[RE-wrenches] Combining Multiple Inverters

[Corey Shalanski]

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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