Generator Sizing: Running kVA vs Motor Starting

A generator that can carry the steady-state load may still be too small to start the largest motor. Preliminary sizing should compare at least two cases: continuous/running kVA with reserve and a starting case that replaces the selected motor's running demand with its starting kVA.

1. Running-load kVA

The reserve margin is a project choice. It can cover uncertainty or planned growth, but it should not be used to hide an unmodelled starting or step-load requirement.

2. First-pass motor-starting case

For a three-phase motor, estimate running current from shaft output, efficiency and power factor:

A simple screening case keeps the other running loads online and replaces the selected motor's running kVA with its starting kVA.

3. Worked example

Assume 100 kW total running load at PF 0.85, 20% reserve, 400 V supply and a largest motor rated 30 kW output, η = 0.92, PF = 0.85, with a 6× starting-current multiplier.

The other running load is 117.65 − 38.36 = 79.28 kVA, so:

The starting case governs over the 141.18 kVA continuous case. In the ElectroDesigner screening list, the next nominal step is 400 kVA — but this is not a final genset selection.

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4. Why manufacturer transient performance still governs

Motor starting imposes both kVA and kW transients. Alternator reactance, excitation response, engine step-load capability and the motor/load torque curve determine voltage and frequency dip and recovery. Reduced-voltage or electronic starting can reduce electrical demand, but it also changes available motor torque and must be checked as a system.

Manufacturer sizing software or transient data should therefore verify the selected set against the project's allowable voltage dip, frequency dip, recovery time, load sequence, ambient conditions and nonlinear loads.

5. Assumptions and limits

• The worked example assumes one governing motor start and all other modeled load remains at running kVA.
• The starting-current multiplier is an input, not a universal value; use motor/starter data.
• Simultaneous or sequenced motor starts need an explicit sequence model.
• UPS, rectifier and VFD loads may require harmonic and alternator-derating review.
• Altitude, temperature, fuel, emissions rating and duty classification can affect available generator output.
• Final selection must use manufacturer transient-performance data.

Adjacent checks

Start from a defensible Load Schedule and document which loads are actually present in generator mode. Where nonlinear loads dominate, use the Harmonic Filter Sizing workflow as a screening aid and confirm genset compatibility with the equipment manufacturers.

Engineering references

Cummins Application Manual — Generator Set Sizing, electrical load impact and Cummins — Understanding Transient Performance of Generating Sets. The references emphasize motor-start voltage dip, recovery and actual genset transient capability rather than running kW alone.