How to calculate transformer load

Written by richard asmus
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How to calculate transformer load
Use smaller transfromers for a smaller load. (transformateur électrique image by Lounatiq from

A transformer changes alternating current (AC) voltage from one level to another for power companies, appliances and chargers. But the size of a transformer has little to do with the voltage, and everything to do with the amount of electricity it provides. Electricians and technicians refer to the equipment a transformer powers as its load, be it machinery, appliances or electronic components. The load can be measured in amps, watts or volt/amps. To calculate load, you must understand certain electrical terms and formulas.

Skill level:

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Things you need

  • Pen or pencil
  • Notepad

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  1. 1

    Inventory all the equipment the transformer powers. Make a list of the components, lights, appliances or machinery that the transformer will operate. Add the amount of current, watts or volt/amps that each draws. There should be a tag or label on all equipment telling the amount of current or power it draws.

    How to calculate transformer load
    The load for a power company transformer is the entire building it feeds. (Power Pole and Rainbow image by Scott Griessel from
  2. 2

    Convert the power to equivalent values. Arrange the values into two columns on the list. Label the first "current" and the second "watts" or "volt/amps." Use the formula power equals volts times amps, or (P=IE) t, make conversions.

    How to calculate transformer load
    The load for a substation transfromer is the entire area it feeds. (High Voltage Transformer image by Andrzej Thiel from
  3. 3

    Add up the total current in amps for the first column and the watts or volt/amps in the second column. The sums equal the transformer load expressed in the three terms.

Tips and warnings

  • The prefix "milli" means thousandths and "kilo" means thousands. For example, 50 milliamps equals .05 amps, 10kW means 10,000 watts and 5 kVA means 5,000 volt/amps.
  • Volt/amps and watts actually mean the same thing because wattage equals volts times amps.
  • Don't add watts to kilowatts without converting to equivalent values. For example, 10kW plus 100 watts equals 10.1kW or 10,100 watts.
  • Don't add amps to watts. Make the conversions first, and only add amps to amps and watts to watts.

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