Transformer Sizing
Transformer kVA for a load given in kVA, kW with power factor, or secondary amps; growth allowance; next preferred rating; full-load current on both sides.
The kVA the load needs, the next preferred rating from a labelled list with the loading it gives, and the full-load primary and secondary currents at that rating.
Example: A 150 kW load at PF 0.85 on 480 → 208 V three-phase is 176 kVA; with 20 % growth the next ANSI rating is 225 kVA — 271 A primary, 625 A secondary at full load.
Rated in kVA,
loaded in amps.
Why the load must become kVA first, how the preferred rating is picked, what the full-load currents mean, and what a nameplate carries that this page does not.
From load to kVA
A transformer carries current whatever the power factor, so it is rated in apparent power. A load in kW becomes kW ÷ PF; a load in secondary amps becomes √3 × V × I (three-phase, line-to-line) or V × I (single-phase). A growth allowance you set (0 = none) is added to give the requirement, and the next preferred rating at or above it is taken from the list you choose. Loading is the present load as a percent of that rating.
Full-load currents
At the chosen rating S, full-load current is S ÷ (√3 × V) on each side for three-phase or S ÷ V for single-phase; primary and secondary VA are equal, so the currents are in inverse ratio to the voltages (the turns ratio). These are the figures for protection and conductor selection at the transformer's rating; the currents drawn by the present load are shown separately.
Preferred ratings
The lists — ANSI three-phase (15 … 5,000 kVA), ANSI single-phase (5 … 500 kVA) and the IEC R10 series (25 … 3,150 kVA) — are the sizes catalogues carry, as commonly published; the standards that define them are licensed and were not consulted. When the requirement exceeds a list the page says so and gives the currents for the requirement itself. A bank of three single-phase units carries one third of the kVA each.
What the number does not settle
Temperature rise and ambient, altitude derating, harmonic loads (K-factor), inrush, impedance and fault duty, voltage taps and vector group are the nameplate's and the code's. Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- Preferred distribution-transformer kVA ratings (ANSI three- and single-phase, IEC R10 series), as commonly published — IEEE C57.12.00 and IEC 60076 are licensed and were not consulted
Last reviewed 18 September 2026. How results are checked: How we verify.