Transformer Size Calculator
Size a transformer for a connected electrical load, in kVA, with a standard safety headroom margin.
How this calculator works
A transformer has to carry more than just the load you can measure today. This calculator takes your connected load in watts, applies a headroom factor above it, and converts the result into kVA — the standard unit transformers are rated in.
The headroom factor is the important part. Instead of matching the transformer rating exactly to today’s load, it pads the number up so the transformer isn’t run at its absolute limit around the clock.
Formula: Minimum transformer size (kVA) = connected load in watts × headroom factor ÷ 1,000.
Worked example
A 24,000 W connected load with a 1.25 headroom factor:
- 24,000 × 1.25 = 30,000
- 30,000 ÷ 1,000 = 30 kVA
Notes
Sizing with headroom — commonly 20-25% above the actual connected load — leaves room for load growth and avoids running a transformer at its absolute limit continuously. The actual load figure you start from should also account for demand factor, since not everything in a building or facility runs at once. Getting that starting load number right matters as much as the headroom multiplier itself.
How to use
Enter your connected load in watts and choose a headroom factor (1.25 is a common default for 25% headroom). The calculator multiplies the two and converts to kVA, giving you a minimum transformer size to work from. Treat the result as a starting point for discussion with an electrician, not a final spec.
Electrical work is code-governed and safety-critical. Confirm circuit, conductor, and equipment sizing with a licensed electrician and your local code.
Frequently asked questions
How do I convert a connected load in watts to a transformer size in kVA?
Multiply the connected load in watts by a headroom factor, then divide by 1,000 to convert to kVA. A 24,000 watt load with a 1.25 headroom factor works out to 24,000 times 1.25, divided by 1,000, which is 30 kVA. The headroom factor is what turns a bare load number into a safe minimum transformer rating.
Why not just size the transformer to match the load exactly?
Running a transformer at exactly its rated load, continuously, leaves no room for load growth, inrush current, or measurement error. Building in headroom, commonly 20-25% above the connected load, keeps the transformer operating below its limit instead of at the edge of it, which is safer and extends its service life.
What is demand factor and does it affect transformer sizing?
Demand factor accounts for the fact that not every connected device runs at full load at the same time. The connected load figure used in this calculator is your starting point; if your actual load study applies a demand factor to reduce that number, use the reduced figure as the input instead of the raw connected load.
Is a kVA result the same as sizing in amps?
No. kVA describes the transformer's total apparent power capacity, while amps describe current draw on a specific circuit at a specific voltage. Once you have a kVA figure from this calculator, you can convert it to amps at your system voltage using a kVA-to-amps calculator if you need current values for downstream equipment.
Estimates only. Verify quantities with your supplier before purchasing.