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Transformer sizing calculator
To size a transformer, add up the load in kVA, divide by 0.8 so it runs at no more than 80% of its rating, and round up to the next standard size. A 208 V three-phase load drawing 50 A is 18 kVA, which becomes 22.5 kVA with the margin, so you buy a 30 kVA unit. The calculator does this for a list of loads in amps, kW, kVA or motor horsepower, single-phase or three-phase.
Updated October 12, 2026
How the calculator sizes it
Each load becomes kVA. Amps use the load voltage (V x A, times 1.732 for three-phase). kW is divided by its power factor. Motors in horsepower take the minimum transformer kVA from Eaton’s and Acme’s motor selection tables (to 200 hp three-phase and 10 hp single-phase); above that, hp x 0.746 / efficiency / power factor with the federal minimum efficiency and a power factor of 0.85.
Single-phase loads on a three-phase transformer are counted on every phase, so no one phase is overloaded. Acme’s worked example: a 28.2 kVA motor plus a 4 kW single-phase heater needs 28.2 + 4 + 4 + 4 = 40.2 kVA, so a 45 kVA unit.
Motor starts. If motors start more than once an hour, the minimum kVA goes up 20%, as Eaton’s and Acme’s selection guides advise.
Growth. Add the share you expect to grow. There is no standard figure; it is your call.
Loading margin. Divide by the loading limit: 0.8 for 80%. Maddox and Alfa Transformer recommend 80%.
Standard size. Round up to the next standard rating below.
Standard transformer sizes
| Phase | Standard kVA ratings |
|---|---|
| Single-phase | 0.5, 0.75, 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333 |
| Three-phase | 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1,000, 1,500, 2,000, 2,500 |
15 to 333 kVA single-phase and 15 to 1,000 kVA three-phase are the DOE ratings for low-voltage dry-type transformers (10 CFR 431.196). Smaller and larger sizes are from Acme’s and Hammond’s catalogs. Some makers also build in-between sizes.
Adjustments the simple formula misses
Motor service factor. When a motor’s service factor is above 1, increase its full-load amps in proportion: 15% for a 1.15 service factor (Eaton and Acme).
Hot rooms. Dry-type transformers are designed for a 40 C maximum ambient. Eaton says to derate the nameplate kVA by 8% for each 10 C above that, or order a unit built for the heat.
Harmonics. Drives, electronic ballasts, computers and UPS units add harmonic current that heats windings beyond what kVA shows. The K-factor panel in the calculator works it out: Eaton’s example of 90% fundamental, 30% third and 20% fifth harmonic gives K = 2.62, which calls for a K-4 transformer. K-rated units come in K-4, 9, 13, 20, 30, 40 and 50.
Short overloads are fine; long ones are not. Eaton states its dry-type units carry 200% of nameplate for 30 minutes, 150% for 1 hour and 125% for 4 hours when a 50% load comes before and after. They are not designed for a continuous overload.
Altitude. The ANSI basic loading conditions Eaton quotes assume 3,300 ft (1,000 m) or lower. Ask the maker above that.
Primary and secondary current
Once you have the size, the full-load current on each side is kVA x 1,000 / (V x 1.732) for three-phase, or kVA x 1,000 / V for single-phase. A 75 kVA three-phase transformer, 480 V to 208Y/120 V, carries 90.2 A on the 480 V side and 208 A on the 208 V side. Overcurrent protection and conductor sizing then follow the NEC® and the transformer maker’s instructions; this calculator does not size breakers or wire.
NEC® is a registered trademark of the National Fire Protection Association. Morsa is not affiliated with NFPA.
Information to gather first
Supply (primary) voltage and phase, and the load (secondary) voltage and phase.
Each load’s nameplate rating: amps, kW and power factor, kVA, or horsepower.
How often large motors start, and whether drives or other non-linear loads are a big share.
Where the transformer sits: ambient temperature, altitude, indoor or outdoor.
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