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Split-phase boat power system with MultiPlus-II 2x120 and Autotransformer

TL;DR: I want to build a system that can take in split-phase 120/240VAC, single-phase 120VAC, or 12VDC, and always output split-phase 120/240VAC. I have a potential solution in mind using MultiPlus-II 2x120 + Autotransformer. Will it work? Are there better options?


I have a boat with a split-phase 120/240VAC system. Normally the two phases come from either 120/240VAC shore power, or from a split-phase generator. I'm considering an inverter setup that would allow me to create both legs from the inverter, and am imagining the following system:

Connect a MultiPlus-II 2x120 to the split-phase system. When the generator is on (or split-phase shore power is connected), it will pass through both legs. When single leg shore power is connected, or no input is provided, it will yield only single phase 120VAC.

I want to then connect the output of the MultiPlus to an autotransformer, such that:

  • When the generator or split-phase shore power is being passed through, the autotransformer balanced loads between the two legs.
  • When only single phase 120VAC is available, the autotransformer will convert it split-phase, creating the second leg.

Is such a configuration viable? Is it possible to wire the autotransformer in a way that achieves both of these purposes?

An alternative is just to use an 240VAC inverter with an autotransformers, but that seems to have the problem that it won't work when only single-phase 120VAC is available from shore power. Is there a workaround for that other than installing another transformer?

EDIT: Added diagram for clarity.

Multiplus-IIAutotransformersplit phase
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1 Answer
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Kevin Windrem answered ·

This won't work with the 2x120. When it is inverting, the two legs are fed from the inverter output IN PHASE so the autotransformer would be trying to make the 0 volts into 240 volts. That can't happen and large currents would flow. If you don't connect the neutral from the inverter to the autotransformer, then when inverting you get no output to your loads.

The 120x2 is intended for RVs where the loads are split between the two legs but there aren't any 240 volt loads. If that fits your system, then you could move the autotransformer to the input side of the Multi. It would only be used for 120 volt shore power.

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That would enable use of the second 120V leg when on single-phase shore power, but would not provide any way of producing split-phase power from the battery, correct?
That is correct On inverter, you get two legs of 120 volts but in phase.

As you suggest, you could use a single inverter with autotransformers in and out.

You could also use two inverters that always feed the loads as split phase and have a single autotransformer on the input to accommodate single phase shore power.