Industrial Generator Sizing Calculator
Enter the running load, the largest motor and your dip limit to size the generator for both running load and starting surge.
Step-by-step
- Enter valid values to begin.
Voltage dip uses the standard X′d approximation, dip = kVAstX′d / (kVAgen + kVAstX′d). Standard sizes follow common 0.8 PF diesel sets. Non-linear loads (VFD input, UPS) usually need extra alternator capacity; confirm with the manufacturer’s sizing software.
Industrial Generator Sizing: Running Load Is Only Half the Job
Core Engineering Principles
Most people size a generator by adding up the running kW and picking the next size up. That works for lighting and heaters. In a plant with motors it fails, because the generator has to survive the biggest moment, not the average one. When a large motor starts across the line it draws five to seven times its running current, and the generator’s voltage sags because the machine has a finite internal reactance, the subtransient or transient reactance X′d. The voltage dips, contactors drop out, drives fault, the lights flicker and the motor may never reach speed.
So there are two numbers to find. The first is the running kVA of all the loads at once, with some growth margin, that sets the generator’s continuous rating. The second is the size needed to keep the voltage dip within limits when the largest motor starts. The starting kVA is the motor’s locked-rotor kVA, reduced by your starter type. A soft starter might cut it to half, a VFD to about 1.5 times full-load. The dip is approximately kVAstart × X′d / (kVAgen + kVAstart × X′d). Rearranged, the generator you need is kVAstart × X′d × (1 − dip) / dip. The bigger of the two answers wins.
Dip = (kVAst × X′d) / (kVAgen + kVAst × X′d) • kVAgen,dip = kVAst × X′d × (1 − dip) / dip
NEC & Standard References
NFPA 110 governs emergency and standby power systems: Level 1 life-safety, Level 2 less critical, and the 10-second transfer limit. NEC 700.4 and 701.4 require capacity and rating to be adequate for the connected load, with a method for sequencing or load-shedding. NEC 445.13 says generator conductors must carry at least 115% of the nameplate current. ISO 8528-1 defines the standby, prime and continuous ratings, and ISO 8528-5 the transient performance classes (G1 to G3), with G2 and G3 limiting voltage dip. A typical specification allows 15 to 20% dip for general loads, 10% for sensitive electronics, and a frequency dip under 10%. The generator manufacturer’s sizing software has the exact X′d for each frame.1. Running kVA = 180 / 0.8 × 1.10 = 247.5 kVA.
2. Starting kVA = 75 × 5.5 × 1.0 = 412.5 kVA.
3. Generator for dip = 412.5 × 0.25 × (1 − 0.20) / 0.20 = 412.5 kVA.
4. The dip requirement governs: choose the next standard size, 500 kVA (400 kW).
If the pump gets a soft starter that cuts inrush by half, the dip-based need falls to 206 kVA and the running load (247.5 kVA) governs, so a 250 kVA set would do.
- Derate for altitude and heat. Engines lose about 3% power per 300 m above 150 m and 1% per 5.5 °C above 25 °C, so a set on a roof in Arizona is not what the brochure says.
- Non-linear loads need a bigger alternator. VFDs, UPS and rectifiers need 1.5 to 2 times the kVA, or an alternator with low X′d.
- Load steps matter, not just the largest motor. Several motors starting together at the transfer switch act like one big one. Sequence them.
- Don’t run lightly loaded for hours. A diesel under 30% load wet-stacks and glazes. Add a load bank or size closer.