CHOOSING THE RIGHT TRANSFORMER

A transformer is a simple machine that is used to step voltage up, or down. 

For example, let’s say the utility provides your building with 3-phase 480v power, but you have a machine that requires 240v 3-phase power.  You would need to wire a step-down transformer to convert the 480v power from the main panel to 240v to power your machine with the proper voltage. 

Another example would be running a secondary panel at a different voltage.  Many buildings have 2 panels with different voltages.   One may be used to run 480/277v loads, while another is used to run 208/120v loads.  Since the utility will only provide a building with power at one voltage, a transformer is needed to step the voltage down to the 208v panel.  See this example pictured to the right.

This short guide helps you choose the right transformer for your application.  This guide is written for people needing dry-type general purpose transformers below 600v, and smaller than 1,000 kVA.  For larger transformers (over 600v) please contact our sales team for help choosing the right unit:  1-800-270-2011.

Follow these 4 simple steps to decide which transformer to buy...

step down transformer

Transformer install example

step 1:  3-phase or 1-phase?

First, you need to determine what kind of power your utility has provided your facility with.  Depending on the type of service your building is equipped with, it can be 1-phase (single-phase), or 3-phase (three-phase).  1-phase is the kind of power found in homes, and small buildings, while 3-phase power is what larger commercial or industrial buildings, which have heavy power needs, are provided with.

1-phase power can be pulled off of a 3-phase transformer, but it will not convert your 1-phase service into 3-phase power.  If you have 1-phase service, and you need 3-phase power for a particular application, buying a 3-phase transformer will not help.  You will need to call your utility and request 3-phase service, or buy a phase converter.  All a transformer does is step voltage up or down.  If you need help understanding your service, call us at 1-866-719-1882.

STEP 2: Determine input voltage

This is the voltage you’re feeding into the transformer. It is also referred to as “primary voltage”, or simply "high voltage".  The voltage of the main distribution panel in your building usually has the voltage labeled clearly. 

Common input voltages in the US are...

3-phase: 600v, 480v, 208v, 240v

1-phase: 480v, 240v, 120v

STEP 3: Decide your output voltage

This is the voltage you need to step the voltage down to. It is also called the “secondary voltage”, or simply "low voltage".

If you are buying a transformer to run a specific piece of electrical equipment, consult the label on the equipment you need to power to determine the operating voltage.

STEP 4: CALCULATE KVA RATING

kVA (kilo-volt-amperes) is the main power rating used for "sizing" transformers.  You can calculate your kVA per the formulas below, or simply use the table below to find your kVA.

3-phase kVA calc: voltage X amperage X 1.732 / 1,000

1-phase kVA calc: voltage X amperage / 1,000

3-phase amps

15 kVA30 kVA45 kVA75 kVA112.5 kVA150 kVA225 kVA300 kVA500 kVA750 kVA1000 kVA
208V42A83A125A208A312A416A625A833A1388A2082A2776A
240V36A72A108A180A271A361A541A722A1203A1804A2406A
380V23A46A68A114A171A228A342A456A760A1140A1519A
400V22A43A65A108A162A217A325A433A722A1083A1443A
416V21A42A62A104A156A208A312A416A694A1041A1388A
480V18A36A54A90A135A180A271A361A601A902A1203A
600V14A29A43A72A108A144A217A289A481A722A962A
V = volts, A = amps

1-phase amps

10 kVA15 kVA25 kVA37.5 kVA50 kVA75 kVA100 kVA167 kVA250 kVA333 kVA
120V83A125A208A313A417A625A833A1392A2083A2775A
240V42A63A104A156A208A313A417A696A1042A1388A
277V36A54A90A1354A181A271A361A603A903A1202A
480V21A31A52A784A104A156A208A348A521A694A
V = volts, A = amps

That's it!

Now that you have your kVA, primary, and secondary voltages, you can order your transformer with confidence!

There are a few other, more advanced topics we could cover such as k Factor, winding conductor, various duty classes, but for 90% of applications this guide suffices.

You can browse available models on our online store here.  Simply select the input and output voltages in the filter on the left-hand side.  Once in the product view, select the appropriate kVA.  If you can't find what you're looking for, or have questions, please call us at 1-866-719-1882.


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