How to Size an Electrical Transformer for Load

Written by richard asmus
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How to Size an Electrical Transformer for Load
Size some transformers in watts or kilowatts. (transformer image by palms from

Transformer load means the amount of electricity the transformer distributes. Transformers change voltage from one level to another for a wide variety of purposes--from less than an amp for a cell phone charger up to thousands of watts for a power company transformer. Load can be sized by current in amps or by power in watts or in volt/amps (VA). The design of a transformer determines its load capacity. You can size a transformer above your load requirement but never below it.

Skill level:


  1. 1

    Size the transformer in amps. Smaller transformers used in chargers and appliances are often sized in amps or milliamps (meaning thousandths of an amp). For example, 50 milliamps means .05 amps or 50 thousandths of an amp. Determine the current your circuit or appliance will need, and size your transformer at or above the amperage level you determine.

    How to Size an Electrical Transformer for Load
    Size some transformers in watts or kilowatts. (transformer image by palms from
  2. 2

    Size the transformer in watts. Medium sized to large transformers are often sized in watts or kilowatts, meaning thousands of watts. The load means the total number of appliances your transformer will operate. For example, if you are travelling out of the US and have a transformer to change 240 volts to 120 for your computer, size the transformer at or above the wattage rating of your computer. For more than one appliance, size the transformer by adding up the watts of all the appliances you will use.

    How to Size an Electrical Transformer for Load
    Size some transformers in kVA. (High Voltage Transformer image by Andrzej Thiel from
  3. 3

    Size the transformer in volt/amps. Larger industrial transformers are sometimes rated in volt/amps (VA). This really means the same as watts, because watts equal volts times amps. Use the terms watts and VA interchangeably. Add up all the power for the equipment you will power with the transformer. Size your transformer at or above the total.

Tips and warnings

  • Most appliances have a tag showing either the current they draw in amps or milliamps, or the power they consume in milliwatts, watts or kilowatts.
  • Larger industrial electrical machinery is sometimes labelled in kilowatts, volt/amps or kilo volt/amps (kVA).
  • To convert sizing data, use the formula: power in watts equals volts times current in amps (P=IE).
  • Sizing a transformer below the load requirments will cause it to overheat and possibly burn out.
  • When making conversions, be careful not to confuse your decimal point when using milliwatts, milliamps and kilowatts. Remember that "milli" means thousandths and "kilo" means thousands.

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