There are times when designing or building an RV, we may want to provide a way to transfer between more than one 120V source. It will become necessary to have a transfer switch to safely switch between them. But what if we need to switch between 3 sources and why would we ever need or want that?

I have come across 2 cases where I've needed to do just that.

Another scenario I have, is one that I designed and use on the Omnia van. In this case, I use 2 Inverters. One that is powered from a LiFePO4 battery bank and one that runs off of the van's electrical system. The first inverter I refer to as the A-Inverter and the later as the V-Inverter. Since I use a 120V residential refrigerator in the Omnia van, the V-Inverter will provide power for it while the van is running. It can also provide power to charge the LiFePO4 battery bank using an AC-DC charger (the same one used to charge with shore power).

When the van isn't running and we don't have shore power connected, then the A-Inverter will provide power from the battery bank. The best way to connect these again is with a 3 way automated transfer switch. Although, with the Omnia van, I use a Slave 3 way switch which is controlled by a Power Management System (PMS). It adds a lot of features such as surge suppression, voltage monitoring, load shedding, transfer override, and more via the Purrfect Campers Area Network (PCAN).

Another option is to use an inverter with self contained switching such as the Victron Multiplus-II. You would need to add a transfer switch if you wanted to use it with a V-Inverter (or generator) and shore power. This is a much more expensive option, but will provide you with some other features like being able to supplement shore power with inverter power. This is nice if you need to operate high demand appliances with only 15A shore power for example.