Generator Sizing Calculator: Load and Fault Current
A free generator sizing calculator that converts kW or kVA into full load amps for single- and three-phase generators, then estimates the fault current the generator can deliver into a downstream short. Use it to size emergency or standby generators, verify breaker interrupting ratings, and check protection coordination.
Generator Load Current Calculator
Calculate generator output current based on load power requirements and operating conditions.
Typical: 0.8 lagging for generators
Generator Sizing Notes
- Generators are typically rated at 0.8 power factor lagging
- Motor starting loads may require larger generator (3-6× motor FLA)
- Altitude and temperature affect output - derate above 3300 ft or 77°F
- Size for continuous loads at 100% + largest motor at 25% of FLA
- Consider harmonic loads - may require oversizing by 20-50%
Need this validated by a licensed PE?
A free calculator gets you in the ballpark. For permit-stamped, defensible work, ClarkTE delivers a PE-stamped generator sizing study from a working engineer — typically within 48 hours of receiving your one-line and load data.
How to size a standby generator
Sizing a generator is more than adding up nameplate watts. You need three numbers: the running load (steady kW), the starting load (the inrush from the largest motor or chiller starting across-the-line), and the load profile (which loads run together vs. sequentially). A 100 kW running load with a 50 HP motor on it can require a 175 kW generator if the motor starts unassisted.
Use the load-current tab above to convert your worst-case kW or kVA to full-load amps. That tells you the breaker frame size and the conductor ampacity per NEC 445.13. Then use the fault-current tab to estimate the generator's contribution into a bolted secondary fault — typically 3–10× full-load amps, governed by the machine's subtransient reactance (X″_d, usually around 0.10–0.18 pu).
Common gotchas: oversizing a standby generator hurts. Below ~30% load, diesel engines wet-stack (unburned fuel coats cylinders, glazes the bores, and shortens engine life). Below ~50% load, 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.
Generator sizing rules of thumb
| Application | Rule of thumb | Code reference |
|---|---|---|
| Emergency lighting only | ≥ connected load | NFPA 110, NEC 700 |
| Building with elevators | Add 1× elevator FLA per car | ASME A17.1 |
| Largest motor < 25% of gen size | Across-the-line OK | Manufacturer guidance |
| Largest motor > 25% of gen size | Use soft-starter or VFD | Manufacturer guidance |
| Healthcare facility | Three branches: life safety, critical, equipment | NEC 517, NFPA 99 |
| Data center | N+1 redundancy minimum | Uptime Institute Tier rating |
Specifying a generator install?
Generator sizing is the easy part. Protection coordination, transfer-switch design, NFPA 110 commissioning, and load-bank acceptance testing are where projects go sideways. ClarkTE has commissioned standby and prime-power systems for hospitals, data centers, and industrial facilities — under one PE stamp, with documented test results.
Generator sizing FAQ
How much does a Generac (or similar commercial generator) cost?
Standby generators for commercial facilities range roughly $25,000 for a small 50 kW unit through $250,000+ for a 1 MW prime-power machine, before transfer switch, fuel system, paralleling gear, and commissioning. Total installed cost typically runs 1.5-2x the unit cost. ClarkTE doesn't sell generators — we specify, commission, and load-bank-test them.
How do I size a generator for commercial loads?
Add up running load (steady kW) and the largest motor's starting kVA (typically 6× run kW for across-the-line). The generator must handle both without exceeding voltage dip limits. For data centers and hospitals, code (NFPA 110, NEC 700/517) imposes additional rules. The Load Current tab returns full-load amps for any kW or kVA you enter.
What's the fault current contribution from a standby generator?
Roughly 3-10× full-load amps in the first cycle, governed by subtransient reactance X″_d (usually 0.10-0.18 pu). Lower than utility fault current but still enough to trip downstream breakers. The Fault Current tab gives a quick estimate; a real coordination study is needed for protection design.