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We have an off-grid cabin. Currently we have a Champion 7500 watt dual fuel generator powered with propane that is connected by a 120/240v cord directly into the cabin's electrical circuit panel. The high wattage and 240v service is needed to power a submersible water pump out in the lake that we use to pump water into attic tanks. When the generator is running, all of the electrical wall outlets in the cabin also are live with regular 120v service.

The current generator is too loud (likely 72+ dba) to run on a regular ongoing basis and we use it only for about 30 minutes every 2-3 days to fill the water tanks. I am interested in adding to our system a very quiet (52-58 dba) low watt 2000-3000 inverter generator for the sole purpose of providing power to an internet microwave receiver, wifi router and 2-3 laptops. The intention is to be able to work a full 8 hr day remotely via computer when at the cabin if necessary. (I know, why not actually take a vacation).

Is it possible to install a transfer switch that would alternate whether the cabin's electrical circuit panel is receiving power from the 120/240 v cord of the Champion 7500 generator or from a L5-30P (120v 30 amp) cord from a 2000-3000 watt inverter generator.

I am hoping to be able to make the regular electrical outlets in the cabin live with 120v service from the 2000-3000 watt inverter generator so we can be flexible where we plug in the internet equipment and our laptops. However the two generators would never be running at the same time. It would be an either or with us only starting one of the generators and using a manual transfer switch to then have only that generator's power feeding the cabin's circuit panel.

If this is doable, would I have the L5-30P go to one 30 amp generator power inlet box and the 120/240 v cord go to a different 30 amp generator power inlet box with the feed from both boxes going into the switch prior to feeding the circuit panel?

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    Can it be done? Yes. How? Depends on your panel (among other things). So start with a picture of the panel showing all breakers. Aug 15, 2021 at 2:40
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    I will not be back to the cabin until September but I will take a picture then. I should also say that I will be having a licensed electrician do the work if there is a feasible way to make it work. Thanks Aug 15, 2021 at 2:42
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    Are any of the 120V outlets in the cabin on Multi-Wire Branch Circuits? Aug 15, 2021 at 4:29
  • 3 or 4 paragraphs would make this an easier read !
    – Tim
    Aug 15, 2021 at 7:36
  • Yes it can be done. You'd need a big three-way switch like the ones in Frankenstein's laboratory. More info would be nice.
    – JACK
    Aug 15, 2021 at 12:05

3 Answers 3

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Super easy... 3PDT switch, 30A rating.

Pretty much along the lines you are thinking. You have the normal transfer switch/generator interlock selecting between "utility" and "generator"... and off the "generator" side of the interlock, you run cable to a 3-pole double-throw safety switch. One leg of the safety switch goes to your 14-30P inlet, and the other one goes to your 120V/30A inlet.

Note that you can use the 120V/30A to supply both poles of power, if, you don't have any "shared neutral" multi-wire branch circuits (MWBCs).

However, I'd encourage a technology review

The small loads you're talking about might really be a better fit for a battery/inverter system (or not even inverter if the loads are 12V-capable).

That would allow you to refill the battery at a time of your choosing using the generator, or even solar panels which have gotten laughably cheap could remove the need to even run the generator some days.

So I would be thinking a lot about scoping and sizing... think about

  • The "watts draw" of each appliance (A Kill-a-Watt energy monitor, $20, can help a lot here).
  • How many hours of running you'd like out of a battery system
  • Those two things tell you how big a battery you'd need (multiply by 2.5 if lead-acid, because you can't use more than 40% of a lead acid's capacity regularly or you'll damage it).
  • Whether solar would make sense given the above.
  • Whether you should run the big generator (only) to charge the batteries.

Or, stay modern tech and think BIG

A larger battery and inverter system could also run the pump.

At that point, you can use a much smaller generator (e.g. your 3KW as your only generator) and run the generator mainly to recharge the batteries, and at a time of your choosing (e.g. when solar isn't delivering).

The smaller generator will be running at its load "sweet spot" at all times when it is running. So your overall fuel efficiency will be much better. Which means less hauling fuel.

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Two generator transfer switches should serve your needs. An automatic switch would power outlets from either generator if one is running, with your choice of which generator takes the full load should you accidentally power both, and from the battery-powered inverter when the generator shuts down. Of course, you would have to plug in a charger for the batteries before the switch, since you can't charge the batteries from the inverter ;-)

Just size your battery storage appropriately. From the usage you describe, I'd estimate the laptops might draw 60 to 100 watts each, and the router and microwave link perhaps 10 to 30 W each. Worst case draw, about 250 W, would draw more than 20 amperes from a 12-volt battery. A single car battery might last an hour; a 100 A-h deep-cycle battery might give four hours of use (fully discharging a battery shortens it's life, so leave a margin). Of course, remember to shut the inverter when not needed, since the inverter still draws some current even unloaded.

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I’d buy some batteries online and get an inverter. Some solar panels. You should not need to the smaller generator. A 3000 watt inverter/charger/solar controller from Amazon cost $500 US. Panels are peanuts now. 800 ah of batteries-$2000.

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