The calculation, without hidden assumptions
Transformer apparent-power relationships depend on phase and voltage basis. For single phase, kVA = V×I/1000; for a balanced three-phase system using line-to-line voltage, kVA = √3×V×I/1000. MAXScanner supports both directions: calculate apparent load from one voltage/current operating point, or start from a transformer kVA rating and calculate ideal full-load current on both primary and secondary sides.
How to use this calculator
Choose single phase or balanced three phase, then choose whether to calculate kVA or currents from a rating.
For kVA from V/I, enter one RMS voltage/current operating point. For rating-to-currents, enter kVA plus primary and secondary voltages.
For three phase, use line-to-line voltage consistently and treat the result as arithmetic/nameplate context rather than protection or conductor sizing.
Where people use it
- •Transformer nameplate current checks
- •Converting a measured balanced load into kVA
- •Comparing primary and secondary full-load current
Example: 75 kVA, 480 V to 208 V, three phase
Primary current is about 90.2 A and ideal secondary current is about 208.2 A using I = 75,000/(√3×V).
What the result does not assume
- •The balanced three-phase formula assumes RMS line current and line-to-line voltage. Unbalance, harmonics, taps, impedance, excitation current and regulation are outside this simple relationship.
- •This page does not select transformer protection, conductors or a transformer rating margin. Those choices depend on the actual installation, applicable rules and manufacturer data.
Frequently asked questions
Why are transformers rated in kVA rather than kW?+
Transformer winding current and voltage determine apparent-power loading; the connected load power factor determines how much of that apparent power becomes real kW.
Which voltage should I use for three phase?+
Use the applicable RMS line-to-line voltage with line current for the √3 relationship used here.
Why is secondary current higher on a step-down transformer?+
For roughly conserved apparent power, reducing voltage requires more current on the lower-voltage side.
Semantic next steps
Continue the calculation
These links move to a different input, formula or project stage rather than a keyword variation of this page.
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