A transformer calculator determines the kVA rating and full-load current a transformer must have for a given supply voltage, load voltage, and load. It answers the question every specifier faces: if my load draws 100 amps at 400 V, what transformer connects it to the 11 kV supply?
kVA = (√3 × Volts × Amps) ÷ 1000
The √3 factor accounts for the three-phase
power distribution.
kVA = (Volts × Amps) ÷ 1000
For control transformers, lighting, and small
single‑phase equipment.
Amps = (kVA × 1000) ÷ (√3 × Volts)
Calculate current on either winding from the kVA
and voltage.
The turns ratio is the relationship between the primary and secondary windings: Np ÷ Ns = Vp ÷ Vs. A step‑down transformer reduces voltage and increases current; a step‑up transformer does the reverse. Generator applications commonly use step‑up transformers to feed medium‑voltage grids from a 400 V generator output.
Supports single‑phase and three‑phase systems at any voltage.
Apply a diversity factor for motor inrush and future expansion headroom.
Results are rounded up to the nearest standard transformer kVA size in your region.
Includes a preset for 400 V → 11 kV generator step‑up applications.
Enter your primary and secondary voltages, load current or kVA, and apply a starting or growth factor. The tool calculates the required kVA, standard rating, full‑load currents, turns ratio, and real power.
Results are estimates for planning. Confirm final sizing with an engineer.
Transformers are manufactured in standard kVA sizes. Always round up to the next available rating. The table below lists common single‑phase and three‑phase ratings, plus full‑load secondary currents at 400 V and 480 V.
| Rating | Single‑Phase | Three‑Phase |
|---|---|---|
| Small | 1, 1.5, 2, 3, 5 kVA | 3, 6, 9, 15, 30 kVA |
| Medium | 7.5, 10, 15, 25 kVA | 45, 75, 112.5, 150 kVA |
| Large | 37.5, 50, 75, 100 kVA | 225, 300, 500, 750 kVA |
| Extra | 167, 200, 250, 333 kVA | 1000, 1500, 2000 kVA |
| Substation | — | 2500, 3000, 4000 kVA |
These four examples show how the formulas come together in practice. Use them to check your own calculation before finalising a transformer specification.
A 60 A three‑phase load at 240 V requires kVA = (1.732 × 240 × 60) ÷ 1000 = 24.9 kVA. Round up to the next standard three‑phase size: 30 kVA. Primary current at 480 V is approximately 36 A.
A 500 kVA generator set feeds an 11 kV grid. The transformer is rated at least 500 kVA (typically 630 kVA with margin). The 400 V side carries about 722 A, while the 11 kV side carries only about 26 A.
A 120 V, 50 A single‑phase control load requires kVA = (120 × 50) ÷ 1000 = 6 kVA. A standard 7.5 kVA single‑phase transformer covers it with margin.
A 40 A three‑phase motor at 400 V has a steady‑state load of about 27.7 kVA. Applying a 1.5 starting factor gives 41.6 kVA, so a 45 kVA transformer is the safe choice rather than a 30 kVA unit.
At Shandong Huali Electromechanical Co., Ltd., we manufacture diesel generator sets from 8 kVA to 4,000 kVA and design complete power systems — including the transformer coordination a generator‑fed network requires. With over 25 years of manufacturing experience and strict pre‑delivery testing on every unit, we help you specify the transformer and the generator as one coordinated system.
A: Add up your secondary load current, convert it to kVA with the formula kVA = (√3 × Volts × Amps) ÷ 1000 for three‑phase, apply a starting or growth factor, then round up to the next standard kVA rating.
A: For three‑phase, multiply line‑to‑line voltage by current by √3 and divide by 1000. For single‑phase, multiply voltage by current and divide by 1000. If you have the load in kilowatts, divide by power factor to get kVA.
A: A transformer’s full‑load current depends on its kVA and voltage. Use Amps = kVA × 1000 ÷ (√3 × Volts) for three‑phase. A 100 kVA transformer draws about 144 A at 400 V but only about 26 A at 11 kV.
A: A step‑down transformer reduces voltage from primary to secondary (and increases current). A step‑up transformer raises voltage (and reduces current). Generator applications commonly use step‑up transformers to feed medium‑voltage grids from a 400 V generator output.
A: Use the motor’s nameplate full‑load amps, then apply a starting factor of 1.5 to 2.0 because motors draw 4–7× running current at startup. Convert the factored load to kVA and round up to the next standard size.
A: Yes. A transformer is rated in kVA, but the real power delivered is kW = kVA × power factor. If you know the load in kW, divide by power factor to get the kVA the transformer must supply. Low power factor loads need a larger kVA rating.
