Looking for a DPDT break-before-make switch to make a simple transfer switch

I am constructing an extended duration backup power solution for my home office computer system. I live in a rural area where power outages are common. A typical outage can last 1 to 1.5 hours. I need to be able to continue my work during these outages.

There are lots of UPS products on the market that are designed specifically for supplying backup power to computer systems. Unfortunately, UPS units with the capacity I require are very expensive. Therefore, I have concocted a hybrid solution that uses an affordable, low capacity, UPS for quick response time, and a large capacity inverter-battery unit to keep my computer system up and running for 2+ hours. See the diagram below.

The computer system is connected to the UPS. The UPS is connected to utility power when available, or to inverter-battery power when utility power is unavailable. Therefore, when utility power fails, I need an easy way to transition UPS input power from utility power to inverter-battery power. I believe this can easily be done using a single DPDT, on-off-on, switch.

I prefer a manual solution as I do not plan on always having the inverter-battery unit active. When utility power is lost, I will turn on the inverter-battery unit and switch the UPS input power from utility power to inverter-battery power. When I turn on the inverter-battery unit, I can easily flip a switch to disengage the UPS from utility power and connect it to inverter-battery power.

How do I choose an appropriate DPDT, on-off-on, switch for my application? I’m using the DPDT switch as a source transfer switch. I have done some online research regarding DIY transfer switches and it has me a bit concerned. On the surface, it seems simple enough. Find a switch with adequate voltage and current ratings, and that is break-before-make. But what I’ve read seems to indicate that switching between two sources and a single load is very different than switching between a single source and two loads. I understand that a switch failure in the two source circuit is potentially more catastrophic than a switch failure in the single source circuit, but is the two source circuit any more likely to cause a switch failure than a single source circuit? Or, is it only that reliability is more critical as a failure is potentially catastrophic? Are there switches that are better suited for the two source circuit? If so, how do I identify such switches?

I’ve estimated my computer system as drawing roughly 150W to 200W. Therefore, the switch will be carrying a relatively low current load.

Thank you.

I’m not an electrician by trade, with that said, what you are proposing should be fairly simple, my only advise is whatever you do would need to be wired up in accordance with local electrical codes with relation to grounding, and potential electrical exposure hazards.

You should be covered by any DPDT Rocker switch rated 120VAC with 5A or greater, On - Off - On switches are all going to be break before make, Click here for examples

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Welcome to the forum, excellent questions, glad to see you are being properly safety conscience.

Failure probability is the same in either configuration it is only the amount of damage from the extremely rare failure mode of shorting between the two throws that makes it a trickier situation. (this is why you never DIY a whole house cutover switch, leave that to the insured pros)

Nearly all decently designed switches will have nearly 100% of their failures be failure to close. So buying high quality switches will be the first line of defense.

For AC line switching in residential applications I almost always choose a 125VAC, 15A or higher switch.
Here’s a search query that gives in stock rocker switches that meet those specs.

The final, and near fool proof, line of defense is to add two fuses to the switch box you are going to build. One fuse in the hot lead of each source. The current rating of the fuses should be based not on the supplies current capabilities, instead it should be based on the load device’s maximum current draw.

However your load device is not the 150/200W PC, it’s the standard UPS. So you need to use the maximum input current requirement of the UPS to choose an appropriate fuse value.


Thank you PaulHutch. I am working to put together a schematic and parts list based on your recommendations. I will post it shortly.

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Thank you Kristof_2649 for the reply and the example switches.

I have completed my design. The circuit should handle 10A @ 120V. The hot lead of each AC input is protected by a 10A fast acting fuse. Figure 1 below shows the circuit schematic. Table 1 below is the parts list.

Figure 1: Circuit Schematic

Table 1: Parts List

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Nicely done!

Thanks for posting the whole circuit and parts list.

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