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michael-rustemeyer avatar image
michael-rustemeyer asked

Autotransformer with MultiPlus-II 2x 120v

I have a newly installed MultiPlus-II 2x120V in my 5th wheel. I usually have 50amp inputs, so it has been running well. But this last weekend I was at a campground where I had to dogbone to 30amp inputs into the 5th wheel. The MPII only draws from line 1 though, which limited me to 30 amps plus battery. We needed more AC than that could support. When split phase 30 amps are available, if they feed the autotransformer first and then the MPII, would that equalize the draws in on L1 and L2 so that I could pull 50amps total, 25 from each? If I then plug into single phase 50amp normal campground with the autotransformer still in line, would it work like it does now? What if I end up on a 30 amp campground and only have one 30a single phase in, can I keep it in line for that scenario? Basically I'm trying to understand if I can add this in to fix my one problem w/o breaking the stuff that works well now...

Multiplus-IIAutotransformer
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Kevin Windrem avatar image
Kevin Windrem answered ·

If you have 30 amps split-phase available, this should work without changing things. But don't used the typical TT30 to NEMA 14-50 dogbone. You will need a NEMA 14-30 to 14-50 adapter. You will only get assist from the Multi on L1 since it's a single inverter core. An autotransformer on the output won't work because when inverting you only get a single phase output (both L1 and L2 are in phase).

If you have 30 amps 120 volt (one leg only), then you'll never get more than 30 amps / 120 volts into the RV even if you add an autotransformer.

You can not safely combine two separate 30 amp 120 volt feeds to get split phase power.

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derrick thomas avatar image derrick thomas commented ·
I have seen many RV pedestals (especially in parks with older infrastructure) that have only 30 amp outlets. Sometimes you can get lucky and find two 30 amp outlets at the same pedestal that are 180 out of phase, in which case you CAN connect with the appropriate adapter and provide 30 amp split phase 240 to the RV. However, I imagine that most management would frown upon this behavior.
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michael-rustemeyer avatar image michael-rustemeyer commented ·
I was able to use the dogbone adapter before the MPII, what I gather from your last sentence is that there isn't a safe way to pull equally from two 30 amp receptacles to get my 50 usable amps by the MPII? I gather I need split phase power coming into the MPII and can't get that from the two 30 amp plugs? The NEMA 14-30 to 14-50, is that adapter just to use one of the 30 amp pedestals?
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