Transformer Sizing Calculator

Choosing a transformer is choosing capacity you cannot easily change later — too small and it overheats under load, too large and you pay for idle copper. The honest number starts from real demand, not nameplate totals.

Transformer Sizing Calculator converts connected load through the demand factor and power factor to a required kVA, adds a growth allowance and starting margin, then returns the next standard size with primary and secondary currents.

It is a preliminary sizing aid. Final selection follows the supply authority and manufacturer requirements.

• Connected load in: Enter total kW of load to serve.
• Demand factor: Adjust for loads that do not run together.
• Power factor: Convert real load into kVA honestly.
• Growth allowance: Reserve room for future load.
• Starting margin: Optional motor-start headroom.
• Primary and secondary current: For both windings at chosen voltages.
• Next standard size: The recommended rating out of common frames.
• Export: Copy or download the sizing as text, CSV or JSON.

1. Enter the connected load in kW.
2. Set the demand factor for simultaneous use.
3. Enter the power factor of the load mix.
4. Set the growth allowance in percent.
5. Enter primary and secondary voltages (or accept defaults).
6. Read the required kVA, the next standard size and both currents.
7. Export the sizing if needed.

Example 1 — General panel. 80 kW connected at 0.6 demand and 0.85 pf with 15% growth: about 65 kVA, next size 75 kVA.

Example 2 — Motor load. 150 kW of pumps at 0.7 demand, 0.85 pf with motor-start margin: near 124 kVA, size 150 kVA.

Example 3 — Currents. A 150 kVA three-phase 11 kV / 433 V unit draws 7.9 A primary and 200 A secondary at full load.

Example 4 — Small commercial. 30 kW office at 0.8 demand, 0.9 pf, 10% growth: about 29 kVA, next size 50 kVA.

• Right capacity: Demand and pf prevent both over and under sizing.
• Growth built in: Future load factored before you buy.
• Both sides shown: Primary and secondary current for the feed and consumer gear.
• Standard sizes: Straight to the size you can order.
• Free and private: No uploads, no accounts, no logging.