I purchased 2 Multiplus ii 12/3000/120 units and a 100amp auto transformer.
My incoming current is 120v 30 or 50 amp.
I want to wire this to send 120v circuit to my electrical panel and a 240v circuit to another location. I will be driving a 240v single phase motor on a dive compressor in a boat.
I need help with wiring. I have a 50amp circuit breaker where the shore power comes into the boat.
How do I wire to the units and then to the auto transformer and then to my circuit panels for 120v leg then 240v leg?
I have read through the installation manuals for the inverter and the auto transformer. It’s not clear to me how to do this with 2 units.
I think @TherealKT could answer this for sure, but from my limited splitphase knowledge, i think you got a device too much.
You can either setup a splitphase system using two MPs, by programming them to 180° splitphase config. This gives you 120/240 output. By then disabling “Switch ACin together” you can make use of the single 120V input from mains. Theres not really a seperate guide, a 180° splitphase config is essentially the same as a threephase config, just with two phases instead of three and 180° phase shift instead of 120°. But thats one selection during setup, the rest is the same.
Or you can use a single MP, that could be connected to the single 120V mains input, and on the ACout you then use the transformer to step up the voltage from 120V to 120/240V. From the transformer manual:
Theres also an example showing two 120V inverters with their ACout neutral tied together and each phase suplying the transformer. While im sure that works, i dont see why you would need this (apart from the additional inverter power), since either of the above setup essentially do the same, but with one fewer device? But i guess thats where actual handson splitphase knowledge comes pays off. Also from the transformer manual:
I had a chat thread in May 2025, I was trying to put together a system on a yacht so we can use a 240v single phase 3hp electric motor with a soft start to run a dive compressor. I also need 120v for the rest of the ac loads used on the boat. Running the dive compressor is not often, but this would be a way to not need a large 7+ KW generator. The boat has a 1320 A/H lithium battery bank and a small DC generator that can charge 12v at a steady 110 A/H. All my research led me to purchasing 2 Multiplus ii 12/3000/120 units which I was told can be set up to do both120v and 240v single phase. The auto transformer is required, but It was recommended that I get this to balance the loads between the 2 Multiplus units especially when operating the dive compressor with the 240v (a large load) and other 120v loads such as washing machine ect.. It’s probably a little over-kill to have the auto transformer. The 240v 3hp motor will take a lot of power during start-up. The soft start will help to reduce the start-up load, and the dive compressor will be mostly unloaded at start up because I can open the pressure control valve.
Now I have the 2 units and the Auto Transformer and I have read through the installation manuals, however There is not an exact design for what I want to do. I know once I get it all wired properly I can use the VE bus system to program the units.
Chrigu suggested that I contact @TherealKT not sure if they want to help me since I didn’t purchase the units from them. I will reach out first to the supplier that I bought the units from. Any other input would be greatly appreciated.
I appreciate the confidence in my knowledge @chrigu but I don’t feel like I have any better handle on this than you have spoke about above. I am a fan of as simple as possible though and like the idea of powering the 240v loads after the 120v through the transformer. A little quick math with the info given seems to indicate the 3hp compressor may draw just under 3000VA. So maybe 2 Muliplus units would be necessary to run other loads at the same time.
I do see discussions about split phase with multiple inverters quite a bit on this site. You should be able to parallel the two units to your electrical panel and then auto transform from there to the compressor.
