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Split Phase Inverter [50-amp RV Service Options]


Split phase inverters could be used for many different applications that need 110-240 volts output. This type of inverter is commonly a setup along with an automatic transfer switch and a battery charger (also known as an inverter-charger).

The input could be 120 volts single phase, 120/240 split phase or 240 single-phase, coming from shore power, generator, or solar panels. Very often inverters like this are used as a home backup system (similar to UPS) and for 50-amp RV or Marine electrical services.

Some inverter-chargers offer multiple types of inputs simultaneously, like this one:

The image was taken from this manual and this is the actual product from schematics Pure Sine Wave inverter charger – AIMS (paid link). It can be purchased on Amazon.

Here is an inverter wiring example for 120/240 volts split-phase:

How my inverter is hooked up in split phase

Some 120-volt inverters could be wired together to produce a 120/240v split phase. This setup is also called the 120-0-120 volt split phase.

Sometimes following types of wiring are confused with one another:

  1. Parallel wiring. Increases Amps.
  2. Split-phase wiring. Increases Volts.

Wiring inverters in parallel will only increase the power capacity of your equipment (amps) and your voltage (of 120v) will not change. Here is a difference in the wiring of parallel and split phase systems:

Please do NOT consider wiring inverters together yourself, unless you are a qualified professional! This type of setup should only be done by professional installers and here is a reference guide from Victron. 

The following examples are for the RV’s 50-amp service, which requires a 120/240 volt split phase inverter setup (NOT 240-volt) and many people have questions about it. So how do we handle our power needs if we want to travel in style?

There are mainly 5 ways to produce 120/240 volt split power with the help of the inverter:

  1. TWO inverters of 120-volts powering each 50-amp power leg. The limitation to this setup is that inverters don’t share loads, which means that one of the legs can be overloaded while another has no load on it whatsoever.
  2. ONE 240-volt inverter and Autotransformer. The autotransformer like Victron is designed to do step up, step down, or split the phase while balancing them all out (which means that you will be able to use your power supply to its maximum potential on whichever leg you would like).
  3. Two stacked 120-volt inverters and Autotransformer. Same as above, only 2 inverters will give you 240-volt power instead of one. Whenever the load of the inverters gets unbalanced, the difference will go through the autotransformer’s neutral wire and windings.
  4. ONE 120-volt inverter and Autotransformer. Probably the best solution for most people since having multiple inverters can be quite costly (not to mention if you have a limited space issue). And YES, autotransformers from Victron make this possible and you only need ONE 120-volt inverter (read below).
  5. ONE 120-volt (or 240-volt) split-phase inverter-charger with a built-in transfer switch. I found several inverters that could handle the task and the link will be included at the end of this article.

** Very Important! Do NOT attempt to install or configure anything yourself if you don’t have proper skills and training.

Option#1. Two 120-volt inverters

If you have two 120v inverters of 3000 watts each (as an example), then you will have an available power supply of 25 amps for each leg:

3000 watts / 120 volts = 25 amp

In order for inverters to work in sync, they are supposed to have a split-phase configuration (NOT parallel). This means that two 120-volt power legs will be created at 180-degree phase angles (also known as 120/240-volt system).

The main disadvantage of this setup (besides the complexity of it) is that:

  1. You will need 2 inverters and they are NOT cheap!
  2. Inverter #1 will power leg A and Inverter #2 will power leg B. Even though they will handle most of the functions in sync (charging, etc.), your amps will be split evenly and you will NOT be able to use all the power on one side only (if you desire).

This situation could be resolved by using an autotransformer, that will balance the loads between the legs (as long as they are within the device’s amp range).

Option#2. 240v Inverter with Autotransformer

If you have one 240v inverter and it is 5000 watts, then you will get about 21 amps worth of service on each of your 120-volt legs:

5000 watts / 240 volts = 21 amps

Total load unbalance cannot exceed 32 amps. This is schematics on how a 240-volt inverter gets connected to an autotransformer:

You can use 240v 60hz inverter to power a lot of things around your house. This includes, but is not limited to, washing machines, water pumps, and some industrial and commercial appliances.

In the United States, 120-volt power is more common as a residential power source, but if you have an outlet that is wired for 240-volt service, you could use all the benefits of a 240-volt inverter-charger. These inverter-chargers could be combined with an autotransformer to produce a 120/240-split phase. 

A 240-volt inverter could be also used well as a backup power source (like UPS or Uninterruptible Power Supply), where it will be in standby mode most of the time and during a power failure situation, it will give electricity to your 240-volt appliances. When the situation goes back to normal, your batteries will be charged.

230-volt 50hz pure sine wave inverters from Victron could be adjusted to 240-volt 60hz via software and require a Mk3USB interface (which is sold separately). There is a limited supply of these units, so please check the website for availability.

Option#3. Two 120-volt inverters (in sync) with Autotransformer

If you have two inverters that can be stacked together (like Victron, or anything that offers “split phase capability”), you can connect them to an autotransformer as well for a perfectly balanced 120/240-volt system:

Option#4. 120-volt inverter with Autotransformer

Yes, you can change the 120-volt input into 120/240 volts split-phase! In this case, you are basically:

Stepping up to 240 volts and splitting it

Here is a wiring diagram for a 120VAC inverter with an autotransformer:

Option#5. 120/240 volt split phase inverter-charger

This type of inverter is much easier to wire than stacked inverters and it does not require an autotransformer. Great space-saving option for many RVer’s!

These units come with either 120, 240, or 120/240-volt split-phase input. All Victron schematics were taken from here.

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