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Transformer kVA Sizing Calculator

Enter voltage, load current, phase and headroom to size the required transformer.

How it is calculated

The load in kVA comes from the line voltage and current: single-phase kVA = V × I / 1000, three-phase kVA = √3 × V × I / 1000, with V the line-to-line voltage in volts and I the load current in amperes per line. The growth margin is added as required = kVA × (1 + margin / 100), with a default of 25 percent if left empty. The tool then picks the first rating in its own list that is at least the required value: 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 150, 167, 200, 250, 300, 500, 750, 1000, 1500, 2000 or 2500 kVA. Above 2,500 kVA the required figure is shown unchanged. Preferred sizes differ between countries and manufacturers, so check the local range. Power factor and inrush are not modelled.

Worked example

A three-phase 400 V supply feeds a load of 120 A per line, with a 25 percent margin. Connected load: 1.732 × 400 × 120 / 1000 = 83.14 kVA. With the margin: 83.14 × 1.25 = 103.92 kVA required. The next rating in the list that is not smaller than 103.92 is 150 kVA, because the list goes 100 then 150. The suggested standard size is therefore 150.0 kVA. If the same load were single-phase 240 V at 120 A, 28.80 kVA would become 36.00 required and 37.5 kVA.

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Common mistakes

  • Choosing single phase for a three-phase load. The √3 factor is applied unless phase is 1, so entering a three-phase current as single phase underestimates the kVA by about 42 percent.
  • Entering phase voltage (for example 230 V on a 400 V system) in the three-phase formula. The tool expects the line-to-line voltage.
  • Using the nameplate current of one motor as the total load. Add up the loads that run together; the tool does not apply diversity or a demand factor.
  • Setting the margin to zero because the load is known today. Without any margin even a small addition forces a new transformer, and starting currents are not modelled.
  • Entering the margin as a fraction such as 0.25. The field is in percent, so enter 25; 0.25 would add only a quarter of a percent.

About this calculator

A transformer's required rating in kVA is calculated from the load it must serve: for single-phase, kVA = (V × I) ÷ 1000, and for three-phase, kVA = (V × I × √3) ÷ 1000, where V is line voltage and I is load current. Adding a headroom margin (commonly 15–25%) for future load growth or inrush before rounding up gives a practical design figure rather than a bare-minimum one.

This calculator takes voltage, current, phase, and your chosen headroom, computes the required kVA, and rounds up to the next standard commercial transformer rating (such as 15, 25, 37.5, 50, 75, or 100 kVA) since transformers aren't custom-built to arbitrary sizes. Electrical engineers and electricians use it during panel or service upgrades to specify a transformer that won't be undersized for the connected load, or oversized enough to waste money and space.

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