Transformers are the backbone of power distribution systems. The VA rating refers to the power distribution rating relative to how much power the transformer can deliver to the load. To calculate VA, you need to know the supply voltage and the current delivered to the load. You get the supply voltage from the voltage specifications associated with the primary and secondary windings. You then apply the voltage and measure the current that appears across the load.
Find the primary and secondary voltage of the transformer by referring to the transformer specifications. As an example, assume that the primary is 480 volts and the secondary is 208 volts.
Turn off power to the system. Put on electrical safety gloves and follow standard electrical safety precautions.
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Connect the secondary side of the transformer to the electrical load. Connect an ammeter in parallel between the secondary side of the transformer and the load. Connect a power supply to the primary side of the transformer. Ensure that the power supply is capable of delivering the primary voltage to the transformer. Using the example numbers, you will ensure that the power supply can deliver 480 volts, which is the primary voltage.
Power up the power supply to the primary voltage level of the transformer. Note and record the amperes registered on the ammeter. Call this value "I," and assume, as an example, that it equals 65 amps.
Calculate the VA rating using the formula "VArating = (Vsecondary x I)/0.8," where Vsecondary is the secondary voltage of the transformer and the 0.8 accounts for the power factor of the load.
Continuing with the example: VArating = (208 x 65)/0.8 = 16,900 volt-amperes, or 16.9 kilovolt-amperes, or 16.9 KVA.