I have two identical 230V single-phase "line interactive" UPS'es, powering a few devices which only have one power supply each – half of the devices are connected to UPS 1, half to UPS 2. (The devices are Ethernet switches and a PC.)

What sort of connection or device would be needed to allow both UPSes to power the same device? (I have a feeling it would be easy for devices taking DC input, but not for those which only have AC in?)

And would it still be possible if the two UPSes were fed from outlets connected to different AC phases? (Which the room happens to have – the building has a standard European 3-phase supply, except it's a bit unreliable – and it's what prompted the question in the first place.)

I'm only looking for theoretical knowledge and not actually planning to do any such connections in reality. I don't want anything to be on fire.

  • (The other reason I'm asking because the rackmounted servers at work have two power-supply modules each, i.e. each server has two AC inputs independent from each other.) – user1686 Sep 2 '19 at 7:19
  • you could use two UPSes if the device has redundant power supplies that you mentioned – jsotola Sep 2 '19 at 9:46
  • Right, but these only have one, and I'm curious about what it would take to do the same thing externally, hence the post. – user1686 Sep 2 '19 at 9:51
  • you could try to connect two power supplies using some kind of an isolation circuit .... or you could use a 3rd UPS ... powered by the two UPSes through a switchover relay of some type ... the third UPS could be small, because it would only be used to maintain power during the switchover – jsotola Sep 2 '19 at 10:00
  • I read the question as, could the two UPS be used serially, with output of UPS 1 inputting / sole power source to UPS 2, UPS 2 connected to and powering the device. Did I misread? Is the setup I describe a usable one? – Old Uncle Ho Sep 2 '19 at 14:29

The "safe & sane" approach is to replace the current PC & switches with models that DO have redundant power supply inputs. If, in fact, there's anything like a reasonable NEED for that. People often get overly concerned with slapping UPSes on non-critical systems, in my experience. If the system was, in fact, critical, you would, in fact, have good reason to get one with multiple power supply inputs. Replacing the PC with a laptop that has a battery is another option for that piece.

Alternatively, you get a single UPS with multiple power supply inputs; if feeling creative you connect battery chargers to each phase and a true-sine inverter (or perhaps a cheaper MSW, depending on the loads you are feeding and what they think of MSW inputs - most computers don't mind) to the battery bank, but you waste power (generate heat) since the conversion in each direction is not 100% efficient - with one battery charger that is a "dual conversion" UPS, and with two it's a dual-input dual-conversion model.

But really - how critical are the systems? If the system is critical, should you move it to a server farm where these things are Standard Operating Procedure rather than running it in your house? As a professional computer support person, I have quite a few systems where I'm quite happy with just a surge suppressor. Many modern systems shut down gracefully on power loss, and a surge suppressor does not need a constant stream of money to replace batteries as UPSes do. Few things are as annoying as a UPS that claims it can run for an hour when you ask its interface, but that dies in 5 minutes when subjected to real-world testing. [Or the "even more annoying" totally killed its battery, no indication, and then trips on a power event so short that non-UPS systems stay up, while the UPS shuts off instantly.] How often is one phase out for long enough that the UPS won't hold things up, while the other phase stays up? Should you just get much bigger UPS batteries instead? Should you just tell your PC to shut down gracefully after a minute or two of UPS operation to cover brief outages?


There are slight differences in rack ups, I have only seen 1 brand ups that was able to sync their output but this was a very high $ product. There may be smaller units out there that can sync their output, the brand I know of was APC , but these were very large units , the baby ones had 4 large deep cycle batteries and the big ones used electric “forklift” batteries. This was 20 years ago so their May be more options like the inverter generators that can be synced and are only about 2kw each. But I know APC made synchronized models and that technology has gotten much cheaper in the last 20 years.


The third best way to do this

Use the two UPS's, but distribute your loads across the two UPS's so their power draw is about equal. Thus, they run out of power at the same time.

Use a Kill-a-Watt type appliance to determine how much each draws. However, I wouldn't be surprised if the vast majority of power is used by the PC, and that even with the PC alone on its own UPS, it uses up its power first.

The second-best way to do this

Scavenge the battery out of one unit to extend the battery of the other unit. Now you have one UPS and one set of inverter losses instead of two, and a battery twice the size, for twice the run-time. This nicely solves the "lopsided" problem of most of the draw being on the UPS for the PC.

The best way to do this

Identify every load that can be powered off 12V directly. For things like DSL modems, routers, etc. - either yours can, or you can buy ones that can. Things that use USB power blocks can use 12V (cigarette lighter) USB blocks.

For the PC, they do make ATX power supplies that accept 12V direct.

You already have the batteries.

If you can get everything on 12V, then just get a quality battery charger big enough to serve 200-300% of all your loads, so it can quickly catch your battery back up after a power failure. Add some solar panels if you like.

For any remaining 120V loads, drive them off an inverter. You already are using a line-interactive UPS, so this is nothing new.

  • Not a good idea to "use a UPS as a Battery Charger" - virtually every one I've used warns that the battery terminals may float to line voltage, or something on those lines - so you do NOT want a "12V" load that people are exposed to connected to that battery. It may have 12 V across it and 230V from "battery - or + " to earth/ground, as opposed to the 12 or 0 volts you might expect. That's one practical difference between a charger and an inverter and the vast majority of UPSes. – Ecnerwal Sep 4 '19 at 1:54
  • @Ecnerwal Good point. Thanks. Edited to advise separates. – Harper - Reinstate Monica Sep 4 '19 at 3:54

The device you're looking for is called an Automatic Transfer Switch (ATS). It takes two power inputs (from the UPSes in your case) and has a single output that you bring to your devices. The ATS will typically have only one of its input active at a time, though (the load will not be shared between both UPSes): if both inputs are powered, the ATS will take power from the primary (default) one, and when this input goes down, it falls back on the secondary input.

This is often used in high-availability server rooms, to provide a way to completely replace a UPS without shutting down the servers. You can easily find such products from most major UPS manufacturers (APC, Eaton, ...). They are not cheap, though, although their task seems rather simple.

Note that going the next step: equally sharing loads between two, possibly out-of-phase, independant AC networks, is quite a challenge electronically. There are some load-sharing automatic transfer switches, but the need for them is much lower and they cost an arm and a leg (certainly much more than your two UPSes combined).

Building an ATS for DC would indeed be typically simpler, and could be as simple as merging all inputs through a diode each, to a common output: that would simply result in having the highest-voltage input winning, and powering the devices (more complicated schemes would of course require a more complex architecture).

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.