Transformer kVA Calculator

How it is calculated

The required apparent power is kVA = V × A ÷ 1000 for a single-phase load and kVA = √3 × V × A ÷ 1000 for a three-phase load, where V is the line voltage in volts and A the load current in amperes. For three-phase, use the line-to-line voltage and the line current. The result is then compared with a built-in list of standard ratings: 1, 1.5, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 112.5, 150, 167, 200, 250, 333, 500, 750, 1000, 1500, 2000, 2500, 3000, 3750, 5000, 7500 and 10,000 kVA. The tool shows the smallest standard size that is equal to or above your calculated value. It adds no safety margin or growth allowance, so if the calculation lands exactly on a standard size, you get that size with no spare capacity. Voltage and current must be above zero, and a load above 10,000 kVA gives no result. Anything other than the three-phase option is treated as single-phase.

Common mistakes

• Leaving the phase option on single-phase for a three-phase load. The result is then about 42% too low, because the √3 factor (1.732) is missing.
• Entering phase voltage instead of line-to-line voltage for a three-phase system. The √3 formula expects the line-to-line voltage, for example 400 V rather than 230 V.
• Entering a power in kW as if it were kVA. The tool works from volts and amperes, so convert from kW only after dividing by the power factor.
• Treating the next standard size as a design margin. If your load computes to exactly 50.00 kVA, the next standard size is also 50 kVA; add growth allowance and inrush current yourself.
• Entering the current of a transformer's primary side with the secondary voltage, or the other way round. Voltage and current must come from the same winding.