How To Determine Current Capacity Of Transformers

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.

Step 1

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

Step 2

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

Step 3

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.

Step 4

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