The Victron Autotransformer does three jobs: it steps voltage up, steps it down, and balances split-phase loads. The last one is what interests most RV, boat and off-grid owners. With a MultiPlus-II, it can also create 120/240V split-phase from a single 120V inverter, as the MultiPlus-II manual notes. Here’s how it works, using Victron’s own example.
Victron Autotransformer: Specs
| Spec | 120/240V 32A | 120/240V 100A |
|---|---|---|
| Max AC feed-through at 240V | 32A | 100A |
| Input breaker | 32A two-pole | 100A two-pole |
| Neutral current, continuous at 40°C | 28A | 28A |
| Neutral current (30 min) | 32A (3,800 VA) | 32A (3,800 VA) |
| Weight | 12.5 kg | 13.5 kg |
| Enclosure | Aluminium, IP21 | Aluminium, IP21 |
Source: Victron Autotransformer product page and datasheet.
Both use a toroidal transformer and meet EN 60076.
The Split-Phase Balancing Problem
Victron’s datasheet uses a 30A 120/240V supply as its example. In theory, that’s 30 x 240 = 7.2 kVA. But 120V loads never split evenly between the two legs. A 1200W hairdryer draws 10A from one leg, and a washing machine can pull more than 20A from one leg.
With a 20A imbalance, one leg hits its 30A limit while the other carries only 10A. Victron calculates the practical maximum at 30 x 120 + 10 x 120 = 4.8 kVA, or 67% of the theoretical maximum.
How the Autotransformer Fixes It
Leave the supply’s neutral unused and let the Victron Autotransformer create a new neutral. The imbalance is then “absorbed” by the transformer. In Victron’s example, a 20A imbalance makes one load leg draw 40A and the other 20A. The 20A difference flows through the autotransformer’s windings, and both supply wires carry 30A. You can use the full 7.2 kVA.
Other Uses
- Step up: 120V to 240V for a single 240V appliance.
- Step down: 240V to 120V.
- Split-phase from one inverter: the MultiPlus-II manual lists creating a split-phase system with a separate autotransformer.
Who It’s For
- 50A and 30A RVs and boats whose 120V loads trip one leg.
- Off-grid cabins with a 120V inverter that need 240V for a well pump.
Good to Know Before You Buy
- Heavy.
- Needs careful wiring of the neutral and breakers. Have an electrician install it.
- IP21, so it needs a dry location.
Want the source figures? The Autotransformer page on Victron’s site has the datasheet.
Worked Example: A 50A RV With Uneven Legs
A 50A RV supply provides two 120V legs, each good for 50A, or 12,000VA in total. Suppose the air conditioner, microwave and water heater all sit on leg 1, adding up to 40A, while leg 2 carries only the fridge and lights at 16A. Without balancing, leg 1 is close to its breaker while leg 2 has 34A unused (estimate).
With the 100A Victron Autotransformer creating its own neutral, the 24A imbalance flows through the transformer windings instead, and each supply wire carries about 28A (estimate, ignoring transformer losses). That 24A sits inside the 28A continuous neutral rating at 40°C. If your typical imbalance runs higher, move one large load to the other leg.
Worked Example: 240V Well Pump From a 120V Inverter
An off-grid cabin has a single MultiPlus-II 120V and a 240V well pump. The autotransformer steps the 120V output up to 240V for the pump. A pump drawing 6A at 240V asks for about 12A at 120V from the inverter, plus a small transformer loss (estimate). Size the inverter for the pump’s start-up surge too, since motors draw several times their running current for a moment.
Worked Example: Split-Phase From One MultiPlus-II
The MultiPlus-II manual lists creating a split-phase system with a separate autotransformer. A cabin with one MultiPlus-II 48/3000 could feed both 120V legs from a single inverter. Loads on either leg are served, and 240V appliances can run as long as the total stays within the inverter’s rating. For larger 240V loads, two MultiPlus-II 120V units in split-phase configuration are the alternative.
Choosing Between the 32A and 100A Models
| Your setup | Good fit |
|---|---|
| — | — |
| 30A RV or small boat supply | 120/240V 32A |
| Step-up for one 240V appliance | 120/240V 32A |
| 50A RV or 100A-class shore supply | 120/240V 100A |
| Full house panel behind a split-phase inverter system | 120/240V 100A |
Both share the same 28A continuous neutral current rating at 40°C. The larger model raises feed-through capacity, not the imbalance it can absorb.
Installation Notes
- Mount it in a dry, ventilated spot. IP21 rules out exposed decks or wet bays.
- At 12.5–13.5kg, fix it to a solid bulkhead or wall.
- The supply neutral is left unconnected on the load side when balancing, so the neutral and earth arrangement must follow the manual and local rules.
- The built-in two-pole breaker protects the transformer; size upstream breakers to match.
- Have a licensed electrician install and test it.
Where It Sits in a Victron System
In a typical RV or boat, shore power comes in, passes through the autotransformer for balancing, then feeds the MultiPlus-II or Quattro’s AC input. On an off-grid system it sits after the inverter output instead, to create 240V or a second leg. Draw your layout before buying so you know which side of the inverter it goes on.
Planning the Load Split
Before buying, list every 120V appliance and note which leg of the panel it’s on. Add up the running amps for each leg in a typical busy hour, such as air conditioner, microwave and water heater on together in the evening. The difference between the two legs is your imbalance. If it stays under about 28A, the autotransformer absorbs it. If one or two loads push it higher, moving a single breaker to the other leg is often enough. Write the result on the panel door so future changes keep the legs reasonably even.
Common Pairings
- MultiPlus-II 2x120V: a split-phase inverter/charger that already has two 120V outputs. Pairing it with an autotransformer on the AC input helps it use more of an unbalanced 50A shore supply.
- Single MultiPlus-II 120V: the autotransformer creates the second leg or a 240V supply.
- Quattro: two AC inputs for shore and generator, with an autotransformer on the input to balance a split-phase source.
FAQ
What does a Victron Autotransformer do?
It steps voltage up or down and balances split-phase loads, so you can use more of a 120/240V supply.
Can it make 240V from a 120V inverter?
Yes. Victron lists step-up use and creating split-phase with a MultiPlus-II.
Which size do I need?
The 32A model for supplies up to 32A at 240V, and the 100A model for larger ones.
Does the autotransformer waste power?
Toroidal transformers are efficient, but there’s a small loss whenever it’s energised. On a battery system, factor in a little extra standby draw.
Can it balance a 30A RV supply?
A standard 30A RV supply is a single 120V leg, so there’s nothing to balance. The 32A model is for 120/240V supplies like Victron’s 30A 120/240V example.
Is 28A of imbalance enough?
For most RVs and boats, yes. Arrange the panel so the largest 120V loads are spread across both legs.
Does it replace a split-phase inverter?
Not exactly. It can create 240V or a second leg from one 120V inverter, but total power is still limited by that single inverter.
Sources: Victron Autotransformer product page and Datasheet Autotransformer 120/240V 32A and 100A; MultiPlus-II 2x120V manual (configuration), victronenergy.com, accessed Oct 2026.
Related: EcoFlow Delta 3 Max RV Boondocking, EcoFlow Delta 3 Max Generator Charging, EcoFlow Delta 3 Max RV Air Conditioner, plus the Victron Energy Guide