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How this calculator works
The sizing calculator works through five steps and shows each factor it applies:
- Running load. It estimates the emergency load from your facility type (for example 4 kW per hospital bed, 3 W per sq ft of office, or 10 kW per data center rack times a PUE of 1.5), or from the loads you list.
- Starting surge. It adds the largest motor starting surge, using the starting method you choose (across-the-line, star-delta, soft starter, or VFD), or a typical surge ratio for the facility type.
- kVA and derating. It converts to kVA at a 0.8 power factor and divides by the altitude and temperature derating for your site.
- Margins. It adds your growth allowance (default 25%) and the 80% loading rule (×1.25), then rounds up to a standard generator size. N+1 and 2N add whole units.
- Fuel storage. It estimates full-load fuel burn at about 0.07 gallons per kWh for a default run time by facility type, then adds a 133% margin. NFPA 110 §5.5.3 sets a fuel-capacity margin above the class run time. How it applies depends on edition and class; confirm with your AHJ. If the result is over 1,320 gallons, it points you to the SPCC calculator.
The default run times (for example 96 hours for hospitals) are planning assumptions, not legal minimums. CMS does not set an on-site fuel quantity, and NFPA 110 leaves the class run time to the AHJ or your emergency plan. Results are estimates; have a licensed electrical engineer confirm the final size.
Sources
- NFPA 110, Standard for Emergency and Standby Power Systems (Type, Class, Level, and fuel supply)
- 42 CFR 482.15, CMS emergency preparedness for hospitals
- 40 CFR Part 112, EPA SPCC rule
- ISO 8528-1, ratings and reference conditions for engine-driven generating sets