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.
I still haven’t been able to figure out all the details. I understand how to wire the 2 inverters for split phase. My understanding is that this will give me 240v with the red and black wires. Calling Black L1 and Red L2 120v each, how do wire this to my 120v distribution panel? My 120v distribution panel has a main breaker (30 amp). I would like to be able to use both inverters to power this panel like they are in parallel even though the system is configured for split-phase. The only way It seems like I can do this is with the auto transformer.
It seems like I could run some of the breakers (loads) with L1 and use L2 for the others. So basically I’m using the inverters independently for different loads?
Yes, but my question is beyond their technical expertise. However I believe I have it sorted. I need to use the auto transformer for the 120v loads by wiring the L1 and L2 Hot (Black and Red) that will be for the 240v load to the AT input and only using the the L1 And neutral out to the 120v loads (circuit panel). This should balance the 120v loads between the inverters. The 240v load will not be through the AT.
This wiring diagram is I the installation manual for the AT when using 2 - 12v inverters in split phase. Unfortunately there are not specific details exactly how I will be wiring it.
