In short. Selection starts with data: measured voltage range and the real load including starting surges. Then choose phase type, input range, kVA, cooling and protection, and confirm behaviour on the generator. Never size from the connected load alone.
Step by step
- Measure the supply. Record voltage on each phase for a week. See monitoring.
- List the load. Running kW, power factor and starting current for motors. See load list.
- Convert to kVA. Apply diversity carefully and add growth margin.
- Choose phase type. See balanced versus independent.
- Choose range and accuracy. See wide-range.
- Choose cooling. Air or oil.
- Add protection and bypass.
- Check generator operation. See DG and stabiliser.
Sizing formula
kVA required = (kW ÷ power factor) × margin factor
Margin covers growth, ageing and the derated output at low input. For motors, check the starting kVA against the stabiliser’s short-time overload rating. See motor starting.
Selection table
| Input | Decides |
|---|---|
| Voltage min and max | Input range |
| Phase differences | Balanced or independent |
| Load kVA and inrush | Rating and overload |
| Ambient and dust | Cooling method |
| Criticality | Bypass, redundancy, monitoring |
See also the kVA guide.
Illustrative example
A workshop has 80 kW at 0.85 power factor, about 94 kVA. With motor starts and 20 percent growth the engineer picks a 125 kVA unit with wide input range and independent-phase correction for uneven single-phase loads.
A complete worked sizing example
A workshop on a 415 V three-phase supply records a minimum voltage of 330 V and a maximum of 440 V. Load list:
| Load | kW | Power factor | kVA |
|---|---|---|---|
| Machine motors | 55 | 0.85 | 64.7 |
| Compressor | 15 | 0.8 | 18.8 |
| Lighting and office | 12 | 0.9 | 13.3 |
| Total | 82 | 96.8 |
- Apply diversity only where the site data supports it. Assume all run together in the worst hour.
- Check the largest motor start: the compressor starts across the line. Compare the starting kVA with the unit’s short-time rating.
- Low input: at 330 V, check the maker’s derated output.
- Growth: add a margin for planned machines.
- Result: a unit of about 125 to 150 kVA with wide input range, subject to the maker’s curves.
All figures here are illustrative. Real selection depends on measurements.
Specification sheet checklist
- Phase configuration and regulation type.
- Input range and output tolerance.
- kVA, overload capability and duration.
- Cooling type and ambient temperature rating.
- Protection list and bypass.
- Metering and communication.
- Enclosure rating and finish.
- Applicable standards and test certificates.
Acceptance tests
- Visual and wiring inspection.
- Insulation resistance test.
- Regulation test over the input range.
- Overload test as agreed.
- Protection and bypass tests.
- Temperature rise at rated load, if practical.
Lifecycle view
Include spares, brush replacement intervals and service support in the purchase decision. A stabiliser that cannot be serviced locally is a risk for a plant that cannot stop.
The master engineering question
If the load grows and the supply sags together, does this selection still hold?
What to send us
Send load list and recordings. Send your application details.
Questions engineers ask
Can I size from the breaker rating?
No. Use measured demand.
How much growth margin?
Base on the site plan; avoid large oversizing.
Do I need a bypass?
For continuous operations, yes.
What about harmonic loads?
Check neutral and thermal rating. See harmonics.
Values are illustrative. Use measured data and maker ratings.