Also in: Disaster Response and Recovery
Generator Sizing for Motor Starting
Required generator kW / kVA from running load and motor-starting kVA at the 30% voltage-dip criterion.
Example
You enter
- Motors hp 25, code letter G; hp 10, code letter F; hp 5, code letter B
- Non motor (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 148.8
- Required starting (kVA) 99.79
- Required gen kW 79.8
- Recommended generator 80
Checked against published worked examples from: NEC 430.110 + manufacturer locked-rotor schedule (project bundled); Cummins Power Generation.
Details, formula, and sources
Formula or tableSteady 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 x X'd x (1 - dip) / dip x starts factor (1.0 / 1.15 / 1.30), the reactance divider dip = X'd S_m / (S_gen + X'd S_m) solved for S_gen, with X'd the generator transient reactance (default 0.25 per unit) and dip_factor (0.30 default per the 30% voltage-dip criterion). Required kW = max(steady, required_starting_kVA × 0.8).
Source and editionNEMA MG-1 (Motors and Generators) by name; engineering-practice 30% voltage-dip criterion for transient motor starts.
Free accessNEMA MG-1 licensed; code-letter principles free in published power-engineering texts.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesmotors, 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 (midpoint of each range)NEMA MG-1 ranges, exact midpoints (Cummins T-030 Table 7 prints rounded averages and differs at A, 2, and S, 16)
- Default dip factor 0.30 (30% voltage-dip criterion)engineering practice
- Generator transient reactance X'd 0.25 per unit default (typical sets 0.15-0.30; the manufacturer's data sheet governs)engineering practice
- Frequent-start derate occasional 1.0 / frequent 1.15 / continuous 1.30manufacturer typical