How to Determine Current Capacity of Transformers

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Transformers change electricity from one voltage level to another. But changing the voltage does not change the power. Power equals voltage times current. So when a transformer increases voltage, it decreases current. Likewise, if it decreases voltage, it increases current. But the power remains the same.

All transformers have two coils of wires called the primary and secondary. The primary induces voltage into the secondary, at a rate determined by the ratio of the number coils of wire in the primary to the number of coils of wire in the secondary. But with a ratio of 1:2, since voltage times current equals power, doubling the voltage halves the current.

The physical size of a transformer and its internal components determines its current rating. A tiny transformer for a cell phone charger operates at about a half an amp. A huge transformer at a hydroelectric dam operates at thousands of amps. If your particular transformer information does not specifically determine the current rating, you can figure it out using other statistics.

    Read the specifications of the transformer. It should show the maximum current ratings for both the primary and the secondary.

    Read the tag on the transformer. It should show the maximum current rating for the primary and secondary.

    Calculate the current from the voltage/amperage (VA) specification or the power rating designated in watts. Both figures mean basically the same thing. Divide either the VA or wattage rating of the transformer by the voltage. This will give you the current rating.

    Perform separate calculations using the statistics of the primary and the secondary, because they are most likely different.


    • The current rating of a transformer tells the maximum amount of current the transformer can handle. If you exceed the rating, you can burn up the transformer. For example, if you bought a 220 volt to 120 volt transformer with a current rating of 5 amps to operate a computer overseas, and you plugged your 20 amp air conditioner into the transformer, you would burn up the transformer because it can not handle the excessive current.

      Never enter a substation, open a power company transformer box or climb a utility pole to look at a transformer unless you are an authorized employee of that power company. If you have questions about your service, call your power company.

About the Author

Richard Asmus was a writer and producer of television commercials in Phoenix, Arizona, and now is retired in Peru. After founding a small telecommunications engineering corporation and visiting 37 countries, Asmus studied broadcasting at Arizona State University and earned his Master of Fine Arts at Brooklyn College in New York.

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