Autotransformers

What Does an Autotransformer Do in an Energy System?

In a renewable energy setup, the inverter typically outputs a single 120V leg. Many loads in Canadian homes and RVs, electric dryers, well pumps, EV chargers, and HVAC compressors require 240V split-phase power.

An autotransformer bridges that gap. It takes the 120V output from your inverter and creates a balanced 120V/240V split-phase supply. This allows you to run 240V appliances without upgrading your inverter or adding a second unit.

It also works in reverse: if your source is 240V, the autotransformer can step it down to 120V for single-phase loads.

Key functions:

A transformer can perform several important tasks depending on your power system and application.

• 120V to 240V Split-Phase: Creates a 240V split-phase output for off-grid homes, RVs, boats, and marine electrical systems.
• 240V to 120V Step-Down: Converts 240V power to 120V for appliances and other voltage-sensitive equipment.
• Balance Split-Phase Loads: Helps distribute electrical loads evenly across both legs of a split-phase system for improved efficiency and stability.
• Extend Inverter Capability: Expands the functionality of a single inverter, allowing it to support additional applications without the cost of installing a second inverter.

Autotransformer vs Isolation Transformer: What's the Difference?

This is one of the most common questions when configuring a power system.

An autotransformer shares a common winding between input and output. It is efficient, lightweight, and cost-effective, but it does not provide electrical isolation. There is a direct electrical connection between primary and secondary circuits.

An isolation transformer uses two separate windings. It provides galvanic isolation, eliminates ground loops, and protects sensitive equipment from shore power faults. It is mandatory in certain marine applications under Canadian and ABYC electrical standards.

Choose an autotransformer when:

• Your goal is voltage conversion, not isolation
• You are running off-grid or RV systems without shore power concerns
• You want to extend a single-output inverter to drive 240V loads

Choose an isolation transformer when:

• You are connecting to shore power on a vessel
• You need to eliminate stray current corrosion risk
• Your application requires full galvanic separation

Victron Energy Autotransformers Models Available at Volts Energies

Volts Energies stocks the Victron Energy autotransformer range for the Canadian market. All units operate at 120/240VAC and are built for continuous duty in demanding environments.

Victron Energy Autotransformer 120/240vac-32a 

Rated at 32A continuous, this unit handles loads up to approximately 3,840W on the 240V output. It is well suited for smaller off-grid cabins, RV systems, and light residential backup setups where a second 240V circuit is required.

Victron Energy Autotransformer 120/240vac-100a 

Rated at 100A continuous, this is the heavy-duty option. It supports larger loads including well pumps, electric dryers, and high-draw HVAC equipment. It integrates cleanly with the Victron MultiPlus-II and Quattro inverter-chargers for full split-phase capability in off-grid homes.

Both models are compact in design, passively cooled, and require no configuration software. They connect directly in-line between the inverter output and the load panel.

Which System Configurations Use an Autotransformer?

Autotransformers are not needed in every system. They are specifically useful in these scenarios:

Off-Grid Home With A Single-Output Inverter 

If your MultiPlus outputs 120V but your home has 240V appliances on the panel, an autotransformer creates the second leg. This is far more cost-effective than purchasing a second inverter or a Quattro.

RV Or Motorhome With 240V Shore Power Hookups 

Some RV parks provide 240V/50A service. An autotransformer allows you to draw balanced power across both legs and distribute it properly to your onboard loads.

Marine Vessels Not Requiring Isolation

For boats where galvanic isolation is not a code requirement, an autotransformer handles voltage conversion cleanly and at lower cost than an isolation transformer.

Generator-Fed Systems 

When a generator outputs 240V but your loads are 120V, the autotransformer steps down the voltage and balances the supply.

How to Size an Autotransformer for Your System

Sizing is based on the highest 240V load current you expect to draw at any point. Use this approach:

1. List all 240V loads (appliances, motors, HVAC)
2. Add up the amperage of loads likely to run simultaneously
3. Select an autotransformer rated at or above that total draw

For most residential off-grid systems with standard 240V loads, the 32A model is sufficient. For larger homes with multiple high-draw circuits running concurrently, the 100A model is the correct choice.

Do not undersize. Autotransformers running above rated current will overheat and fail prematurely.

Why Buy Your Autotransformer from Volts Energies?

Volts Energies is Canada's largest online retailer of renewable energy equipment. We have been supplying Victron Energy products to off-grid homeowners, marine installers, and RV builders across Canada for over 20 years.

Every autotransformer we sell is sourced directly through authorised Victron Energy distribution channels. You receive the full manufacturer warranty, Canadian-compliant documentation, and free shipping on orders over $297.

Our team includes certified energy system designers. If you are unsure which autotransformer fits your setup, or whether you need an autotransformer or an isolation transformer, contact us before purchasing. We will give you a straight answer.

FAQs

Can I Use An Autotransformer With Any Inverter? 

Yes, as long as the inverter produces a stable sine wave AC output. Victron autotransformers are optimised for use with Victron MultiPlus and Quattro inverters but will work with other pure sine wave inverters producing 120VAC output.

Does An Autotransformer Need To Be Grounded? 

Yes. Proper grounding is required for safe operation and compliance with Canadian electrical codes. Follow local NEC/CEC requirements and consult a licensed electrician during installation.

Will An Autotransformer Work With A Modified Sine Wave Inverter? 

It is not recommended. Autotransformers are designed for pure sine wave input. Modified sine wave output can cause overheating and reduce the efficiency of the transformer.

Is An Autotransformer The Same As A Step-Up Transformer? 

Not exactly. A step-up transformer has two separate windings. An autotransformer uses one shared winding with a tap point to determine the output voltage. It achieves the same voltage conversion but is lighter, more efficient, and less expensive for this application.

Can I Use An Autotransformer To Run A 240v Well Pump Off-Grid? 

Yes. This is one of the most common use cases. Pair a Victron MultiPlus inverter with the 100A autotransformer for well pumps drawing up to 100A at 240V. Confirm the pump's starting surge current and ensure your battery bank can handle it.