Distribution Transformer Sizing Calculator
Choose the kVA rating for a site or building transformer.
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Distribution Transformer Sizing Calculator by TheCalcTools
How the Distribution Transformer Sizing Calculator works
Enter the connected load, how much of it runs at once (demand factor) and the power factor.
Add a growth allowance for future expansion.
Divide by the target loading (commonly 70–80%) and pick the next standard rating.
The formula
Worked example
Using the default values shown in the calculator:
| Connected load (kW) | 150 |
|---|---|
| Demand factor | 0.8 |
| Power factor | 0.9 |
| Future growth (%) | 20 % |
| Maximum loading target (%) | 80 % |
- kVA = 150 × 0.8 ÷ 0.9 × 1.2 = 160.0.
- At 80% target loading you need 200.0 kVA, so choose 200 kVA (loading 80.0%).
Result:
| Required load (kVA) | 160.0 |
|---|---|
| Transformer needed at target loading (kVA) | 200.0 |
| Standard rating (kVA) | 200 |
| Loading at standard rating | 80.0% |
Frequently asked questions
What loading is best?
Transformers are most efficient at about 40–70% load and can run higher temporarily, but sizing for 70–80% leaves headroom and extends life.
Who approves the connection?
In Pakistan the distribution company (DISCO) and utility rules decide transformer capacity, sanctioned load and metering. Check their requirements.
Sources and further reading
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This calculator gives estimates for planning only. Electrical installations must be designed, checked and carried out by qualified personnel following local codes and the equipment manufacturer's data. Never rely on this result alone for safety-critical decisions. See our disclaimer.