How to size a transformer kVA

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How to size a transformer kVA
Choose a transformer based on the requirement of the load. (Stockbyte/Stockbyte/Getty Images)

Transformers convert input voltage to the voltage required in the installation. They may step up (increase) or step down (decrease) the input voltage, which is applied to the primary side of the transformer. At home, connect a continental or US 110V appliance through a transformer to use it in the UK. A transformer is sized or rated according to its kVA or kilo-volt ampere capacity to serve the voltage and current required by the secondary or output side.

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Primary voltage

The line or primary voltage describes the voltage supplied from the source. For example, the standard UK domestic supply voltage is 240V; this is the input voltage supplied to the transformer.

Secondary voltage

The secondary or load voltage is the voltage needed to operate the appliance or equipment. An appliance purchased in some European countries or the US has a standard voltage of 110V; to use it the UK, the transformer needs to step down the input voltage of 240V to the output voltage of 110V.

Load frequency

The load frequency is expressed in Hertz (Hz) and is typically 50 to 60 Hz for domestic appliances in the UK. The frequency of both primary and secondary voltages must be the same as a transformer doesn't change the frequency. The frequency of your appliance may be shown on the electrical information plate or in the manufacturer's manual.


The electrical load or power rating of an appliance is measured in amps. Calculate the capacity of the transformer required in kVA by multiplying output volts by amps and dividing the result by 1,000 for a single phase supply. For a three phase supply, multiply volts by amps by 1.732 and divide the result by 1,000. For example, a 110V, 5A appliance requires a transformer of at least (110 x 5)/1000 which equals 0.55 kVA on a single phase supply and (110 x 5 x 1.732)/1000 or 0.95 kVA minimum for a three-phase supply. Choose a transformer with a capacity equal to or greater than the minimum kVA. If the appliance is motor-based, increase the minimum kVA by 20 percent to account for the surge in power drawn when the motor starts.

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