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Transformer Sizing Calculator

Calculate transformer kVA and currents

kW
V
V
Result
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Formulas

kVAkVA = kW / PF
I (1φ)I = kVA×1000 / V
I (3φ)I = kVA×1000 / (√3×V)

Standard Sizes (kVA)

Single-Phase: 5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500

Three-Phase: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500

Related

kVA to AmpskW to kVAMotor Current

How to Size a Transformer

Proper transformer sizing is critical for safe and efficient operation. An undersized transformer will overheat and fail prematurely, while an oversized transformer wastes money and operates inefficiently at low loads.

Step-by-Step Sizing Process

1. Calculate total load: Add up all connected loads in kW. For motors, use nameplate HP × 0.746 for kW.

2. Apply diversity factor: If not all loads run simultaneously, apply a diversity factor (typically 0.7-0.9).

3. Convert to kVA: Divide kW by power factor. Use 0.8 for mixed loads, 0.85 for mostly motor loads.

4. Add safety margin: Add 20-25% for future growth and inrush currents.

5. Select standard size: Choose the next standard kVA rating above your calculated requirement.

Important Considerations

Motor starting: Motors draw 5-7× full load current when starting. If many motors start simultaneously, size transformer accordingly.

Harmonics: Non-linear loads (VFDs, computers, LED lighting) create harmonics that increase transformer heating. Derate by 10-15% for harmonic-rich environments.

Future expansion: Plan for 20-25% load growth to avoid premature replacement.

Frequently Asked Questions

For single-phase: kVA = (Volts × Amps) / 1000. For three-phase: kVA = (Volts × Amps × 1.732) / 1000. If you know kW and power factor: kVA = kW / PF. Always add 20-25% safety margin and select the next standard size up.

kW (kilowatts) is real power that does actual work. kVA (kilovolt-amperes) is apparent power that includes both real power and reactive power. They're related by: kW = kVA × Power Factor. Transformers are rated in kVA because they must handle both components regardless of the load's power factor.

The 25% margin accounts for: motor starting currents (5-7× normal), future load growth, harmonic heating from non-linear loads, temperature derating, and load measurement uncertainties. Operating continuously above 80% rated capacity shortens transformer life significantly.

For single-phase: I = (kVA × 1000) / Secondary Voltage. For three-phase: I = (kVA × 1000) / (√3 × Secondary Voltage). Example: 100 kVA 3-phase at 415V secondary = 100,000 / (1.732 × 415) = 139.1 Amps per phase.

Common 3-phase sizes: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500 kVA. Common 1-phase sizes: 5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500 kVA. Always select the next size up from your calculated requirement.

Turns ratio = Primary Voltage / Secondary Voltage. It determines the voltage transformation and is inversely proportional to the current ratio. Example: 11kV/415V transformer has a turns ratio of 26.5:1, meaning secondary current is 26.5× primary current for the same power.

Yes, but with limits. Most transformers can handle 10-15% overload for 2 hours, or 25-30% overload for 30 minutes, depending on ambient temperature and prior loading. Repeated overloading accelerates insulation aging and shortens transformer life. Check manufacturer specifications for exact limits.

Lower power factor requires larger transformer for the same kW load. A 100 kW load at PF 0.8 requires 125 kVA transformer (100/0.8), while the same load at PF 0.95 requires only 105 kVA (100/0.95). Improving power factor with capacitors can allow smaller transformer sizing.