I'd to deploy a UPS system as shown below. My primary need is the setup shown in black, a simple setup feeding directly to my equipment. The transfer switch is 15A which is more than I need.

Considering the black setup:

  1. At point "A", what can I use to isolate the batteries?

  2. Also at point "A" or after, what breaker(s) can I use to limit amps and damage to the inverter? On page 11 of its manual, it states a 250A ANL fuse but I'd rather use a breaker.

  3. Should I combine the wires to the inverter into one or just screw all three to its inputs?

  4. How do I ground the charger and inverter (G1, G2)? The manuals only speak of DC ground, as in a vehicle. Can I just use the AC ground? Wire size?

My optional need is to feed three 15A circuits from the main panel. Note that the total amp load of all three of these curcuits is only 10A, if that. The blue setup shows one feed off the UPS APC transfer switch. The APC has several female outputs, again, 15A total output. My desire here is to have a switch to select between the UPS setup and the main panel. This will allow me to switch to the main if I need to service the UPS:

  1. What should I use for the select switch(es)?

  2. If a subpanel is more appropriate here, feeding off one APC output, please advise on how to wire that up.

Charger is Xantrex TC 4012 - data sheet - owner's manual

Inverter is Xantrex ProWatt SW 2000 - data sheet - owner's manual

Battery is 3 x lead-acid Lifeline GPL-31XT AGM

Sorry about the links. Perhaps this will work:



Again, I already had the electronics and the batteries were given to me for free. I am not forcing their usage after an unwise purchase. I will not be buying Chinese junk, I only used the links above as examples.

The manuals also refer to permanent, non-vehicle use for the electronics. So, I am good there. I will ground to the AC ground as advised.

Again on the isolation, you say nobody does that, regarding the diodes, as long as the batteries are identical. Well, my batteries are identical. But, what prevents current from flowing between two batteries if they are "off balance" instead of all current flow going to the inverter?

I appreciate your advice about using a 24V or 36V inverter with series batteries. My need is long UPS duration not power capacity. Will the series, 24V or 36V setup (batteries and inverter, the charger is the same) give me equal duration as the parallel setup? If so, great, I'll invest in a new inverter. Any brands recommended?

Finally, the Siemens setup is ideal. I will look into that for my long term solution. For now, I will focus on getting it all to work in the simple, non-panel setup.

ups layout

  • See also diychatroom.com/threads/… Commented Nov 30, 2022 at 2:11
  • 4
    Why all this instead of a plug-in 2kva UPS or using an inverter-charger? Either will require less integration than this setup.
    – KMJ
    Commented Nov 30, 2022 at 2:12
  • 1
    Also are those all diodes? You don't need those and they will throw the charger off.
    – KMJ
    Commented Nov 30, 2022 at 2:21
  • 3
    Second @kmj's puzzlement. An off-the-shelf UPS performs all the charging, inverting, and automatic cut-over functions, plus in many cases having the hardware and software to monitor the battery charge and do a clean shut-down before loss of power crashes the PC. And they're available in a wide range of watt-hour capacities. You are asking about a way of solving a problem... What is the real problem you are trying to solve?
    – keshlam
    Commented Nov 30, 2022 at 2:41
  • 2
    Looks like an XY Problem to me.
    – FreeMan
    Commented Nov 30, 2022 at 13:17

2 Answers 2


DC is difficult to interrupt, unlike AC which goes to zero volts 100 or 120 times per second. However, 250A DC Circuit Breaker does bring up results these days.

Your arrangement of paralleling 3 small batteries is guaranteed to cause you grief, based on decades of accumulated off-grid system battery experience. Get one big battery, or two to six lower-voltage but higher amp-hour batteries you can put in series. Or an inverter that takes higher voltage input so you can put 3 or 4 12V batteries in series. Decent 48V input inverters are dirt common, and not incidentally cut down the required input current and wire and fuse size by a factor of 4 for the same power.

This is virtually certain to be more trouble and expense .vs. just buying an off-the-shelf UPS of the proper size.

The grounding is called out for fixed locations as follows:


Snippet from device manual

Which tells you to connect to your main AC grounding point. With cable as large as your battery cables.

Use of multiple batteries is diagrammed here, and has no diodes in the current path:


enter image description here

  • These batteries I got for free and I already had the other equipment. So, I'd like to stick with this gear. So, any thoughts of using rectifier diodes to isolate each positive leg and then breakers as I noted? Like this diode: Rectifier Diode 5Pcs 70HFR120 5Pcs 70HF120 amazon.com/… Like this breaker: Erayco 250 Amp Circuit Breaker amazon.com/…
    – Art
    Commented Nov 30, 2022 at 17:42
  • 1
    70A diodes for a 250A load should be interesting, even with 3 sharing the load, assuming they share equally (they often don't.) The 0.6V typical drop across a diode is 5% of your battery voltage, so that will probably screw up the charger behavior on that side, and on the output side will amount to a sizable amount of power lost as heat. Looking at Vishay's data sheet, they call out 0.79-1.35V drop for their version of this part, so that will be worse than "typical" at as much as 10% battery voltage. Likely your inverter won't be happy with that.
    – Ecnerwal
    Commented Nov 30, 2022 at 17:59
  • Hmm...I thought the diode link was for 100A version. Of course I wouldn't use 3x70A.
    – Art
    Commented Nov 30, 2022 at 21:05
  • Something like this should work: www.ebay.com/itm/325230868670
    – Art
    Commented Nov 30, 2022 at 21:58
  • 2
    @Art dude, what is with your links? Your eBay link isn't hot, you Amazon links go to a search result page and include the text of the item description for no earthly reason. And over on the other site you linked to your personal copy of the product manuals instead of to the manufacturer sites like you ought to (I didn't download; who's to say the hosting site you used didn't add a virus or something). Just post the URL. If it doesn't start with https:// don't post it lol. Commented Nov 30, 2022 at 22:47

At point "A", what can I use to isolate the batteries?

No one does that. They choose a matched set of batteries. Or much preferably, they run the batteries in series and choose appropriate equipment. I honestly think you'll spend more trying to make this equipment work with your flawed design than you would simply buying the right equipment. "Make it work with this unfit crud I already bought, so I can avoid admitting error with a prior impulse buy" is a very common refrain around here, and it usually ends in project failure because vanity is a higher priority than success. I really respect Martin of the Marble Machine for avoiding that while pursuing a passion.

Your idea of diodes is problematic, but at the least you'll need to seek out ones with the lowest possible voltage drop, such as Schottky diodes. You're still dealing with 0.5V drop.

Cooling is critical. The forward voltage will tell you how much voltage drop you'll have through these diodes; that x current is a number of watts you will need to dissipate. Preferably not by turning the diode cherry red.

This all but guarantees a "HOT" heat sink. If one is smart about diode choice, the hot heat sink will be on the common side, so you get to use it as a conductor.

Also at point "A" or after, what breaker(s) can I use to limit amps and damage to the inverter? On page 11 of its manual, it states a 250A ANL fuse but I'd rather use a breaker.

Alternately, use a fuse and don't blow the fuse.

A 250A breaker is going to be a monstrously costly thing. This drives us crazy in home electrical; people with >200A services would very much like to have master 250-300-400A breakers but no sanely priced breaker exists. So they are forced to dual main breakers feeding 2 separate panels. So this is another example of having to spend an outsize amount of money merely to avoid admitting error with prior hasty purchases.

Anything you use should be UL Listed, UR Recognized or Coast Guard certified. The place to go for stuff like this (on the DC side) is automotive or marine. Stay away from that Chinese garbage off eBay, Amazon Marketplace, Banggood, wish.com, DealExtreme etc. Go to marine suppliers like West Marine, automotive suppliers, or quality electronics suppliers like Mouser, Digi-Key or Galco.

Should I combine the wires to the inverter into one or just screw all three to its inputs?

NEC 110.3 requires you follow the instructions and labeling which came with the UL Listed inverter. So if that does not authorize the input lug for multiple wires, then no-go. By the way you should take a keen interest in whether those lugs are rated for aluminum wire. 200A copper wire is costly. Another on the long list of expenses due to salvaging this equipment instead of choosing 36V equipment.

Tell me you're not one of those Youtubers who is obliged to use particular pieces of equipment due to a sponsorship deal. Once, Linus Tech Tips built a whole-house PC cooling system (great idea) and needed an outdoor fan/radiator. A normal person would just hit an automotive wrecking yard, those radiators have big 12V fans already bolted on. But Linus was obliged to use a sponsor's product so zip-tied a bunch of them together into a disaster, which failed, end of cooling project and back to window air conditioners in the PC rooms.

How do I ground the charger and inverter (G1, G2)? The manuals only speak of DC ground, as in a vehicle. Can I just use the AC ground? Wire size?

Back to 110.3 you follow the instructions. And it looks to me like all these instructions are telling you to use these components in vehicles only, and not in a home. That is a deal-breaker. Unless you want to mount the whole shebang onto a "vehicle" lol.

Hmm, y'know what, I actually like that.

What should I use for the select switch(es)?

Well the trick is that the furnace must be hard-wired.

You could use DPDT switches. For each circuit, wire the hot and neutral of the loads into the common on the switch. Wire the hot and neutral from the panel onto one of the throws, and then common up all the other throws and bring that to an inlet.

If you don't switch neutral, you create a monumental neutral looping problem that could in certain circumstances burn the house down. Which naturally is a Code violation.

If a subpanel is more appropriate here, feeding off one APC output, please advise on how to wire that up.

Wire a subpanel with a generator interlock - the Siemens ECSBPK01 interlock is the best value there. Wire the subpanel to switch neutral also, for same reason as above.

enter image description here

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