Transformer Size Calculator (kVA Rating from Load)
By Santaji GadepatilinUpdated October 28, 2026
Choosing a transformer means turning the connected load into a kVA figure, allowing for diversity, power factor and future growth. Too small and it overheats; too large and it wastes money and no-load losses. This transformer size calculator follows the usual method — see how to calculate transformer kVA rating.
How to use the transformer size calculator
Enter the total connected load in kW, the demand (diversity) factor, the power factor you expect after correction and a growth margin. The calculator gives maximum demand and the required kVA. Choose the next standard rating above the result.
Transformer sizing formula
kVA = (connected kW × demand factor) ÷ PF × (1 + growth margin)
Example
500 kW connected, 0.7 demand factor, 0.9 PF, 25% growth:
- Maximum demand = 350 kW
- kVA = 350 ÷ 0.9 × 1.25 = 486 kVA → choose 500 kVA
Standard distribution transformer ratings
16, 25, 63, 100, 160, 200, 250, 315, 400, 500, 630, 800, 1,000, 1,250, 1,600, 2,000 and 2,500 kVA (IS 1180 / IEC 60076).
Typical demand factors
| Installation | Demand factor |
|---|---|
| Continuous process plant | 0.8–0.9 |
| General manufacturing | 0.6–0.8 |
| Commercial buildings | 0.5–0.7 |
| Residential complexes | 0.4–0.6 |
After sizing
- Check full-load current and protection with the transformer current calculator.
- Check fault level with the transformer short-circuit calculator.
- Plan capacitor banks with the power factor correction calculator — a better PF reduces the kVA you need.
Frequently asked questions
How do I calculate transformer size?
Multiply connected load by the diversity factor to get maximum demand, divide by power factor for kVA, add a growth margin, then choose the next standard rating.
What diversity factor should I use?
It depends on how loads operate together. Continuous process plants may be 0.8–0.9; workshops and buildings 0.5–0.7.
Why not size exactly to the load?
Transformers last longer and are most efficient at 50–80% load, and margins allow for growth and motor starting.
Should harmonics be considered?
Yes. With many VFDs, UPSs or rectifiers, use a K-rated transformer or derate a standard one.