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Transformer Sizing

Demand, diversity and future reserve sizing.

Calculation inputs

Illustrative example values — replace them before any engineering decision.

Engineering results

Required capacity incl. reserve304.94 kVA
Next standard capacity315 kVA
Existing transformer loading— %
Existing capacity minus requirement— kVA

⚠ Some results are unavailable because optional or compatible inputs are missing.

Method and traceability

kVA = kW × demand × simultaneity / cosφ × (1 + reserve). The next listed rating is shown.

Visible assumptions: copper ρ₂₀ 0.0175 and aluminium 0.0282 Ω·mm²/m; copper α 0.00393 and aluminium 0.00403/°C. Reactance, density, factors and thresholds remain editable.

Screening lists — sections: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630 mm²; breakers: 2, 4, 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600 A; transformers: 25, 50, 100, 160, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150 kVA.

Final design must verify applicable IEC 60364 requirements, actual thermal conditions, short circuit, shock protection, selectivity, starting, harmonics and local rules.

IEC 60364-5-52 ↗Guide — voltage drop ↗

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Engineering reference

Transformer Sizing Calculator

Estimate coincident kVA, future reserve and the next configured transformer rating.

Calculation method

Active load is multiplied by demand and simultaneity factors, divided by power factor, then increased by the entered reserve. Existing transformer loading is assessed against coincident demand before reserve.

Formulas used

  • Required kVA = kW × demand × simultaneity / cosφ × (1 + reserve)
  • Existing loading = coincident kVA / existing kVA × 100

Worked example

Example: 300 kW × 0.8 × 0.9 / 0.85 with 20% reserve gives about 305 kVA before choosing a configured standard rating.

How to interpret the result

The recommendation is a preliminary standard-rating step. Validate load profile, motor starting, harmonics, temperature and redundancy strategy.

Common mistakes

  • Applying diversity twice.
  • Sizing only from installed nameplate power.
  • Ignoring future load and harmonic duty.

Scope and limitations

  • The calculator does not model inrush, voltage regulation, impedance, losses or fault level.

Frequently asked questions

What is the difference between demand and simultaneity?

Demand reduces individual installed loads to expected use; simultaneity represents how much of that demand occurs together.

Why size in kVA?

Transformer thermal duty follows apparent power, which includes active and reactive components.

Can an oversized transformer be inefficient?

Low loading can make fixed no-load losses more significant; interpret this with measured load and loss data.