A focused landing for “how to size a standby or prime generator for a commercial application.” Walks through the four numbers that drive sizing — running load, largest motor starting kVA, redundancy requirement, and code adders for healthcare or data center occupancies — then routes to the working calculator for kW-to-amps and fault-current math.
Where the calculator lives. The full Generator calculator handles kW-to-amps, kVA-to-amps, and fault-current contribution. Open it, then come back here for the sizing rules-of-thumb if you need them.
Open the generator calculatorSum the kW of every load that must run during an outage. For lighting and receptacles, multiply VA by power factor (typically 0.95). For motors, use nameplate running kW (not starting). This is your steady-state requirement.
Across-the-line motor starts pull 5-7× rated current at low voltage (locked-rotor kVA). The generator must absorb that without exceeding its voltage dip limit (typically 25%) and its momentary kVA capability. If the largest motor's starting kVA exceeds 25% of the generator's continuous kVA, you need a soft-starter, a VFD, or a bigger generator.
For most commercial applications, N (one generator). For data centers per Uptime Institute Tier ratings: N+1 minimum at Tier III (one extra unit), 2N at Tier IV (full redundancy). For healthcare per NFPA 99/110, the system must be sized for the Type 1 emergency power supply system (EPSS) load on a single generator.
Healthcare (NEC 517, NFPA 99): three branches — life safety, critical, equipment — must run continuously. Elevators (ASME A17.1): add 1× elevator FLA per car for life-safety operation. Fire pump (NEC 695): the fire pump must be on the same generator, sized to handle pump locked-rotor.
A free calculator gets you in the ballpark. For permit-stamped, defensible work, ClarkTE delivers a PE-stamped generator sizing study with NFPA 110 acceptance plan from a working engineer — typically within 48 hours of receiving your one-line and load data.
It depends on which loads must run during an outage. For emergency lighting and life-safety only (NEC 700 / NFPA 110), the generator can be modest — typically 20-100 kW for a small commercial building. For full backup including HVAC and elevators, sizing scales with running load and the largest motor's starting kVA. The Generator calculator returns full-load amps and fault contribution; use the rules-of-thumb table to size the unit itself.
Standby commercial generators range roughly $25,000 for a 50 kW natural-gas unit through $250,000+ for a 1 MW diesel prime-power machine. Total installed cost (including transfer switch, fuel system, paralleling gear if multiple units, commissioning, and load bank acceptance testing) typically runs 1.5-2× the equipment cost. ClarkTE doesn't sell generators — we specify, commission, and document them.
Not really, no. Below ~30% load, diesel engines wet-stack — unburned fuel coats cylinders, glazes the bores, and shortens engine life dramatically. Below ~50%, fuel efficiency tanks. The right size is the smallest unit that handles your worst-case starting load with margin, not the largest unit that fits in your yard.