How to Size a Transformer

Electrical transformers are used in everyday life. At times when an electrical device fails it is more often than not the transformer that is the problem. Replacing the transformer is generally an easy to do. You must be sure that you select the proper size so as not to ruin that particular piece of equipment. Sizing an electrical transformer is an easy task once you understand the nomenclature or language that is used. There are no hidden codes on the transformer, just abbreviations.

Step 1

Understand the language used in transformer calculations. Certain terminology is used when sizing the transformer. The terms are easy to understand since they use abbreviations of voltage which is V and amperage which is A. The letter K stands for Kilo which is equal to 1000. If a transformer is rated at 120VA it means that it can handle 120 volts at 1-ampere or 1 amp of current. The VA is short for Volt-Ampere a designation of power. A transformer rated at 1.2 KVA is another way of saying 1200 VA or 120 volts at 10 amps of current.

Step 2

Calculate the load you want to place on the transformer. All electrical devices come with a rated nameplate power usage. This is always described as voltage and load amps. The load amps are calculated after the device is started and running. All electrical devices have a starting current and must be accounted for, when sizing the transformer. When figuring the load, always multiply the voltage times the load current and multiply a 125 percent start factor for the load.

Step 3

Calculate an example as follows. A 120-volt motor has a load amperage of 5 amps. Multiply 120 volts times 5 amps this equals 600VA now lets multiply the 125 percent start factor. Take 600 times 1.25 this equals 720VA and most transformers are sized by a factor of 25VA or 50VA. The required transformer would be a 750VA or .75KVA transformer. Always figure a little higher in the VA capacity, never go lower.

Step 4

Size the transformer for any voltage or amperage. You can plug any voltage or current into the VA rating of the transformer. The nameplate rating will have a maximum voltage rating for the transformer, generally this is 600 volts. If you want to operate a transformer at 24 volts and it is rated at 120VA, then divide 120 VA by 24 volts. The amperage allowed by the transformer would be 5 amperes.

Things You'll Need

  • Pencil

  • Paper

  • Calculator


Certain state and local codes may apply when wiring any electrical device, be sure to check with local authorities before beginning any electrical work.