Commercial generator sizing is the process of calculating the maximum simultaneous electrical load (in kW and kVA) a facility will demand during a utility outage, then selecting a generator set (genset) with adequate continuous and starting capacity to support that load without voltage or frequency collapse. Getting this calculation right dictates your Automatic Transfer Switch (ATS) amperage rating, the feeder wire gauge (e.g., choosing 350 kcmil vs. 500 kcmil THHN copper), the physical concrete pad dimensions, and the fuel line diameter for natural gas or tank capacity for diesel. The most common mistake in this process is confusing real power (kW) with apparent power (kVA), or mistakenly ordering a unit based on its standby rating when the application actually requires a prime or continuous rating.
The Core Concept: Real Power vs. Apparent Power
Before running load calculations, you must separate kW from kVA. Generators are limited by both the engine's mechanical horsepower (which dictates kW) and the alternator's thermal capacity (which dictates kVA).
- kW (Kilowatts): Real power. This is the actual work being done (heating, lighting, mechanical torque). The diesel or natural gas engine must be sized to supply the total kW.
- kVA (Kilovolt-Amperes): Apparent power. This includes the reactive power (kVAR) required to magnetize motors and transformers. The alternator must be sized to supply the total kVA.
- The Bridge: Power Factor (PF). In commercial 3-phase systems, PF is typically assumed to be 0.80 for sizing unless specific load data dictates otherwise. kVA = kW ÷ 0.80
If you size a generator purely on kW and ignore kVA, large inductive loads (like 3-phase HVAC compressors) will cause severe voltage dips, tripping your UPS systems and stalling the engine.
The Math: A Worked Numeric Example
Let’s size a standby generator for a small commercial facility. We need to calculate both the Running kVA (RKVA) and the Starting kVA (SKVA). SKVA is critical because 3-phase motors draw 5 to 7 times their running current for the first few cycles when starting across-the-line.
The Load Profile
- 50 HP 3-Phase HVAC Compressor: 37.3 kW mechanical output. Assuming 90% motor efficiency and 0.85 PF, the electrical input is roughly 41.4 kW. At 0.85 PF, that is 49 kVA RKVA. Across-the-line starting SKVA is 6x running, so 294 kVA SKVA.
- Lighting and Receptacles: 20 kW at 0.90 PF = 22.2 kVA (Both RKVA and SKVA).
- Server Room Double-Conversion UPS: 10 kW at 0.95 PF = 10.5 kVA (Both RKVA and SKVA).
| Load Description | Running kW | Running kVA (RKVA) | Starting kVA (SKVA) |
|---|---|---|---|
| 50 HP HVAC Compressor | 41.4 kW | 49.0 kVA | 294.0 kVA |
| Lighting & Receptacles | 20.0 kW | 22.2 kVA | 22.2 kVA |
| Server Room UPS | 10.0 kW | 10.5 kVA | 10.5 kVA |
| Totals | 71.4 kW | 81.7 kVA | 326.7 kVA |
Selecting the Genset
Your total running load is 71.4 kW (81.7 kVA). A naive approach might select a 100 kW (125 kVA) generator. However, when the 50 HP compressor kicks on, the generator must instantly supply 326.7 kVA. A 125 kVA alternator will instantly collapse in voltage, likely tripping the server UPS offline.
To handle a 326.7 kVA starting surge while maintaining voltage dip within acceptable limits (typically < 20% for motor starting, per NFPA 110 standards), you must step up to a 275 kW (344 kVA) or 300 kW (375 kVA) standby-rated generator. The engine is oversized for the running kW, but the alternator is properly sized for the starting kVA.
Where You Meet This in Practice
Commercial generator sizing extends far beyond the nameplate on the genset itself. The calculated kW and kVA values ripple through the entire electrical installation:
- Automatic Transfer Switch (ATS) Sizing: If your 300 kW generator outputs 480Y/277V 3-phase, the full load amps (FLA) are roughly 360A. You will need a 400A ATS. If you miscalculated and the actual load peaks at 450A, you are forced to install a more expensive 600A ATS and heavier busbars.
- Feeder Wire Gauge and Voltage Drop: A 400A ATS requires parallel runs of 350 kcmil copper THHN or a single 600 kcmil run per phase. If the generator pad is 200 feet from the main switchgear, voltage drop calculations might force you to upsize to 500 kcmil parallel runs to keep the drop under 3% at full load.
- Fuel Delivery Infrastructure: A 300 kW diesel generator consumes roughly 22 to 25 gallons per hour at full load. This dictates the size of your belly tank or the external bulk storage tank and the diameter of the fuel polishing lines. For natural gas units, you must coordinate with the utility to ensure the gas meter and supply line can deliver the required inches of water column (WC) pressure under full transient load.
Motor Starting and Inrush Current
Starting a large 3-phase compressor on a generator is like merging a fully loaded semi-truck into fast-moving highway traffic; the engine must have enough reserve torque to absorb the sudden load without stalling the flow. In electrical terms, this "stall" is a voltage and frequency dip.
If your facility relies heavily on large motors, you can artificially reduce the SKVA requirement—and therefore buy a smaller, cheaper generator—by altering how the motors start:
- Across-the-Line (DOL): 600% inrush. Highest SKVA, lowest equipment cost.
- Soft Starters: Reduces inrush to 250% - 300%. Moderately lowers SKVA requirement.
- Variable Frequency Drives (VFD): Limits inrush to 100% - 150%. Drastically lowers SKVA, allowing you to size the generator closer to the actual running RKVA. Note: VFDs introduce harmonic distortion, which may require you to oversize the alternator by 15-20% to handle the heat generated by non-linear loads.
Commercial Generator Sizing FAQ
How do I size a commercial generator for a building with large 3-phase HVAC motors?
Focus on the Starting kVA (SKVA) rather than just the running kW. Calculate the SKVA for the largest motor starting across-the-line, add the RKVA of all other background loads, and select a generator whose alternator can handle that peak SKVA without dropping voltage below 80% of nominal. If the required generator is too large or expensive, install a VFD or soft starter on the HVAC compressor to reduce the inrush current.
What is the difference between standby, prime, and continuous generator ratings?
These ratings are defined by ISO 8528-1. Standby is the maximum power a unit can deliver for a short duration (usually up to 500 hours a year) during a utility outage. Prime is the power it can deliver for an unlimited number of hours per year, typically at a slightly lower kW rating than standby, used for peak shaving or off-grid sites. Continuous is the baseline power it can run 24/7/365 without interruption, which is lower still. Never size a backup generator using the continuous rating; use the standby rating for emergency applications.
Does commercial generator sizing change if I use a natural gas versus diesel genset?
Yes, primarily in transient response and fuel delivery. Diesel generators accept heavy block loads (sudden large SKVA demands) much better than natural gas units because of the mechanical energy stored in the diesel engine's heavy flywheel and the immediate torque response of fuel injection. Natural gas engines can suffer from "turbo lag" and gas line pressure drops during sudden load steps. If you must use natural gas for a high-SKVA application, you often need to oversize the generator by 15% to 20% compared to a diesel equivalent, or sequence your loads using a multi-step ATS to prevent stalling.
How does NEC Article 700 affect my generator sizing calculation?
Under NFPA 110 and NEC Article 700, Emergency (Life Safety) loads must be transferred to the generator within 10 seconds of a utility failure. This means all life safety loads (egress lighting, fire alarms, elevator cab lighting) must be calculated as starting simultaneously. Optional standby loads (like general HVAC or convenience receptacles under Article 702) do not have this strict time limit, allowing you to use load-shedding or sequenced starting to keep the physical size and cost of the generator down.
