Transformer Size Calculator
Enter the load in kW or amps, the power factor, the secondary voltage, single or three phase and how much growth to allow. The calculator gives the minimum kVA, the next standard size and its full load current.
- kW or amps in
- Standard kVA sizes
- Full load current
Use the maximum demand you expect, not the sum of every nameplate, unless everything runs at once.
Transformers are rated in kVA, not kW. Divide the real load in kW by the power factor to get kVA, then add room for growth. A 100 kW load at power factor 0.85 with 20% growth needs about 141 kVA, so a 150 kVA transformer.
Transformer size chart
Power factor 0.85, 20% growth. Search for a load.
| Load (kW) | Needed (kVA) | Standard size (kVA) |
|---|---|---|
| 10 | 14.1 | 15 |
| 25 | 35.3 | 37.5 |
| 50 | 70.6 | 75 |
| 100 | 141.2 | 150 |
| 200 | 282.4 | 300 |
| 400 | 564.7 | 750 |
| 800 | 1129.4 | 1500 |
Standard sizes vary a little by region and maker.
Load and kVA
PF 0.85, 20% growth. Slide the load.
The gauge shows how heavily the standard size is loaded.
How to size a transformer
kW to kVA. Step through it.
Total the load
Maximum demand.
For three phase, current = kVA × 1000 ÷ (√3 × volts).
Dry type, oil filled or pad mounted
Transformer types. Compare them.
Air cooled, indoor.
Oil cooled, efficient.
Sealed outdoor cabinet.
Indoor transformers are usually dry type for fire safety.
Before ordering
Spec checks. Tick each item.
Tick each item as you go.
Motor starting and welders draw large inrush currents. Check voltage dip as well as rating.
What you get
Transformer sizing. Each tile explains one result.
Load ÷ PF.
Next up.
Secondary.
Today.
Allowed.
Running a transformer at 50% to 80% of rating is the most efficient and gives longer life.
Frequently Asked Questions
No question matches that search.
How do I size a transformer?
kVA = kW ÷ power factor, plus growth, then pick the next standard size.
What size transformer for 100 kW?
About 150 kVA at power factor 0.85 with 20% growth.
Why are transformers rated in kVA?
Their heating depends on current, whatever the power factor.
How do I convert amps to kVA?
Three phase: volts × amps × 1.732 ÷ 1000.
What is a standard transformer size?
Common sizes include 25, 50, 75, 100, 150, 300, 500, 750 and 1,000 kVA.
How much spare capacity should a transformer have?
About 20% to 25% for growth.
What is full load current?
The current at the rated kVA, which sizes the cables and breakers.
Power with room to grow
Convert your load to kVA, allow for growth and choose the next standard size. Then check the current for cables and protection.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.