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The formula for finding the required KVA or transformer size for single-phase power is the following:
Volts x Amps / 1,000 = KVA
Based on the equation, one would need to plug in the proper load/output (secondary) voltage and current (amps) to compute for KVA. Note that load voltage is not the same as line voltage, which is also known as primary voltage or input.
Example: Find the KVA or transformer size for load voltage of 120V 1PH and a load current of 50A.
120 x 50 / 1,000 = KVA 6,000 / 1,000 = KVA = 6 KVA
Businesses that require three-phase power need to add an extra component in the formula to arrive at the correct transformer size, i.e., square root of 3 (√3) or 1.732. This figure is a constant found in three phase, as the phases do not generate the same amount of power (simultaneously). Furthermore, three-phase transformers handle three lines of AC power, with each of the three lines 120 degrees out of phase from the other two lines.
With this in mind, the new formula can be found below:
Volts x Amps x √3 / 1,000 = KVA
Example: Find the KVA or transformer size for load voltage of 240 3PH and a load current of 60 amps.
240 x 60 x 1.732 / 1,000 = KVA = 24.94 KVA (or 25 KVA after rounding up)
Computing for the required KVA is not the final step in determining the proper transformer size. Most computations (especially for three-phase loads) do not provide a whole number. As a result, the value must be rounded up, as seen in the sample above. It is best practice to always round up and not down. Next, in order to factor in future expansion and prevent risks associated with accidental overloading, one should add 20 percent of spare capacity. Taking the three-phase sample again, we simply add 20 percent to the rounded figure:
25 KVA + 5 = 30 KVA
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