Step up, step down transformers, and reverse feeding


Step-up vs. step-down, what's the difference?

Simply put, transformers are machines that step voltage up or down so that electricity can be moved and used more efficiently. In this article we will primarily review the importance of and differences in step-up and step-down transformers.

A “step-down transformer” is used to step voltage down, while a “step-up transformer” is used to step voltage up.

The voltage coming into the transformer from the power source is called the primary voltage and the voltage going out of the transformer is called the secondary voltage.

What is a step-down transformer?

A step-down transformer is one which has a primary voltage that is higher than the secondary voltage.

To illustrate, let’s say your company receives 3-phase 480v power from the power company, but you have a piece of equipment that requires 208v 3-phase power. To make this work, you need a step-down transformer to convert the 480v power to 208v to run your machine with the right voltage.

What is a step-up transformer?

A step-up transformer is one which has a primary voltage that is lower than the secondary voltage.

In this case, let’s say your building is wired with 208v but you need 480v to power a large machine, you would need a step-up transformer to boost the voltage from 208v to 480v.

These examples are small industrial applications. But the principle applies no matter the size. For example, power companies use massive substation transformers called GSU transformers (generator step-up) to step voltages up from power plants at 7,200v to extra-high voltage like 345,000v for large-scale power transmission over many miles. Once the power has reached its destination, a substation transformer is used to step the voltage back down for distribution.

Because transformers operate with AC electricity, all transformers are technically capable of both step-up and step-down operation. In this sense, the “step-up” and “step-down” designation just refers to the way the transformer is being used.

NOTE: AC stands for alternating current, which means that the direction of the current flowing through the system literally changes direction 60 times per second. That frequency of change is measured in hertz, which is why AC systems in the US are called 60 hertz. Read more about the history of AC power v. DC power here .

What are the design differences between step-up and step-down transformers?

Any transformer can theoretically be used for either step-up or step-down operation. But, there are some notable differences in the way that step-up and step-down transformers are designed. These are not hard-and-fast rules by any means, but are standards the transformer industry tends to adhere to. Further, design differences tend to be more pronounced in low-voltage transformers (<600v) as compared to their medium-voltage counterparts (>2400v).

NOTE: Step-up transformers designed specifically for solar and wind farms have their own set of design standards, which we will explore in a future article.

Here are the main design differences broken out by transformer type.

1. Winding and voltage tap location

Low voltage transformers

Low voltage, step-down transformers usually have the high-voltage windings on the outside, and low-voltage windings on the inside. Step-up transformers have the opposite configuration. The main reason for this is that voltage adjustment taps are usually located on the primary windings, and since the windings are concentric (one inside the other), the windings with the voltage taps have to be physically located on the outer coils.

Below is an example with the high voltage windings shown as red, and the low voltage windings shown as blue.

Medium voltage transformers

When it comes to medium voltage transformers, there is virtually no difference in winding or tap locations. HV windings are always on the outside, and taps are always on the HV side.

2. Vector grouping

Low voltage transformers vector grouping

Low voltage transformers are usually built with a DELTA-WYE vector group regardless of step-up or step-down operation, with DELTA being the connection on the primary side, and WYE being the connection on the secondary side.

Below is an example showing the same low voltage transformer, but one is designed for step-down, and the other for step-up.

Medium voltage transformers vector grouping

Medium voltage transformers are usually built with a DELTA-WYE vector group when designed for step down operation, and with a WYE-WYE vector group when designed for step up.

Below is an example showing the same medium voltage transformer, but one is designed for step-down, and the other for step-up.

3. Reverse feeding

Any step-down transformer can technically be used as a step-up transformer by “reverse feeding” the transformer.

Reverse feeding is simply a matter of having the power supply entering at the low voltage windings.

Given the greater availability of step-down transformers, reverse feeding is a common practice in the industry. With that said, here are a few considerations when reverse-feeding a step-down transformer.

  • Code compliance: While there’s nothing in the NEC that directly prohibits reverse-feeding, your local electrical inspector may decide they want to see language on the transformer nameplate like “suitable for step up operation” before approving the installation.
  • Inrush: Inrush current (the amount of current the transformer draws when initially energized) is greater when reverse-feeding, which could result in a flipped breaker. This is rare given that modern breakers usually have enough of a time-delay to sustain the inrush without nuisance tripping. Nuisance tripping usually only occurs when either the breaker feeding the transformer is antiquated, and/or the transformer kVA is very high.

    The greater the kVA the greater the inrush, which is why some manufacturers recommend you only reverse feed 75 kVA and below. But as long as the breaker has a sufficient time delay, you can reverse feed larger transformers.
  • Vector grouping / No neutral: The most common transformer vector group is DELTA-WYE, with DELTA being the configuration on the primary side, and WYE on the secondary side. Therefore, reverse feeding a transformer originally designed for step-down operation will result in a DELTA secondary connection, which lacks a neutral. If this is your situation, you need to ensure that the load does not require a neutral, and the secondary may need to be corner grounded.

    See article Putting It In Reverse by James Stallcup for more reading on the subject.

NOTE: Transformer terminals are always labeled as H for the high voltage windings, and as X for the low voltage windings. Regardless if it’s being used as a step-up or step-down transformer. H is always the higher of the two voltages, and X is always the lower.

With these things in mind, our recommendation is that you buy a purpose-built step-up transformer instead of reverse feeding when possible. And as always, if you have questions or need more help when it comes to understanding or finding the right step-up or step-down transformer, don’t hesitate to contact us or call our team at 800-270-2011.


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