Transformer Sizing Calculator
Estimate the kVA rating of a distribution transformer from your load, power factor, demand factor, growth margin, motor starting and site conditions — then jump straight to matching products.
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- Demand load
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- kVA incl. growth margin
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- Site derating factor
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- Required kVA (after derating & motor start)
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- Full-load secondary current
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- Initial loading of chosen unit
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Estimate only. Confirm with a load schedule, fault-level study and the utility's requirements before ordering. Motor-starting allowance assumes 6× starting current with a 25 % kVA contribution; adjust for soft starters or VFDs.
How the calculator works
The tool follows the same method described in our guide How to Size a Distribution Transformer. Connected load is multiplied by the demand factor to obtain the demand load, divided by the power factor to convert kW to kVA, and increased by the growth margin. If a large direct-on-line motor is entered, an allowance for its starting kVA is added. The result is divided by a derating factor for altitude (0.4 % per 100 m above 1,000 m) and ambient (1 % per °C above 40 °C), then rounded up to the next preferred IEC rating: 25, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150, 4000, 5000, 6300, 8000, 10000, 12500, 16000, 20000, 25000 or 31500 kVA.
Worked example
A factory with 850 kW connected load, power factor 0.85, demand factor 0.8 and 20 % growth margin: demand load = 680 kW; kVA = 680 / 0.85 = 800 kVA; with growth = 960 kVA; at sea level and 40 °C no derating applies, so the next standard rating is 1000 kVA, loaded initially at about 96 %. Adding a 110 kW DOL motor pushes the requirement up and the tool would recommend 1250 kVA.
Browse oil-immersed and dry-type distribution transformers, or read which type to specify.
Calculator FAQ
Should I size from connected load or demand load?
Demand load. Connected load is the sum of nameplates; the transformer only sees what runs at the same time, so multiply by a demand (diversity) factor — typically 0.6–0.9 for industrial plants — before converting to kVA.
What power factor should I assume?
Use the measured or contracted power factor. Motor-heavy plants without correction run around 0.8; with capacitor banks 0.92–0.95. A lower power factor means more kVA for the same kW.
How much growth margin is normal?
20–25 % for a facility expected to expand; 10 % where the load is fixed. Oversizing raises no-load losses for the life of the unit, so avoid margins above 30 % without reason.
Why does altitude reduce the rating?
Thinner air cools less effectively. IEC 60076 applies a derating above 1,000 m — roughly 0.4 % per 100 m for naturally cooled oil-immersed units — or the transformer is designed for the altitude from the start.