Transformer Sizing Calculator
Size a distribution transformer from your connected load. Enter the total load in kW, the load power factor, a growth allowance and an optional diversity factor, and this calculator returns the required kVA and recommends the next standard transformer rating that covers it.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the total connected load P in kW.
- Enter the load power factor (default 0.9) and a spare/growth allowance in percent (default 25% for headroom and future load).
- Optionally enter a diversity factor below 1.0 if not all loads run at the same time; the tool recommends the next standard kVA rating at or above the required value.
How it works
Transformers are rated in kVA (apparent power), so the first step converts the real load to apparent power: load kVA = P / pf. The required rating then applies the diversity factor (how much of the load runs together) and the spare allowance for headroom: required kVA = load kVA × diversity × (1 + spare/100). Finally the tool picks the next transformer up from the standard series (10, 15, 25, 50, 63, 100, 160, 200, 250, 315, 400 … kVA) that meets or exceeds the required value.
Worked example
Worked example. A site has 80 kW of connected load at a power factor of 0.9, all running together (diversity 1.0), with a 25% spare allowance. Load apparent power = 80 / 0.9 = 88.89 kVA. Required = 88.89 × 1.0 × 1.25 = 111.11 kVA. The next standard rating above 111.11 kVA is 160 kVA, so a 160 kVA transformer is recommended.
Common mistakes
- Sizing in kW instead of kVA — a transformer is rated in kVA, so you must divide the kW load by the power factor first.
- Leaving out any spare capacity, so the transformer runs at its thermal limit with no room for growth or inrush.
- Applying a diversity factor of 1.0 when loads clearly do not all run at once (over-sizing), or ignoring diversity where it genuinely applies.
Frequently asked questions
Why is a transformer rated in kVA, not kW?
A transformer's limits are set by voltage (core saturation/insulation) and current (winding heating), and the product of the two is apparent power in kVA — independent of the load's power factor. A 100 kVA transformer supplies 100 kVA whether the load is 100 kW at pf 1.0 or 90 kW at pf 0.9, which is why sizing is always done in kVA.
How much spare capacity should I allow?
A common rule of thumb is around 20–25% spare so the transformer runs comfortably below its rating, tolerates inrush and leaves room for load growth. The default here is 25%. If you expect significant expansion, or heavy harmonic/non-linear loads, allow more and consider the transformer's K-factor and cooling.
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