Quick answer: Transformer kVA = (total connected load × demand factor) ÷ power factor, then round up to the next standard IS 2026 rating (e.g. 100, 160, 250, 315, 500, 630, 1000 kVA). Add spare capacity (typically 20–25%) for future growth.
How to size a transformer
The transformer must supply the maximum demand of all connected loads, not their arithmetic sum — because loads rarely peak together. That is captured by the diversity/demand factor.
Maximum demand (kW) = Connected load × Demand factor
Required kVA = Maximum demand / Power factor
Selected kVA = next standard rating ≥ required kVA × (1 + spare%)Steps
- List all loads and total the connected kW.
- Apply a demand factor (0.6–0.8 typical for mixed loads).
- Divide by power factor (0.85–0.9) to get kVA.
- Add spare margin and pick the next standard IS 2026 rating.
Worked example
Connected load 700 kW, demand factor 0.7, PF 0.9: demand = 490 kW → 544 kVA. With 20% spare → 653 kVA → select a standard 630 kVA (or 800 kVA if growth is expected).
Enter your loads and get the standard IS 2026 kVA rating with diversity applied. Free tool.
Open the Transformer Sizing Calculator →Frequently Asked Questions
How do you calculate transformer kVA?
Multiply connected load by the demand factor to get maximum demand in kW, divide by power factor to get kVA, then round up to the next standard IS 2026 rating with a spare margin.
What is a good spare capacity for a transformer?
A spare capacity of 20-25% is common so the transformer is not continuously loaded near its rating and can accommodate future load growth.
Why divide by power factor?
Transformers are rated in kVA (apparent power). Real load in kW is converted to kVA by dividing by the power factor, since kVA = kW / pf.