Generator Load and Motor Start Estimator
Compare running and starting kW, kVA, and line current using equipment data.
Operating details
Use line-to-line voltage for balanced three-phase loads. Single-phase current is calculated across the entered voltage.
This simplified model assumes lagging or unity power factor.
Enter electrical input power, not mechanical shaft output.
Use total motor starting kW. The calculation replaces the running motor load in the starting state.
Use manufacturer starting power factor or derive it from manufacturer kW and kVA data.
Generator ratings
Starting capability depends on the specific generator and conditions. Continuous ratings are not substituted.
Running and starting demand
Motor-start state
Steady running state
Running demand is within the entered kW and kVA values. Transient performance remains unverified.
Equipment and installation checklist
0 of 8 review prompts checked. This count is not an approval or readiness determination.
About this estimator
Enter other running loads, the motor's running and starting kW, each power factor, the supply voltage, and the generator's continuous ratings. The tool estimates running and motor-start kW, kVA, and line current. A separate transfer-system checklist records model, drawing, and commissioning references for review.
Motor-start capability is checked only when manufacturer starting kW and kVA data is entered. Continuous ratings are not substituted for missing starting data.
Method and assumptions
For each load group, reactive power Q = P × √(1 − PF²) ÷ PF. The calculator adds real and reactive power for other loads and the largest motor, then derives apparent power S = √(P² + Q²).
During startup, the calculator removes that motor's running kW and kvar and uses its total starting kW and starting power factor instead. Single-phase current = kVA × 1,000 ÷ voltage. Balanced three-phase current = kVA × 1,000 ÷ (√3 × line voltage).
The estimate assumes lagging or unity power factors, one motor starting at a time, and all other entered loads remaining constant. It does not model transient voltage recovery, alternator response curves, harmonics, or nonlinear loads.
Interpret the comparison
Compare steady demand separately with generator continuous kW and kVA ratings. Compare motor-start demand with manufacturer starting capability for both kW and kVA. If starting capability data is missing, the motor-start comparison remains unchecked.
A comparison within ratings is not a complete generator sizing result. Confirm engine power, alternator transient response, voltage and frequency limits, fuel, site conditions, load sequence, transfer equipment, and driven-equipment requirements with the manufacturers. The checklist records review topics only; checked items do not approve the installation.
FAQs
Why does the calculator ask for both kW and kVA?
The generator engine is limited by real power in kW, and the alternator is limited by apparent power in kVA. A load can be below one rating and exceed the other.
What starting data should I enter for the motor?
Enter total starting kW and its starting power factor from motor or manufacturer data. Starting current and power vary with the motor, driven load, voltage, and starting method; do not infer them from horsepower alone.
Why is generator starting capability optional?
Momentary starting performance depends on the specific generator, pre-load, start duration, environment, and allowable voltage and frequency dip. If manufacturer data is unavailable, the calculator shows demand but does not claim the motor-start check passes.
Does this include two motors starting together?
No. The estimate replaces the largest motor running load with its starting load while other entered loads continue running. Simultaneous starts and staged load sequences need their own equipment data.
Sources
- Cummins, Liquid-Cooled Generator Set Application Manual T-030, Issue 9 (2015) — discusses engine kW and alternator kVA limits, motor starts, and transient response (§3.3.1–3.3.2).
- Use current generator and transfer-equipment nameplates, manufacturer instructions, approved one-line diagrams, and commissioning procedures for the actual installation.