How to Use the Transformer kVA Sizing Calculator
Undersizing a transformer risks overload and outages as load grows, while oversizing wastes capital on equipment you don't need. This calculator divides your actual real load (kW) by power factor to get the required apparent power (kVA), applies a safety margin, and recommends the nearest standard NEMA transformer size.
Real power (kW) and apparent power (kVA) differ because of power factor. The lower the power factor, the more apparent power a transformer must carry to deliver the same real power, which is why kVA = kW รท PF is the standard sizing formula used by electrical engineers.
Dry-type transformers are only manufactured in fixed standard NEMA sizes โ 15, 30, 45, 75, 112.5, 150 kVA and so on โ so a raw calculated value can't be ordered directly; you round up to the next standard size. Building in a safety margin (commonly around 20%) on top of that also leaves headroom for future load growth or momentary peaks.
Based on the recommended standard size, the calculator also computes the three-phase rated current (I = kVA x 1000 / (โ3 x V)) at your selected NEC voltage โ a useful reference point when sizing the feeder breaker and conductors downstream.
Frequently Asked Questions
A margin (typically around 20%) covers future load growth, momentary peak demand, and keeps the transformer from running fully loaded all the time. After adding the margin, you round up to the nearest standard kVA size.
Dry-type transformers are manufactured in standard NEMA kVA ratings (15, 30, 45, 75, 112.5, 150 kVA, and so on). If your calculated value falls between two standard sizes, round up to the next one.
This calculator gives a preliminary sizing estimate only. Actual maximum demand, planned load growth, and service configuration all need review, so have a licensed electrical engineer confirm the final design before ordering.