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Transformer size calculator

Select a transformer by load: power (kW), cos φ, efficiency and secondary voltage — required rating, standard size and current.

Formulas

Required rating: S = P/(cosφ·η), where P is the active load, cos φ the power factor, η the transformer efficiency (0.95–0.98). Secondary current: I = S/(√3·U) for 3-phase.

Standard rating series

  • 1.6 / 2.5 / 4 / 6.3 / 10 / 16 / 25 / 40 / 63 / 100 / 160 / 250 / 400 / 630 / 1000 kVA
  • Choose the next standard rating UP from the calculated value
  • Recommended loading 60–80% — reserve for load growth

Example

Load 20 kW, cos φ = 0.9, η = 0.95: S = 20/(0.9×0.95) = 23.4 kVA → 25 kVA transformer, secondary current at 400 V: 25000/(1.73×400) = 36 A.

  • kVA to kW — kva-kw
  • Load calculation — load-calculator
  • Power factor correction — power-factor

FAQ

What transformer for a 50 kW load?

At cos φ = 0.9 and η = 0.95: S = 50/0.855 ≈ 58.5 kVA → 63 kVA transformer.

Why are transformers rated in kVA?

Because the rating is limited by winding current: I = S/(√3·U). Active power depends on the load's cos φ.

What is the secondary current at 400 V?

I = S×1000/(√3×400). For 100 kVA: 100000/692 ≈ 144 A.

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