// FREE CALCULATORS
Generator Sizing and Step Load Calculator
Size a generator set for steady load plus the largest motor start. Enter the load, the motor and its starting method, the generator reactance X"d, and the voltage dip you can accept. Works at 50 and 60 Hz.
// INPUTS
// STEADY LOAD
Total demand load the generator carries when all loads, including the largest motor, are running
Project choice. Leaves headroom below the rating for transients and aging
// GENERATOR AND LIMITS
Altitude and ambient derating from the manufacturer's rating table. Enter 0 if the quoted rating is already at site conditions
Used only to convert kVA to the engine kW rating
From the generator data sheet, on the generator's own kVA base
From the project specification and the most sensitive load (contactors, UPS, drives)
Identical sets paralleled to carry the load. Each is sized for the total divided by N
// LARGEST MOTOR START
Everything on except the motor that is starting
DOL locked-rotor current is commonly 5 to 8 x; use the motor data sheet. Soft starter and drive values depend on the setting
Running Load-kVA
Required for Steady Load (total online)-kVA
Motor Full-Load Apparent Power-kVA
Motor Starting Apparent Power-kVA
Total kVA at the Instant of the Start-kVA
Required for the Voltage Dip Limit (total online)-kVA
Governing Requirement-
Required Rating per Unit-kVA
Selected Standard Size per Unit-kVA
Engine Rating per Unit-kW
Units Installed-
Steady Loading (N units online)-%
Estimated Voltage Dip at the Start-%
Dip Check-
Assumptions and limits: the voltage dip uses a series-reactance model, dip = X"d S / (G + X"d S), where S is the motor starting kVA and G is the online generator kVA after derating. It ignores the load already on, the voltage regulator and exciter response, cable impedance, and the motor's reduced starting torque at low voltage. Frequency dip and engine load acceptance are not calculated: check the engine's block-load limits. Paralleled sets are treated as one source of the combined kVA. Standard sizes are typical catalog steps and differ by manufacturer. Use the manufacturer's sizing data for the final selection.
Disclaimer: For reference and preliminary estimation only. Engineering judgment is required. Verify results against the applicable code, project specification, and manufacturer data before use in design documents. Found an error or an outdated reference? Let us know at [email protected].
Please check inputs: loads, efficiency, power factors, reactance, and the dip limit must be positive, power factors and efficiency must not exceed 1 and 100%, and N must be at least 1.