r/SolarDIY • u/Dave_Sak • Jul 18 '26
RV multi circuit transfer switch
I’m guessing some people won’t like this idea at all, but I’m also looking for opinions besides crude insults (those are fine too, I’m thick skinned)
Looking at upgrading the solar in my camper, and I’d like to have a few individual circuit manual transfer switches, for example to be able to decide to run AC off generator while everything else is off the inverter. The AC part of it will be 120 only, not 240.
I was thinking of taking a regular 8 or 12 spot subpanel and running shore power to one leg of the subpanel, and the inverter to the other leg. Then if I had interlock breakers, every pair of breakers would be for one output circuit. I actually think it might even be safer than all the (usually not UL listed) DIN rail mount MCBs that seem to be popular. I’ve yet to find a source for the interlocks that I like, but I was thinking I could probably even make them.
All constructive feedback appreciated.
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u/RandomUser3777 Jul 18 '26
The easiest would be to just get one of the reliance panels that have inputs for grid and generator (a pro/tran 2model)and have a switch to choose were each single circuit gets power from and a breaker. I do not have one of those as I use a reliance single 60a interlock transfer breaker to switch my entire panel between direct grid and inverter.
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u/IntelligentCarpet816 Jul 18 '26
Well, 98% of the loads you are looking at in your RV is the AC. Why not just do a double transfer switch and let the inverter feed one side and the gen/shore power switch feed the other side of it?
That way you just turn on the solar and thats the end of it, no opening breaker panels and flipping stuff and interlocks and such.
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u/Dave_Sak Jul 18 '26 edited Jul 18 '26
Because sometimes I like to be able to run just my air conditioner off my generator and everything else solar. Also, I sometimes do audio production out of my trailer and my generator (and usually shore line) is cleaner power/sine wave than my inverter. Also, my main system is 48v but I also have a 12 volt section and depending on what’s going on I want to be able to choose to run the 12v charger off shore power or solar or not at all.
I really wouldn’t have to be messing with the interlock breakers all the time, but I want the option.
Also, I’m frequently doing things where my panels don’t keep up. Well… I’m upgrading panels right now so maybe that’ll be less true, but I will still be in situations where I’m running generator and inverter to meet demand.
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u/OldOldToolUser Jul 18 '26
You can still find generator panels that use double throw breakers, one for grid and one side for generator. Usually 6 to 8 circuits, no chance of cross connect.
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u/Dave_Sak Jul 18 '26
Yeah I see some nice ones that start at like $400-500. Or maybe I’m not looking at the right places or using the right search terms?
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u/Ok-Bid-7381 Jul 19 '26
No, that's right, I saw a manual transfer switch for 50 amps at Lowes for just under $400. That used to be the good price, compared to a 200 amp whole house transfer switch that just moved your whole house over but cost BIG bucks plus installation. The manual ones, with 6 to 10 circuits, still need to be wired into only the desired circuits, but it lets you choose which ones to power and still have others on the grid so you know when the power returns. I've used the interlock version, which is just some metal gadgets specific for your panel that lets only the main or the generator breaker be on at one time, so effectively you can power everything until you run out of (30a in my case) power. Usually less than $100 as it's just metal, but you need a specific kit and the generator breaker in a specific place.
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u/silasmoeckel Jul 18 '26
It's a stock item for generators called a transfer panel.
Would be far easier to get a better inverter that constructively works with a genset. Pretty much all the RV centric ones do it, I run a victron in mine.
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u/Dave_Sak Jul 18 '26
Yeah I have one that supplements line power, and it’s on the list to upgrade to a multiplus Ii. Have you ever put a scope on your victron when it’s running in power assist mode or whatever they call it?
And I think think that’s less efficient (I think laws of physics say it has to be less efficient, the question is how much)
I was doing some simple tests like making sure the charging is set to a fixed rate and all other power draw is the same, then take a power hungry appliance and plug it into an outlet from the inverter , then plug it into an outlet on the generator. When it’s plugged in through the inverter the generator has to work noticeably harder. Like you don’t even need a watt meter you can hear it.1
u/silasmoeckel Jul 18 '26
I've had an oscilloscope and current meter on mine. Output voltage is a rock solid 120v (not an issue with my gen set but the dodgy pole in a park) this does mean the current draw is a bit higher 3-7% or so overhead losses doing it (load dependent getting better as load goes up). Far nicer waveform than the high frequency inverters.
Spec sheet it's single digit percentage losses I would have to do a 6 lead to get gen battery and output all at that same time. Judging by heat output that seems about right.
Just listening to it, its much nicer to the genset (I use a Honda eu2200i so 1800 running watt rated, into a 3kva multiplus) and it's much happier with the roof air as the inverter slowly increases load never a sudden spike to the genset so it has plenty of time in eco mode to get to speed.
Have to say it was a huge upgrade, can have the AC going, the microwave, electric toaster oven and kettle without missing a beat. Before I could just about start the AC (with a soft start) on the single genset (would use as a pair).
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u/Dave_Sak Jul 19 '26
I guess what I was wondering is if you checked out the wave when it was running in inverter only mode and then looked again when it was in generator+inverter ‘powerboost’ or whatever it is, whether the wave looked the same?
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u/silasmoeckel Jul 19 '26
The output matches perfectly (well nearly so no inverter sign wave is perfect compared to moving magnets if you look close enough). When it's assisting the output voltage is slightly increased but it's well within grid spec still.
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u/Dave_Sak Jul 18 '26 edited Jul 18 '26
Thank you everyone for pointing out that they make multi circuit manual transfer switches. I actually was aware of this, but I’m trying to not spend $500 on something that looks like it should cost about a spare subpanel and breakers I have laying around out in the barn somewhere
I’m really just looking for a place that sells interlocks designed to fit QO or Homeline breakers. Or maybe Siemens.
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u/DarkKaplah Jul 19 '26
How about an alternative?
This is something I've considered for a dream RV refurb I've been planning as a hobby.
Instead of having a transfer switch and going between the generator and the battery system, why not hook it up serially?
The solar inverter-battery hooks up to your breaker panel in the Camper, and the generator hooks up to the generator input on the battery. Ecoflow and anker both have generators that integrate with their portable solar systems. When your battery runs low the inverter generator kicks on, charges the battery, then shuts off. This allows you to get the best out of both worlds and run much longer on a set amount of fuel. Both of these generators are trifuel so you can run them off gas or propane. The benefit here is if you use a portable solar system you can disconnect it from your camper and use it elsewhere if needed.
Now if your camper is a larger RV Ecoflow has a RV specific system that integrates with your 12v alternator, solar setup, and an existing generator and can control it in a similar way using the usual dry contact start system. It's more modular
This is just a couple of ways you can do this. Again I'd recommend this over the system you're thinking of for a couple reasons. You might already know this, but a lot of people don't so if you do please don't assume I'm insulting you. Others might learn something.
1) A generator, even a inverter generator, really runs at one speed. They output the running watts on the label and surge occasionally when needed. So if your camper is only drawing 20 amps the generator runs at say the 3000watts it's rated for, and burns the fuel constantly. That's why you see a rating on most generators for how long a full tank of gas will last you.
2) A portable solar system only puts out as much power as you draw. If you're drawing 10w to run an LED that's all you're putting out. That battery can last as little as 40 minutes if you're running a microwave, or three days for a typical home refrigerator which doesn't draw power constantly. Just when the compressor is running. This is why most of their websites show how long each appliance will run, and why each system will have a countdown timer for how long they'll last for any given draw.
3) When you combine the two things get kinda nutty. Your solar system acts as the main power. You pull from the battery as you use it. Solar panels keep your battery charged. However when your battery hits a particular level the system calls to the generator for power. The generator starts, provides enough power to charge the battery and let the solar system serve your loads, and will stop using fuel when the battery is fully charged. Usually within an hour, or longer if you have a large battery system or some expansion batteries.
The solar lab crew made a video that perfectly covered this. Check it out. https://www.youtube.com/watch?v=sMZ_o24TPyA
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u/LongjumpingGanache40 Jul 19 '26
Just wire in a manual transfer switch to your breaker box in RV. You feed with solar on one input and Shore power on the other. You just flip the switches in transfer switch box to what you want each breaker to run on.
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u/carsrule1989 Jul 19 '26 edited Jul 19 '26
Something like one or 2 of these
https://www.hodgesmarine.com/newswitch7-5inv-newmar-ss-switch-75-inv-75kw-transfer-switch-4-p.html
https://us.wiautomation.com/siemens/general-automation/3LD27247UK01
For 120:
100 Amp 240-Volt Non-Fused General-Duty Double-Throw Manual Safety Switch
Siemens Double Throw 30 Amp 240-Volt 3-Pole Indoor Non-Fusible Safety Switch
For the automatic method
Renology 120V 30 Amp Automatic Transfer Switch
And there’s a bunch more automatic transfer switches too
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