at lists.re-wrenches.org> wrote:
>
>>
>> Message: 5
>> Date: Fri, 27 Jun 2014 11:32:02 -0400
>> From: Dave Click <daveclick at fsec.ucf.edu>
>> To: re-wrenches at lists.re-wrenches.org
>>
>> Subject: Re: [RE-wrenches] Combining Multiple Inverters
>> Message-ID: <53AD8E72.4080009 at fsec.ucf.edu>
>> 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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-- 
Chris Mason
NABCEP Certified Solar PV Installer™
Solar Design Engineer
Generac Generators Industrial technician

www.cometsolar.com <http://www.cometenergysystems.com>
264.235.5670
869.662.5670
Skype: netconcepts
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