Transformer Sizing Calculator

Your details

Choose single-phase for residential and light commercial loads; three-phase for industrial and large commercial.
Choose the values you already know. kW mode needs a power factor; kVA mode skips it; amps mode uses load current directly.
Total connected real power load in kilowatts.
kW
Ratio of real power to apparent power (0.8 to 0.95 is typical for industrial loads). Enter as a decimal, e.g. 0.85.
Fraction of the connected load that will actually run simultaneously (0.5 to 1.0). Use 1.0 if all loads run at once.
Multiply by this factor to reserve capacity for future load additions (NEC best practice recommends 1.25).
Voltage on the supply (primary) side of the transformer, e.g. 11 000 V for medium-voltage distribution.
V
Voltage delivered to the load (secondary) side. Common values: 120, 208, 240, 480, 415, 600 V.
V
Minimum kVA requiredAt minimum rating
73.53kVA

Apparent power the transformer must supply based on your load, demand factor, and growth factor

Recommended standard size75kVA
Primary full-load current3.86A
Secondary full-load current106.13A
Turns ratio (Np : Ns)27.5
Transformer typeStep-down
Power factor used0.85
Minimum kVA73.53
Standard size75

Minimum 73.5 kVA - recommend 75 kVA standard size

  • Your load requires at least 73.53 kVA. The next standard three-phase commercial rating is 75 kVA.
  • Turns ratio 27.50:1 (step-down). Primary full-load current: 3.86 A; secondary: 106.13 A.
  • The 1.25x growth factor adds future capacity headroom per NEC best practice.
  • Always verify the final selection with a licensed electrical engineer and local code requirements (NEC Article 450).

Next stepVerify winding connection (delta/wye), insulation class, temperature rise, and impedance with the transformer datasheet before installation.

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