I had a thought. What about just backing up one of the two 200A panels with an off-the-shelf residential MID, and install an automatic transfer switch on the other 200A panel with the primary source being the grid, and auxiliary source being the inverter output, possibly through a feeder from the first 200A panel.
This would allow backup of all loads on a single 200A MID. I would comply with 710.15(A) (NEC 2020) without any required load management. Better yet, make the auxiliary source for the second 200A panel a feeder from a smart load breaker in the MID for load management purposes. The EG4 GridBOSS can send a 125A feeder to the other panel. That would be more than sufficient for practical purposes in a grid outage. Jason Szumlanski Principal Solar Designer | Florida Solar Design Group NABCEP Certified Solar Professional (PVIP) Florida State Certified Solar Contractor CVC56956 Florida Certified Electrical Contractor EC13013208 On Thu, Feb 5, 2026, 8:48 AM Jay <[email protected]> wrote: > I reread your question. > > I think the only way to be sure the internal transfer switches in the > inverters all switch at the same time on a big high load ( exceeding > potentially the max current rating of a single relay) multi stack inverter > system is to use external transfer switch’s. > > The system I installed is never going to exceed 200 amps, probably never > exceed 80 amps. 2 inverters can more than handle the load. The 3rd was > installed as built in redundancy to make it easier in case one fails. As > the system won’t run on 1. So I didn’t have to worry about exceeding relay > capacity. > > I think this is one of the potential problems with paralleled systems vs > larger single 120/240 inverter’s which we don’t really have here in the US. > > > > On Feb 4, 2026, at 7:05 PM, Jay <[email protected]> wrote: > > > I have a 3 stack eg4 18k and while watching the inverters online they are > totally balanced. > > Jay > > On Feb 4, 2026, at 4:29 PM, Jason Szumlanski via RE-wrenches < > [email protected]> wrote: > > > Hi all, > > When backing up a 400A service with multiple paralleled all-in-one > inverters by combining the AC inputs and outputs, the individual inverter > transfer switches must operate in a synchronized manner. Come to think of > it, the same thing applies to backing up 200A service with paralleled > inverters. Are these tested for this function? – I assume so. > > I'm really asking because Sol-Ark seems to be the only brand that > specifically addresses 400A service in documentation, although not very > clearly in the manual itself. Their training and literature support passing > through additive grid pass-through with multiple inverters, which implies > that they are additive in their ability to transfer power. See attached > examples. > > I have not found anything from EG4 (for the 18kPV) or the Midnite AIO that > specifically addresses this. > > In other words, how can we assume that paralleled inverters will be > additive in terms of their ability to transfer high levels of current that > exceed the rating of a single unit? Is that even addressed in listings? The > documentation is quite poor from all manufacturers (no surprise there!) > > I do have a couple of grid-interactive Midnite AIO systems with four > paralleled AIOs backing up 400A services, and so far, so good. My engineer > signed off on that. > > > Jason Szumlanski > > > * <http://www.floridasolardesigngroup.com/>* > <Image 1171.jpg> > <Image 1172.jpg> > _______________________________________________ > List sponsored by Redwood Alliance > > Pay optional member dues here: http://re-wrenches.org > > List Address: [email protected] > > Change listserver email address & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > There are two list archives for searching. When one doesn't work, try the > other: > https://www.mail-archive.com/[email protected]/ > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > http://www.re-wrenches.org/etiquette.htm > > Check out or update participant bios: > http://www.members.re-wrenches.org > >
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