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How to buy the right transformer (600v class)

When choosing a general purpose low voltage dry type transformer (below 600v), you'll need to consider phase type, kVA rating, and input and output voltage.

Written by:
Mac Spiller

September 14, 2022

Low voltage dry type transformer

As anyone who’s worked with transformers knows, you can’t use just any unit for any given application. Every application has unique specifications, which inform the design of the transformer you need to use.

For example, if a building is provided with 3-phase 480v power, but you have a machine that requires 3-phase 240v power, you’d need to install a step-down transformer to convert the 480v power to the 240v electricity to get the machine going. Or let’s say you need to power a secondary panel in a building at a different voltage than the primary. Since the utility provider will only provide electricity at a single voltage, you’d need to get a transformer that matches the precise voltages necessary between the two panels.

So before you order a transformer, you need to know the exact demands of the project you’re working on. This short guide will walk you through three steps to help you collect the information you need before you get in touch with a transformer company. With this information, they’ll be able to size your unit correctly and make any necessary modifications.

This guide is for specifying needs for dry-type general purpose transformers below 600v and smaller than 1,000 kVA. If you need a larger transformer (anything over 600v), contact our team of experts.

Step 1: Choose between 1-phase and 3-phase

First, you’ll need to determine what kind of power your utility provider supplies to your facility. It will be either 1-phase (single phase) or 3-phase (three phase) power. 1-phase power is found in homes and small buildings, while 3-phase power is found in larger commercial and industrial settings. Once you know whether your provider supplies you with 1-phase or 3-phase power, you have a few options for choosing the right transformer. The simplest approach, of course, is to match your transformer’s phase type to the power you have in the building. If you’re working with 1-phase power, order a 1-phase transformer. If you’re working with 3-phase power, order a 3-phase transformer.

If your utility provider is supplying 3-phase power, but you need 1-phase, you can use a 3-phase transformer to convert it into 1-phase power, which means that larger, industrial transformers can take an industrial power source and use it to supply non-industrial applications. However, you cannot use a 3-phase transformer to convert 1-phase power into 3-phase power. For your transformer to put out 3-phase power from a 1-phase source, you’ll either need to call your utility provider and upgrade to 3-phase service or install a phase converter. To learn more about identifying your service’s phase type, call us at 1-866-719-1882.

Step 2: Determine your input and output voltage

Also known as “primary voltage,” your transformer’s input voltage is the voltage of the source electricity that needs to be changed. The voltage of the main distribution panel of a given building will typically have the input voltage clearly labeled. But if you’re not sure or can’t find the voltage, get in touch with your utility provider. Common input voltages in the U.S. are:

  • 3-phase: 600v, 480v, 240v, 208v
  • 1-phase: 480v, 240v, 120v

Your output voltage, also known as the “secondary voltage,” is the voltage being supplied by the transformer. Depending on the project the transformer is powering, this voltage can be:

  • 600v
  • 480v
  • 460v
  • 430v
  • 416v
  • 400v
  • 380v
  • 240v
  • 230v
  • 208v
  • 120v
  • Etc.

If you’re going to use the transformer to power a specific piece of equipment, consult the label on the equipment you need to power to find out what the operating voltage is.

Learn more about common industry voltages.

Step 3: Calculate the kVA rating

Short for “kilo-volt-amperes,” kVA is the main power rating system used for sizing transformers. This doesn’t have anything to do with the physical dimensions of the transformer. Instead, sizing a transformer is the process of identifying a given unit’s maximum output voltage and current. If you don’t size your transformer correctly, it will be either too large, in which case you’ll have overpaid for something you don’t need, or it will be too small, which can lead to overloading and damaging the transformer.

To calculate the kVA you need in your transformer, you can use the formulas below, or our free online transformer kVA calculator. We also have these values in the tables under the formulas.

  • 3-phase kVA calculation: voltage X amperage X 1.732 ÷ 1,000
  • 1-phase kVA calculation: voltage X amperage ÷ 1,000

3-Phase sizing chart (amps)

15kVA 30kVA 45kVA 75kVA 112.5kVA 150kVA 225kVA 300kVA 500kVA 750kVA 1000kVA
208V 42A 83A 125A 208A 312A 416A 625A 833A 1388A 2082A 2776A
240V 36A 72A 108A 180A 271A 361A 541A 722A 1203A 1804A 2406A
380V 23A 46A 68A 114A 171A 228A 342A 456A 760A 1140A 1519A
400V 22A 43A 65A 108A 162A 217A 325A 433A 722A 1084A 1443A
416V 21A 42A 62A 104A 156A 208A 312A 416A 694A 1041A 1388A
480V 18A 36A 54A 90A 135A 180A 271A 361A 601A 902A 1203A
600V 14A 29A 43A 72A 108A 144A 217A 289A 481A 722A 962A

1-Phase sizing chart (amps)

10kVA 15kVA 25kVA 37.5kVA 50kVA 75kVA 100kVA 167kVA 250kVA 333kVA
120V 83A 125A 208A 313A 417A 625A 833A 1392A 2083A 2775A
240V 42A 63A 104A 156A 208A 313A 417A 696A 1042A 1388A
277V 36A 54A 90A 1354A 181A 271A 361A 603A 903A 1202A
480V 21A 31A 52A 784A 104A 156A 208A 348A 521A 694A
416V 21A 42A 62A 104A 156A 208A 312A 416A 694A 1041A
480V 18A 36A 54A 90A 135A 180A 271A 361A 601A 902A
600V 14A 29A 43A 72A 108A 144A 217A 289A 481A 722A

If you’re looking for a transformer for a crypto-mining application, you should also check out our article on sizing for crypto-mining.


Now that you know your phase type, input voltage, output voltage, and kVA size, you’re ready to start the ordering process.

Our webstore makes it easy for you to buy the transformer you need for your operation. With a simple, user-friendly interface, it lets you click on the appropriate input and output voltages and select the necessary kVA, then pulls up available units.

And if you order a unit and find out that it's not exactly what you needed, we will exchange it for free. See our exchange policy here.

Choosing unit specifications

If you can’t find what you’re looking for on our website or if you have any questions, give us a call at 1-866-719-1882.

There are other (more technical) elements to transformers that may need to be taken into account for your application such as the K-factor rating, winding conductor material, duty class, oversizing for harmonic distortion, and more, but covering all of these elements in any depth is beyond the scope of this article. You’ll be able to determine if any of these elements are necessary and how to customize each one as you work with the transformer provider.

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