Calculating Transformer & Generator Sizing

You may have noticed that there are quite a few different voltage levels in different countries- even on the same continent. For example, in the US, high voltage in commercial buildings use 480V, yet in Canada it is 600V- even though both countries are considered North America. Since most equipment is built for a specific voltage, when using it in various locations where the applicable voltage is not available, you’ll need a transformer or generator. Generators are also good for when there is not enough power available on the site or if the power must be turned off while the work is being done.

This blog will help you to find the right sized transformer or generator for your equipment.


What You Need to Know

When a transformer or generator is required, it is important to select one that is the right size for your equipment or else you could run into some issues. A common confusion we hear with transformers and generators, is how they are sized when they are being used to power electric motors. Generally, home generators are sized in Watts (Voltage multiplied by Amperage), do to how electric motors draw power.

In order to properly size a generator for an electric motor load, you need to know the kVA(equations below) that the motor in your equipment requires. The kVA takes into account the extra part of power the motor draws, that is not measured in the normal fashion. For simplicity sake, we won’t go into the details here, but this portion is represented by the power factor of the motor. In our previous blog post about reading nameplates and connectors we showed you how to identify the power factor of your equipment’s motor.

In order to calculate the kVA required for your machine, you'll need to know the following information:

  • Voltage (Volts)
  • Amperage (Amps)
  • Power Factor (PF)
  • Single or Three Phase

If you are unable to find the power factor of the equipment you are calculating for, 0.8 is a good, general value for electric motors.


Below you'll find the 2 basic equations that are used and when to use them. Below the equations, you will find a calculator to do the math for you.

For single phase equipment


For three phase equipment


kVA Calculator

Enter the information that you have collected in the fields below and click calculate:

Please enter the voltage
Please enter the current
Please select phase
Please enter the Power Factor

* DISCLAIMER: Whenever complex electrical calculations are involved, it is always wise to seek the assistance of a certified electrician. The calculator on this page is offered to assist in generating general estimations only. In most instances, there are numerous additional factors that should be accounted for.


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