Calculator Inputs
Example Data Table
| Load group | Units | kW each | Subtotal kW | Start multiplier |
|---|---|---|---|---|
| Water pumps | 2 | 4.50 | 9.00 | 3.00 |
| Air compressors | 1 | 9.00 | 9.00 | 3.00 |
| Lighting circuits | 1 | 4.00 | 4.00 | 1.00 |
| Heating load | 1 | 12.00 | 12.00 | 1.00 |
| Control electronics | 1 | 6.00 | 6.00 | 1.00 |
Formula Used
Connected load: Resistive kW + Motor kW + Lighting kW + Electronics kW
Running load: Connected load × Demand factor
Running kVA: Running kW ÷ (Power factor × Efficiency)
Largest motor running kVA: Largest motor kW ÷ (Power factor × Efficiency)
Largest motor starting kVA: Largest motor running kVA × Starting multiplier
Starting total kVA: Running kVA − Largest motor running kVA + Largest motor starting kVA
Reserve adjusted kVA: Base required kVA × (1 + Reserve margin)
Available derating factor: (1 − Altitude derate) × (1 − Temperature derate)
Recommended generator kVA: Reserve adjusted kVA ÷ Available derating factor
Line current: (kVA × 1000) ÷ (√3 × Line voltage)
Recommended generator kW: Generator kVA × Generator power factor
How to Use This Calculator
- Enter each connected load in kilowatts.
- Enter the largest motor that creates the highest startup surge.
- Provide line voltage, power factor, and efficiency.
- Set demand factor for expected simultaneous operation.
- Add reserve margin for expansion and uncertainty.
- Enter derating percentages for site altitude and heat.
- Click the calculate button to view the result above the form.
- Download the final result as CSV or PDF if needed.
Three Phase Generator Sizing Guide
Why Accurate Generator Sizing Matters
A three phase generator must match real operating demand. Small units struggle during motor starts. Oversized units waste fuel and capital. Good sizing improves voltage stability, runtime, and equipment life. This calculator helps estimate generator capacity from connected load, current, power factor, efficiency, and surge demand. It supports balanced systems, mixed loads, and reserve planning.
Key Inputs That Affect Capacity
Load mix is the first factor. Resistive loads behave differently from motors and compressors. Motors create starting current that can be several times higher than running current. Line voltage also changes required current. Power factor affects apparent power. Efficiency affects input demand. Demand factor reduces connected load to expected simultaneous load. Reserve margin adds safety for future expansion and unexpected peaks.
How the Calculator Interprets the Numbers
The tool first combines resistive, motor, lighting, and electronic loads. It then applies the demand factor to find expected running kilowatts. Apparent power is derived from running kilowatts, power factor, and efficiency. Starting demand is estimated from the largest motor and chosen surge multiplier. The calculator then adds either the running requirement or the higher starting requirement. After that, reserve and derating adjustments are applied. The final answer shows recommended generator size in kVA, kW, and line current.
Better Planning for Real Installations
Use the result as a planning estimate for buildings, workshops, farms, and backup systems. Check site temperature, altitude, and fuel type before final selection. Review manufacturer data for transient response and motor starting capability. A healthy margin often prevents nuisance trips and voltage dips. When loads are sensitive, compare several generator ratings and select the next practical size above the estimate. This method gives a stronger starting point for procurement, budgeting, and technical review.
Useful Outputs for Decisions
The output table summarizes running kilowatts, starting kilovolt amperes, adjusted current, and the recommended generator rating. These values help compare standby models, alternator frames, and transfer switch limits. They also support discussion with electricians, facility managers, and vendors. Because every installation differs, confirm conductor sizing, breaker coordination, harmonic content, and local code requirements before purchase.
Document assumptions so maintenance teams understand future capacity limits and operating intent.
FAQs
1. Why does the calculator use kVA and kW?
Generators are commonly rated in kVA, while loads are often listed in kW. Both are needed because power factor changes the apparent power the generator must support during normal running conditions.
2. Why is the largest motor entered separately?
The largest motor usually creates the highest startup surge. Entering it separately helps estimate the temporary demand spike that can force a larger generator selection.
3. What demand factor should I use?
Use a lower percentage when all loads do not run together. Use a higher percentage when most equipment operates at the same time. Site measurements give the best estimate.
4. What does reserve margin do?
Reserve margin adds planning space above calculated demand. It helps cover future expansion, load uncertainty, and brief operational peaks that may not appear in a simple connected load total.
5. Why do altitude and temperature reduce capacity?
Hot air and high elevation reduce cooling and combustion performance. Many generator installations lose usable capacity in these conditions, so derating prevents undersized selections.
6. Is the suggested standard size always the final answer?
No. It is a practical planning output. Final selection should still be checked against manufacturer data, motor starting curves, site code requirements, and transient performance needs.
7. Can this calculator be used for standby and prime applications?
Yes, as an estimate. Prime duty often needs more careful review because the generator may operate longer and under wider load variation than standby service.
8. Does this work for perfectly unbalanced systems?
It is best for typical three phase planning. Strongly unbalanced systems may need deeper phase-by-phase analysis, because one line can become overloaded before total kVA looks critical.