In short. Convert every load to kVA, combine them with realistic diversity, then add margins for starting surges, growth and low-input derating. Pick the next standard rating above the result. The connected load is a starting point, not the answer.
Conversion formulas
Single phase: kVA = V × I ÷ 1000
Three phase: kVA = √3 × V × I ÷ 1000
From kW: kVA = kW ÷ power factor
Method
- Take nameplate kW or current for each load.
- Convert to kVA with the realistic power factor.
- Add loads that run together; reduce others by diversity.
- Check the largest motor start separately. See motor starting.
- Apply a growth margin from the site plan.
- Check low-input derating. See wide-range stabilisers.
Worked example (illustrative)
| Load | kW | pf | kVA |
|---|---|---|---|
| Motors | 45 | 0.85 | 52.9 |
| Lighting and small power | 10 | 0.9 | 11.1 |
| Air conditioning | 20 | 0.8 | 25.0 |
| Total | 89.0 |
With 20 percent growth: about 107 kVA. The next standard size above is chosen, subject to the maker’s rating at the lowest input voltage and the largest motor start.
Common mistakes
- Adding kW instead of kVA.
- Ignoring the starting current of compressors and pumps.
- Using breaker size as demand.
- Forgetting the derated output at low input voltage.
- Oversizing heavily; light load can reduce efficiency and accuracy.
See the selection and sizing method.
Handling motor starting in kVA terms
Starting current depends on the starting method. A direct-on-line motor can draw several times its full-load current; a star-delta or soft starter reduces it; a variable-speed drive limits it to about full load.
| Starting method | Typical effect on stabiliser sizing |
|---|---|
| Direct on line | Largest surge; check short-time overload rating |
| Star-delta | Reduced surge, with a possible transition spike |
| Soft starter | Controlled; sizing close to running |
| Drive | Running current plus harmonics |
A useful check: starting kVA ≈ running kVA of the motor × the ratio of starting to full-load current. Compare this plus other running loads with the unit’s overload capability for the duration of the start. See motor starting.
Second worked example: single-phase workshop
| Load | Voltage | Current | kVA |
|---|---|---|---|
| Welding transformer (intermittent) | 230 V | 35 A | 8.1 |
| Air compressor | 230 V | 14 A | 3.2 |
| Lighting and fans | 230 V | 10 A | 2.3 |
Total running is about 13.6 kVA. The welder is intermittent, so the unit must handle its peak. Include growth, then select the next standard size, with the maker’s low-input rating checked.
Efficiency and heat
A stabiliser’s losses become heat in the room. A larger unit than needed has slightly higher no-load loss and takes more space, so avoid gross oversizing while keeping real margins for starts and growth.
Verification after installation
- Record output current at the busiest time.
- Calculate the loading as a percentage of rating.
- If it exceeds the comfortable range, plan for load transfer or an upgrade.
Common reference values
- kVA = kW ÷ power factor.
- Current (three phase) = kVA × 1000 ÷ (√3 × V).
- Always use measured demand rather than connected load where possible.
The master engineering question
At the worst combination of low supply voltage, full load and a large motor start, is the stabiliser still inside its rating?
What to send us
Send nameplates and measurements. Send your application details.
Questions engineers ask
Is kVA the same as kW?
No. kW = kVA × power factor.
How do I include motor starts?
Check starting kVA against the short-time overload rating.
Should I add 25 percent always?
No. Use data and the site plan.
What if my loads are nonlinear?
Check thermal rating and neutral current with harmonic data.
Values are illustrative. Use nameplate and maker data.