Generator Sizing for Motor Starting

Required generator kW / kVA from running load and motor-starting kVA at the 30% voltage-dip criterion.

Run the calculator

Example

You enter

Motors hp 25, code letter G; hp 10, code letter F; hp 5, code letter B
Non-motor steady kW 15
Allowable voltage dip (0-1, default 0.30) 0.3
Starts per hour frequent

You get

Steady running kW 44.8
Worst starting kVA 140
Required starting (kVA) 536.67
Required gen kW 429.3
Recommended generator 500

Details, formula, and sources

Steady kW = Σ running_kW + non_motor_kW. Worst starting kVA = max over motors of starting_kVA(motor); starting_kVA = HP × code_kVA_per_HP from the NEMA MG-1 code letter, OR LRA × V × √phases / 1000 if the user supplies LRA. Required gen kVA = worst_starting_kVA / dip_factor (0.30 default per the 30% voltage-dip criterion). Required kW = max(steady, required_starting_kVA × 0.8).

NEMA MG-1 (Motors and Generators) by name; engineering-practice 30% voltage-dip criterion for transient motor starts.

NEMA MG-1 licensed; code-letter principles free in published power-engineering texts.

Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.

Field names used by the API: motors, non_motor_kW, dip_factor, starts_per_hour, running_kW, worst_starting_kVA, required_starting_kVA, required_kW, recommended_kW

Code-letter table data/electrical/nema-mg1-code-letters.json keyed to A through V (lower bound of each range)NEMA MG-1
Default dip factor 0.30 (30% voltage-dip criterion)engineering practice
Frequent-start derate occasional 1.0 / frequent 1.15 / continuous 1.30manufacturer typical