Enter the load either as power or as current, add a margin for growth, and pick the standard. The calculator gives the required kVA, the next standard transformer size and how heavily it will be loaded.
Recommended transformer
Load in kVA
kVA
Required with margin
kVA
Full-load current of this size
A
Loading at present load
%
How a transformer is sized
A transformer is chosen by its apparent power in kVA. Work out the kVA of the load, add a margin for future loads, and pick the next standard size:
Three-phase: S = √3 · V · I ÷ 1,000
Single-phase: S = V · I ÷ 1,000
From power: S = P ÷ PF
Required size = S × (1 + margin)
Worked example
A workshop runs 120 kW of machines at a power factor of 0.85 from a 480 V three-phase supply. The load is 120 ÷ 0.85 = 141 kVA. With 25% margin it needs 176 kVA, so the next standard ANSI size is 225 kVA. That transformer carries 271 A at full load and runs at 63% today.
Three-phase: kVA = √3 × V × I ÷ 1,000, or kVA = kW ÷ power factor. Single-phase: kVA = V × I ÷ 1,000. Add a margin of 20 to 25% for growth and pick the next standard size up.
What size transformer for 100 amps at 480 V three-phase?
1.732 × 480 × 100 ÷ 1,000 = 83 kVA. With 25% margin that is 104 kVA, so the next standard size is 112.5 kVA.
Why are transformers rated in kVA, not kW?
Transformer losses depend on current, regardless of how much of it does real work. kVA (voltage × current) therefore describes the load on the transformer better than kW, which depends on the power factor of the load.
What about motor starting and harmonics?
Large motors draw five to seven times their running current when starting direct on line, which causes a voltage dip. Loads with many drives, rectifiers or LED drivers create harmonics that heat the transformer; use a K-rated transformer or derate it. Have the final selection checked by an electrical engineer.