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Old 08-07-2012, 08:57 PM   #1
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UPS battery backup to generator panel


Hi all,
I've got an interesting thing I'd like to set up but not exactly sure if it's even possible. what I'd like to do...I have a separate panel that feeds a portion of the house and is used as a generator panel with a proper backfeed switch. What I'd like to do, is feed that panel from a UPS system first so in the event of an outage the gen panel remains fed until the batteries go or the generator is fired up. The tricky bit in my opinion...currently the generator (120/240v 4 wire) feeds into the panel (240v 4 wire) but the UPS it only a 3 wire setup (208)....can this be done?
As an aside as well, can this be done with 2 120v UPSs? Or would that mess up the phasing or sine waves or something? (one feeding each leg of the panel)

Thanks to all in advance for your input!

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Old 08-07-2012, 09:55 PM   #2
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UPS battery backup to generator panel


You might an inverter from outback or similar. They are designed for what you want to do and might be compatable with a 240/120 volt system.


Last edited by Auger01; 08-07-2012 at 09:59 PM.
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Old 08-07-2012, 10:14 PM   #3
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UPS battery backup to generator panel


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Originally Posted by Mr2malv View Post
Hi all,
I've got an interesting thing I'd like to set up but not exactly sure if it's even possible. what I'd like to do...I have a separate panel that feeds a portion of the house and is used as a generator panel with a proper backfeed switch. What I'd like to do, is feed that panel from a UPS system first so in the event of an outage the gen panel remains fed until the batteries go or the generator is fired up. The tricky bit in my opinion...currently the generator (120/240v 4 wire) feeds into the panel (240v 4 wire) but the UPS it only a 3 wire setup (208)....can this be done?
As an aside as well, can this be done with 2 120v UPSs? Or would that mess up the phasing or sine waves or something? (one feeding each leg of the panel)

Thanks to all in advance for your input!
This sounds like a small version of what's done in any data center. What you'd want is the commercial power and the generator connected to the transfer switch inputs, the switch output feeding the UPS, and the UPS feeding the load.

Normal operation is commercial power through the transfer switch, through the UPS, to the load. During power loss, the UPS feeds the load until the generator comes up, at which time the transfer switch switches the UPS input to the generator. When commercial power comes back up, the transfer switch swings the UPS's input back to commercial power and shuts down the generator. The UPS batteries/inverter will supply steady load power during the initial outage and again during the re-transfer switchover.

Now the bad news: Unless your load is extremely small (on the order of a couple kilowatts), a 208v UPS is going to cost you an order of magnitude more than the generator. You might get a 7.5kW generator for a few hundred dollars, while a 7.5kW UPS is going to cost several thousand.

And two smaller UPSes together would have no ability to maintain phasing, so that option is definitely out.

Bottom line is this is really impractical (and very costly) unless you have a very specific need. You'd do much better putting in a generator with an automatic transfer switch (ATS), then using a couple smaller load-point UPSes wherever needed (computer room/office, living room AV equipment, etc.)

That's just the very basics. There are a ton of other considerations and options, all of which will drain your wallet very quickly.

EDIT: Auger01's suggestion is pretty good, but again, I'm betting a couple kilodollars at least...

Last edited by slickwilliam; 08-07-2012 at 10:20 PM.
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Old 08-08-2012, 05:53 AM   #4
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UPS battery backup to generator panel


Thanks for the response!

Lol, very true. As I happens I've got a line on a used 5000 va unit at a very compelling price which I think would be plenty enough for the smaller gen panel, I also have a 3000va unit that needs a battery...not sure if that one would be big enough or not, assuming it would be very tight if it was...the gen panel is a much smaller sub panel to run a minimal powerset in the house during an outage, though the generator is 7800w and appears to be able to run the entire house when needed :o)
I've set up a manual transfer switch in my last house but when we purchased the new one it had the code compliant breaker setup already in place. Ats would be nice but the gen is a large portable unit so not likely the best set up that way...the batteries would make an ideal middle stage if it can be done.
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Old 08-10-2012, 09:39 AM   #5
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UPS battery backup to generator panel


Anyone able to outline how this could be done? I'd love to hear your thoughts folks!
Thought about inverters and a battery bank at first, but I like the charging circuit with the ups, seems like fewer pieces and less complication in the overall system.
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Old 08-10-2012, 11:06 AM   #6
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UPS battery backup to generator panel


Need to see a spec sheet on the ups. Normally a 208 volt application is designed for three phase.
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Old 08-10-2012, 11:03 PM   #7
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UPS battery backup to generator panel


These are apc smart ups 3000 rt and 5000 rt units. Will see if I can find specs anything in particular that would help?
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Old 08-11-2012, 12:42 AM   #8
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UPS battery backup to generator panel


find a wiring diagram to see if it's compatible with regular house power.
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Old 08-11-2012, 12:50 AM   #9
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UPS battery backup to generator panel


the spec sheet i see on the 5000va unit is that it'll run on anything from 220v to 240v. It's a 5000va only rated at 3500w, at 3500w it will keep you in power for about 4min depending on the amount of batteries it has in it. the 4 battery model looks like it'll hold for about an hour and a half. I'm not sure if this is going to work out for you. What happends when the power goes out and your ups kicks in and sees that it needs to supply much more than 3500w. I guess it all boils down to what you have in your generator panel. You would need to size the ups to the load in that panel. It's not like a genny where power goes out and you pick what you want to have on. Your dealing with a seamless transfer that just automatically has to have enough power to deliver to whatever happens to be running at the time of the outage.

as mentioned before running two at once would be a no no. Getting the waves in sinc would be very difficult.
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Old 08-11-2012, 07:04 AM   #10
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UPS battery backup to generator panel


Hmm not sure on the battery count. Not much on the panel though to be sure. Half dozen lights @ 100w and some outlets, no significant load on those but will check for a more accurate count. Would be nice to throw the fridge on and or the well pump, but those are pretty high draw. Maybe best to install a second panel under the gen panel for a contained load with X number of lights and outlets to cover phones and Internet, etc the heavier items like the pump and such could be genny only without much problem, certainly not as convenient but more wish list types than essential immediately after the power goes out. Power to the lights in the garage would be handy when going out to the panel and gen lol. Of course a flashlights not so bad once you dig it out.
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Old 08-11-2012, 07:07 AM   #11
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UPS battery backup to generator panel


Definately see what's meant about the cost effectiveness in comparison to an invertor setup though whoa!
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Old 08-11-2012, 10:55 AM   #12
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UPS battery backup to generator panel


A few years back I worked on system that had two seperate rooms. Keep in mind this was for a bank.Power went into a massive ups system to take the load until 4 cat genny's kicked on supplying 8000hp at idle each. 46 story building with seamless transfer of not just certain draws but the entire building stayed online. Was an interesting job. The time spent syncing those engines was crazy. I wouldn't even want to guess at the pricetag for the whole system.

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