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Transformer kVA Calculator

Work out transformer kva instantly with clear inputs, formula shown and shareable results.

About Transformer kVA Calculator

Transformer rating is in kVA, not kW, because the winding and core losses depend on current rather than useful power. Dividing the design load by power factor converts kW to kVA, then diversity and growth allowances set the design load before selecting the next standard rating up.

Formula

Transformer sizing

kVA = kW x diversity x (1 + growth) / power factor, then round up to a standard rating

Worked example

With the default values below, Transformer kVA Calculator returns Transformer rating required of 52.94. Change any input above and every figure here is recalculated in your browser.

Inputs used

Connected load
45 kW
Power factor
0.85
Future growth allowance
25 %
Diversity factor
0.8 x

Results

Transformer rating required
52.94 kVA
Design load after diversity and growth
45.00 kW
Next standard rating
75 kVA
Loading on the standard size
70.6% %

Step by step

  1. 1Values used
  2. 2Transformer sizing
  3. 3Transformer rating required= 52.94 kVA
  4. 4Design load after diversity and growth= 45.00 kW
  5. 5Next standard rating= 75 kVA
  6. 6Loading on the standard size= 70.6 %

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why is transformer rating in kVA?

Heating is caused by current in the windings and flux in the core, neither of which cares about the load's phase angle. Only apparent power captures both.

Is loading a transformer to 100% acceptable?

Continuous full load leaves no margin for ambient temperature, harmonics or growth. Most designs target 70-85% of nameplate for the expected peak.

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