Transformer Size Calculator
Size a transformer in kVA for a given load, with headroom and diversity applied.
About Transformer Size Calculator
Connected load overstates demand because not everything runs simultaneously, so diversity is applied first. Headroom and a continuous-loading limit are then added, and the result rounded up to a standard size. Transformers loaded continuously above about 80 per cent age rapidly, so the loading figure at the chosen standard size is the number that determines service life.
Formula
Transformer Size
Required kVA = connected load × diversity ÷ power factor × (1 + headroom) ÷ max loading
- diversity factor
- Share of connected load actually running at once
- max loading
- Continuous loading limit, typically 80% for thermal life
Worked example
With the default values below, Transformer Size Calculator returns Transformer Rating Required of 344.67. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Connected loadminimum 0 kW
- 250 kW
- Average power factoraccepts 0.01–1
- 0.85
- Diversity factoraccepts 0–100 %
- 75 %
- Headroom for future loadaccepts 0–100 %
- 25 %
- Maximum continuous loadingaccepts 0–100 %
- 80 %
Results
- Transformer Rating Required
- 344.67 kVA
- Maximum Demand After Diversity
- 187.500 kW
- Maximum Demand
- 220.588 kVA
- Next Standard Size
- 400 kVA
- Loading at That Standard Size
- 55.1%
Step by step
- 1Values used
- 2Transformer Size
- 3Transformer Rating Required= 344.67 kVA
- 4Maximum Demand After Diversity= 187.500 kW
- 5Maximum Demand= 220.588 kVA
- 6Next Standard Size= 400 kVA
- 7Loading at That Standard Size= 55.1
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Transformer Size calculated?
Required kVA = connected load × diversity ÷ power factor × (1 + headroom) ÷ max loading Connected load overstates demand because not everything runs simultaneously, so diversity is applied first. Headroom and a continuous-loading limit are then added, and the result rounded up to a standard size.
Why does Transformer Size matter?
Transformers loaded continuously above about 80 per cent age rapidly, so the loading figure at the chosen standard size is the number that determines service life.
What values do I need to enter?
This calculator takes 5 inputs: Connected load, Average power factor, Diversity factor, Headroom for future load, Maximum continuous loading. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.