Diesel Generator Sizing for Pump and Motor Loads

By LANDTOP Technical Team

A diesel generator serving a pump or motor must satisfy continuous kW and kVA requirements while also supplying enough transient current and engine power during acceleration. Select the set from verified motor, pump, starter and alternator data—not from motor kW alone—and validate voltage dip, frequency recovery, load sequence and site derating.

Record nameplate, driven-load and starting-method data.
Calculate running kW, running kVA and starting kVA separately.
Check alternator voltage response and engine frequency recovery.
Apply ambient, altitude and load-sequencing conditions.
Require supplier calculations and transient-performance evidence.

Collect the required inputs

Obtain voltage, phase, frequency, rated kW or hp, full-load current, efficiency, power factor, locked-rotor or starter-limited current, acceleration time, pump torque curve, duty, simultaneous loads, altitude and temperature.

Calculate continuous and starting demand

For three-phase loads, kVA = √3 × V × A ÷ 1000. Estimate motor input kW from shaft kW ÷ efficiency, and kVA from input kW ÷ power factor. Use verified starting current for starting kVA. See detailed motor starting current calculations.

Branch by starting method

DOL normally imposes the strongest electrical step. Star-delta, autotransformer and soft starters reduce current but also affect available torque. A VFD controls acceleration but adds nonlinear-load and compatibility checks. Compare motor starting methods before fixing capacity.

Validate transient response

Ask the supplier to model the actual load step, allowable voltage dip, frequency excursion and recovery time. If results are marginal, sequence loads, change the starter or increase alternator and engine capacity. Use this motor starting troubleshooting reference.

Verify the offered configuration

Request the engine rating basis, alternator reactance and excitation data, derating curves, controller settings, starter limits, calculation sheet, test procedure, warranty and service scope.

Can generator capacity be selected from motor kW alone?

No. Motor kW does not define starting current, acceleration torque, transient kVA or acceptable voltage and frequency response.

Does a soft starter always permit a smaller generator?

Not automatically. The current limit must still provide enough torque to accelerate the pump within an acceptable time.

Which result should appear in a supplier quotation?

The quotation should state running load, starting load, sequence, site conditions, predicted voltage and frequency response, rating basis and assumptions.

Standards, application fit and reference table

The ISO overview of ISO 8528-1 explains the application, rating and performance framework for generating sets. This supports specifying the duty and rating basis rather than comparing headline kVA alone. The ISO overview of ISO 8528-5 covers generating-set design and performance, supporting the need to verify transient voltage and frequency behavior for motor starts.

Starting method	Primary check	Typical risk
DOL	Locked-rotor kVA	High voltage dip
Star-delta	Transition and torque	Failure to accelerate
Soft starter	Current limit and ramp	Long acceleration
VFD	Input current and harmonics	Control incompatibility

Application conditions differ for irrigation, municipal, industrial and fire pumps; review pump application sizing rules. For fire-pump projects, the authority having jurisdiction and the applicable edition of NFPA 20 must be checked because this standard addresses stationary fire-pump installations; ordinary pump assumptions should not be substituted for project requirements.

Procurement decision

Compare compliant configurations using documented pump generator sizing cases. Buyers should not approve a set until the supplier identifies every assumption and provides evidence for the offered engine, alternator, controller and starter combination.