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Autotransformers: applications, advantages, & disadvantages

An autotransformer is an electrical transformer that uses a single coil to adjust voltage, meaning there's no isolation between primary & secondary circuits.

Written by:
Ben Gulick, Mac Spiller, Josh Long, & Nathan Stenzel

June 28, 2023

A picture of three Autotransformers

What is an Autotransformer?

An autotransformer is a type of electrical transformer that utilizes a single coil to adjust voltage, meaning there is no isolation between the primary and secondary circuits.

Similar to a typical isolation transformer, autotransformers transfer electrical energy from one circuit to another while changing the voltage. But unlike isolation transformers where the windings are completely separate, the primary and secondary of an autotransformer are electrically connected.

Is an autotransformer right for my application?

Autotransformers are often employed as a cost-effective substitute for 3-phase general purpose distribution transformers for adjusting supply voltage for particular load requirements, provided that isolation from the supply line is not necessary.

Below are some examples of autotransformer applications, and installation considerations.

Voltage Adjustments for Stand Alone Equipment

Autotransformers are ideal for voltage adjustment for commercial and industrial machines. They provide an efficient, low cost way of serving the proper voltage to motors and compressors, lathes, CNC machines and other industrial equipment requiring a step up or down from a building’s service voltage. Their smaller profile is often half the size of a standard two winding transformer, which makes for a discreet installation.

An autotransformer downstream from an electrical panel connected to a CNC Machine.

Autotransformers Supplying Branch Circuits

For applications deriving branch circuits from autotransformers, a connection to a grounded conductor from the system supplying power to the autotransformer must be made at the load side terminal of the transformer. This means there will be four wires connected at the primary and secondary of the autotransformer–three hot wires (ungrounded conductors) and a neutral (grounded conductor). The electrical code allows for two exceptions to this rule as follows:

“Exception No. 1: An autotransformer shall be permitted without the connection to a grounded conductor where transforming from a nominal 208 volts to a nominal 240-volt supply or similarly from 240 volts to 208 volts.”

“Exception No. 2: In industrial occupancies, where conditions of maintenance and supervision ensure that only qualified persons service the installation, autotransformers shall be permitted to supply nominal 600-volt loads from nominal 480-volt systems, and 480-volt loads from nominal 600-volt systems, without the connection to a similar grounded conductor.”

When going from 240V to 208V and vice versa, the NEC allows autotransformers to derive a branch circuit with a three wire connection on the primary and secondary sides–meaning you don’t need to supply a fourth wire (grounded conductor) from the system supplying power to the autotransformer.

This same rule can be applied in industrial occupancies with voltage transformations going from 480 to 600 and vice versa with the stipulation that there is proper maintenance and supervision present in the facility to ensure only qualified persons can service the transformer installation.

Customer review

“We have used Maddox autotransformers on several ground-mount solar installations. It is quite common for commercial-scale PV inverters to produce 600 volts, but the auxiliary equipment requires lower voltages such as 380, 400, or 480 volts. Autotransformers make it very easy to achieve that voltage conversion.”
Mason Werbelow, Assistant Project Manager at LightEdison

Grounding & Bonding Autotransformers

Because autotransformers do not isolate the incoming and outgoing line and load circuits they are connected to, it is generally not recommended that the neutral point of the windings be bonded to ground. Grounding the neutral point may create objectionable ground current paths with an already present neutral ground bond in the system.

This should not be confused with the equipment grounding conductor (or bare copper wire) that is connected to the metal enclosure of the transformer (commonly referred to as a “case ground”). The case of the transformer should be grounded per standard electrical code regulations.

Using the Neutral Terminal

The HV and LV neutral connections on Maddox’s autotransformers are typically brought out as a single common H0/X0 terminal. When an autotransformer supplies power to a load that requires a neutral (fourth wire), a grounded conductor (neutral) from both the line and load sides of the circuit must be landed at the H0/X0 terminal. In other words, you cannot supply a load from the transformer with four wires if you only bring three into it from the supply end–you have to have four wires in and four wires out.

A three wire connection on the incoming and outgoing terminals is commonly used as well. In such cases, the neutral ground bond at the source supplying power to the autotransformer is referenced from its point of origin all the way through to the load being served by the autotransformer.

All of these restrictions and rules can be a lot to take in if you’ve never dealt with autotransformers before, so we’ve boiled the basic rules of thumb down with quick reference table as follows:

Pros Cons
Simple 1 winding per phase design No isolation
Smaller, more efficient design Does not create a neutral
No phase shift No adjustment taps
Lower cost

Wires Approved
3 wires in, 3 wires out OK
4 wires in, 4 wires out OK
4 wires in, 3 wires out OK
3 wires in, 4 wires out NOT OK

Customer review

“Alpha builds custom hydraulic elevator controls, and we occasionally need to supply a transformer with the package to change the building voltage to work with our controls. Autotransformers are great because they’re less expensive than regular isolation transformers while getting the same exact job done. We pass the savings on to the customer.”
Joel Licea, Business Development at Alpha Elevator Control

How do I know if my machine requires isolation?

If you are unsure if an autotransformer is compatible with a particular piece of equipment, we recommend contacting the equipment manufacturer. However, as a general rule, this issue goes beyond the scope of an equipment manufacturers’ specifications since it is largely dependent on the existing electrical system configuration, where the equipment is installed, and local code requirements which we covered in the first part of this article.

One hard and fast rule around autotransformer use, is that if the machine requires a neutral, and your system does not have one, an isolation transformer is required to create that neutral. Autotransformers do not create their own neutrals.

Beyond that, machine manufacturers are often more focused on the what (volts and amps) rather than the how (isolated or not) when it comes to power supply. But there are two types of equipment that we have noticed tend to prefer isolation transformers:

  1. Anything non-linear (like electronics) are usually better served by a delta-wye isolation transformer which would 1) deal more effectively with balancing loads that produce harmonics and 2) establish a fresh ground reference close to the load, isolating it from any pesky power quality issues that may be present in the grounding system upwind of the transformer.
  2. Certain solar inverter manufacturers have strict transformer specifications, so it’s recommended to check with them before installing an autotransformer with their equipment.

Learn more about transformer sizing and design requirements —inverters, harmonics, DC bias, overload, bi-directionality, and more.

How is an autotransformer different from an isolation transformer?

There are four main ways in which autotransformers differ from isolation transformers: construction, voltage transformation, voltage regulation and impedance, and size and efficiency.


  • Autotransformer: An autotransformer has a single winding that serves as both the primary and secondary winding. It typically consists of a common winding with taps at different points to provide various voltage ratios.
  • Isolation Transformer: An isolation transformer has two separate windings—the primary winding and the secondary winding. These windings are electrically isolated from each other.

Voltage Transformation:

  • Autotransformer: In an autotransformer, the voltage transformation occurs by tapping at different points along the winding. By changing the point of connection, different voltage ratios are achieved. The primary and secondary voltages are interconnected, which means the secondary voltage is derived from the same winding as the primary voltage.
  • Isolation Transformer: In an isolation transformer, the voltage transformation occurs through mutual induction between the primary and secondary windings. The primary voltage induces a magnetic field, which then induces a voltage in the secondary winding. The primary and secondary voltages are electrically isolated from each other.

Voltage Regulation and Impedance:

  • Autotransformer: Because of the direct connection between primary and secondary windings, autotransformers generally provide better voltage regulation. However, they have a lower impedance compared to isolation transformers, and as a result, they may not provide as much protection against voltage spikes or fluctuations. Due to the lack of any phase shift, autotransformers provide no assistance in removing minor harmonic distortion.
  • Isolation Transformer: Isolation transformers offer complete electrical isolation between the input and output, making them suitable for applications where safety and protection against electrical faults are critical. They have higher impedance, which can help reduce voltage spikes and provide isolation from the main power supply. In systems with minor harmonic distortion, an isolated transformer provides phase shift (and potentially a grounded neutral conductor in D-Y systems) which can help in removing minor harmonic distortion.

Size and Efficiency:

  • Autotransformer: Autotransformers are typically smaller and lighter than isolation transformers since they require only a single winding. Autotransformers typically have increased efficiency despite being exempt from DOE 2016 energy efficiency requirements, due to their low impedance and common conducting path.
  • Isolation Transformer: Isolation transformers are bulkier and heavier than autotransformers because of the two separate windings. They are required to meet current DOE efficiency guidelines and are not as efficient as autotransformers.

What conductor is used in autotransformers?

Transformer windings are made out of either aluminum or copper; there’s no substantial advantage to either winding material. Here at Maddox, we use copper windings for all of our autotransformers.


If you need help deciding if an autotransformer is right for you or need a custom autotransformer built, please fill out the form below.

You can also purchase autotransformers directly from our online store.

Maddox padmount transformer loaded on truck

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