Transformer Sizing Calculator

The formula multiplies voltage, current, and (for three-phase) √3 by the Demand Factor before dividing by power factor — the demand factor represents the ratio of actual maximum simultaneous demand to total connected load, typically 0.70–0.90 for commercial loads, so it scales the raw V×I product down to reflect that not every connected load draws its full current at the same moment.

The calculator picks the smallest transformer from a fixed industry list of standard nameplate ratings (15 KVA up through 5,000 KVA) that is still greater than or equal to your Required KVA, since transformers aren't custom-wound to arbitrary sizes. If your requirement falls just above a standard size, the next available size can be a noticeably larger jump — for example, just over 100 KVA rounds up to 112.5 KVA.

Percent Loading compares your actual demand current against the full load current the selected standard-size transformer can supply, but the displayed value is capped at 100% — so if your calculated demand actually exceeds the transformer's rated capacity, the calculator won't show a number above 100%; check that Required KVA doesn't exceed the Next Standard Size shown, since that would mean the transformer is undersized regardless of what Percent Loading displays.

Power factor is internally floored at 0.1 to prevent divide-by-zero errors in the KVA formula, but a power factor that low is unrealistic for any real facility — resistive loads run near 1.0, motors typically 0.80–0.90, and mixed commercial loads 0.85–0.90. If you get an unexpectedly large Required KVA, double-check that your power factor input reflects real measured data rather than an extreme placeholder value.