GuideCalculators and sizing12 min read
How to size a generator for a building or a plant
A generator for a building or a plant is sized from four things: the running load in kW that must be carried, the largest motor that has to start and the voltage dip the site can accept while it does, the share of the load that is drives, UPSs and other non-linear equipment, and the conditions the set will run in. Add up the running load, allow 20 to 25% for growth and for the steps the transfer switches add, choose the next standard size, then check that the alternator can start the biggest motor and carry the harmonic load. A 400 kW running load with a 100 HP motor started across the line lands on a 500 kW standby set; the calculator below does the first pass.
The short version
- Running kW is the load that will actually be on the generator, from metering or the demand on utility bills, not the connected load; divide by the power factor (0.8 by convention) for kVA.
- Standard sizes run 20 to 3,000 kW; a set’s kVA is its kW ÷ 0.8, and its amps at 480 V are kVA × 1,000 ÷ 831.
- A motor started across the line draws about six times its running current; the alternator must supply that starting kVA within the allowable dip (15 to 20% is usual), which often sets the size on a plant with one big motor.
- Drives, UPSs and switch-mode loads distort the current; a rule of thumb is an alternator rated 1.5 to 2 times the non-linear kVA, or a larger set.
- Standby (ESP) ratings are for emergency use at variable load; prime (PRP) for unlimited hours; continuous (COP) for constant base load. A set carries 10 to 15% less on a prime rating than on standby.
- Derate for altitude and heat (a few percent per 1,000 ft above about 3,000 ft and per 10 °C above 40 °C), and keep the set above about 30% load in service to avoid wet stacking.
Calculator
Enter the running load, a margin, the largest motor and how it starts. The calculator gives the standard standby size, its kVA and amps, and the starting kVA the motor will ask of the alternator.
500kW standby, standard size
400 kW × 1.25 = 500 kW; a 500 kW set is 625 kVA at 0.8 power factor and delivers 752 A at 480 V. The 100 HP motor draws about 600 kVA while starting; check that against the set’s motor-starting (skVA) rating at the voltage dip you can accept.
The six steps
List the loads the set must carry
Everything, for a plant that must keep running; or the emergency loads (egress lighting, fire pumps, life safety: NEC 700), the legally required standby loads (701) and the optional ones (702), each on its own transfer switch, for a building. The list sets both the size and the sequence.
Find the running load
Metering over a few weeks, or the demand on the utility bills, for the loads listed. Connected load with a demand factor only where there is nothing better. Convert to kVA at the power factor (0.8 if unknown).
Add the margin and pick a standard size
Twenty to 25% for growth and for the way the load arrives in steps as transfer switches close. The next standard size up; the chart below has the sizes and their amps.
Check motor starting
The largest motor, and the worst combination that can start together, against the alternator’s starting capability at the voltage dip you accept. This can push the set one or two sizes up, or argue for soft starters.
Check the non-linear load
UPSs, drives, LED lighting and computer loads distort the current and heat the alternator; keep them under about half the set’s kVA, or specify an oversized alternator.
Derate for the site and confirm the rating class
Altitude, ambient temperature, fuel (natural gas sets give less than diesel of the same engine), and whether the duty is standby, prime or continuous. Then run it through the maker’s sizing tool, which models the transient behavior of the actual set.
Standby kW ≥ running kW × (1 + margin); then check motor starting and non-linear load
Chart: generator kW to kVA and amps
Generator sets are rated in kW at 0.8 power factor, so the kVA is the kW divided by 0.8 and the full-load amps follow from the voltage. The common commercial and industrial sizes:
| kW (standby) | kVA | Amps at 480 V | Amps at 208 V | Amps at 4,160 V |
|---|---|---|---|---|
| 100 | 125 | 150 | 347 | 17.3 |
| 150 | 188 | 226 | 520 | 26 |
| 200 | 250 | 301 | 694 | 34.7 |
| 250 | 313 | 376 | 867 | 43.4 |
| 300 | 375 | 451 | 1,041 | 52 |
| 400 | 500 | 601 | 1,388 | 69.4 |
| 500 | 625 | 752 | 1,735 | 86.7 |
| 600 | 750 | 902 | 2,082 | 104 |
| 750 | 938 | 1,128 | 2,602 | 130 |
| 1,000 | 1,250 | 1,504 | 3,470 | 173 |
| 1,250 | 1,563 | 1,879 | 4,337 | 217 |
| 1,500 | 1,875 | 2,255 | 5,204 | 260 |
| 2,000 | 2,500 | 3,007 | 6,939 | 347 |
| 2,500 | 3,125 | 3,759 | 8,674 | 434 |
Amps = kVA × 1,000 ÷ (1.732 × V). The breaker on the set is usually 100 to 125% of these amps; the feeder conductors per NEC 445.13. The kVA to amps guide has every size.
Motor starting and voltage dip
A motor started across the line draws its locked-rotor current, about six times its running current, until it comes up to speed. From the utility that’s nothing; from a generator, whose alternator is a limited source, it pulls the voltage down for everyone else on the bus until the voltage regulator and the engine catch up. The two questions are how far the voltage dips and whether the engine can supply the kW the motor needs during acceleration.
| Motor | Starting kVA, across the line (code G) | With a soft starter | Standby set for about a 20% dip |
|---|---|---|---|
| 25 HP | 150 | 60 | 100 kW |
| 50 HP | 300 | 120 | 150 to 200 kW |
| 100 HP | 600 | 240 | 300 to 400 kW |
| 200 HP | 1,200 | 480 | 600 to 750 kW |
| 300 HP | 1,800 | 720 | 1,000 kW |
| 500 HP | 3,000 | 1,200 | 1,500 to 2,000 kW |
Starting kVA from the motor nameplate’s code letter (G is 5.6 to 6.3 kVA per HP). The last column is a rule of thumb for a standard alternator; makers’ sizing tools give the dip for a specific set, and an oversized alternator on the same engine improves it.
Where one motor sets the size, the alternatives cost less than the next size of generator: a soft starter that ramps the motor up at two to three times running current; a drive that starts it at little more than running current; or sequencing, so the big motor starts first on an unloaded set and the rest follow. A fire pump is the exception: NFPA 20 requires it to start across the line, and the set is sized for it.
Drives, UPSs and other non-linear loads
A drive’s rectifier, a UPS’s charger and a building full of switch-mode power supplies draw current in pulses rather than sine waves. On a stiff utility supply that barely shows; on a generator, whose alternator impedance is ten times a transformer’s, the pulses distort the voltage, heat the alternator windings, and can confuse the voltage regulator and the UPS itself, which may refuse to synchronize. The rules of thumb:
- Keep non-linear loads under about half the set’s kVA, or specify an alternator one or two frame sizes larger than standard (makers quote this as a UPS or harmonic option).
- For a UPS with a six-pulse rectifier, size the set at 2 to 2.5 times the UPS kVA; with a twelve-pulse or an active (IGBT) front end, 1.2 to 1.5 times.
- Drives with line reactors or active front ends draw far cleaner current than bare six-pulse drives; the drive specification matters to the generator.
- Use a permanent-magnet excitation system and a digital regulator, which hold voltage through the distortion better than self-excited sets.
Standby, prime and continuous ratings
| Rating | Hours | Load | Overload | Use |
|---|---|---|---|---|
| Standby (ESP) | Limited: typically up to 200 hours a year | Variable, averaging under 70% | None | Emergency and standby power when the utility fails |
| Prime (PRP) | Unlimited | Variable, averaging under 70% | 10% for 1 hour in 12 | Primary power where there is no utility, or long outages; rental sets |
| Limited-time prime (LTP) | Up to 500 hours a year | Constant, at the rating | None | Peak shaving, utility interruptible programs |
| Continuous (COP) | Unlimited | Constant, at the rating | None | Base load: a plant running on its own generation |
The same engine and alternator carry a different number under each class: a 500 kW standby set is about 450 kW prime and about 400 kW continuous. A used set’s nameplate gives one or more of these, and a set sold as “500 kW” is usually its standby figure.
Derating, load factor and fuel
- Altitude
- Thinner air means less oxygen and less cooling; engines lose on the order of 3 to 4% of output per 1,000 ft above a threshold the maker sets (often 3,000 ft for turbocharged diesels, lower for naturally aspirated and gas engines).
- Temperature
- About 1 to 2% per 10 °C (or 3% per 10 °F) above the rated ambient, usually 40 °C; the radiator’s capacity matters as much as the engine’s.
- Fuel
- A natural gas engine of the same displacement makes less power than diesel and responds to load steps more slowly, so gas sets are sized larger and loaded in smaller steps; propane and dual-fuel have their own factors.
- Emissions tier
- EPA Tier 4 Final sets (required for prime and some standby applications) carry aftertreatment that needs the exhaust hot, which argues against oversizing; Tier 2 standby sets are more tolerant of light load.
- Load factor
- A diesel run below about 30% of rating for long periods wet-stacks. Standby sets are exercised at load (NFPA 110 asks for monthly runs at 30% or more, or a yearly load-bank test) for this reason; a prime set should carry 50 to 80%.
- Step loading
- A diesel will take its full load in one step if the set is sized for it; NFPA 110 Level 1 systems must pick up the emergency load in 10 seconds. Gas sets take 25 to 50% steps. Transfer switches with staged closing manage it.
- Fuel tank
- Runtime at the expected load: NFPA 110 classes (2, 6, 48, 72 hours) set the on-site fuel for emergency systems; a sub-base tank sized for 24 hours at full load is common for standby.
A worked example
A water treatment plant needs a standby set for the whole process: 320 kW running (metered), a 150 HP high-service pump started across the line, two 50 HP pumps on drives, a 30 kVA UPS for the controls, and the site is at 5,000 ft.
- Running load: 320 kW; at 0.85 power factor, 376 kVA. Margin 25%: 400 kW, 470 kVA.
- Standard size: 400 kW standby (500 kVA, 601 A at 480 V).
- Motor starting: the 150 HP pump draws about 900 kVA across the line. On a 500 kVA alternator that’s a dip over 30%; the controls would drop out. Either a 750 kW set (938 kVA, dip near 20%) or a soft starter on the pump (about 360 kVA starting, dip around 12% on the 400 kW set). The soft starter costs a fraction of the larger set.
- Non-linear load: 30 kVA UPS and two 50 HP drives, about 120 kVA of distorting load on a 500 kVA set, under a quarter: fine with a standard alternator; the six-pulse UPS rule (2 × 30 = 60 kVA) is covered.
- Altitude: 5,000 ft is about 2,000 ft over the threshold, a 6 to 8% derate. The 400 kW set delivers about 370 kW; the 400 kW load after margin was conservative, and the running load is 320 kW, so it holds. A 500 kW set would be the choice if the plant expects to grow.
- Rating and fuel: standby duty, Tier 2 allowed for emergency standby in most states; a 48-hour sub-base tank per the plant’s NFPA 110 class.
The specification: 400 or 500 kW diesel standby, 480Y/277 V, with a soft starter added to the 150 HP pump, derated for 5,000 ft, 48-hour tank. The generator listings show what’s available, with hours and tier, and Magic Search finds the sets for sale across the web that match it.
Questions people ask
How do you size a generator for a building?
List the loads the generator must carry (all of them, or the emergency and legally required loads on their own transfer switches), find their running kW from metering or the utility bill’s demand, add 20 to 25% for growth and for the sequence in which transfer switches add load, and choose the next standard size. Then check the largest motor can start within the voltage dip you accept, that the alternator can carry the UPS and drive load, and that the set still delivers its rating at your altitude and temperature. The makers’ sizing programs do all of it once the loads are listed.
What size generator do I need for 400 amps at 480 V?
400 A three-phase at 480 V is 333 kVA, which at 0.8 power factor is 266 kW. A 300 kW set (375 kVA, 451 A) carries it with a little margin; a 350 or 400 kW set if the 400 A is a demand figure that will grow or the load includes large motors or a UPS. For a 400 A service at 208 V the figures are 144 kVA and 115 kW: a 125 or 150 kW set.
What size generator for a house?
A different calculation from the one on this page. A whole-house standby set for a typical American home is 20 to 26 kW; the makers’ residential calculators size it from the air-conditioning tonnage and the circuits you want backed up. This guide is for the commercial and industrial sets, 100 kW and up, that we list.
What is the difference between standby, prime and continuous ratings?
How long and how hard the set may run (ISO 8528-1). Standby (ESP) is the highest rating, for emergency use with a variable load averaging under 70% and a few hundred hours a year, with no overload allowed. Prime (PRP) is for unlimited hours at a variable load averaging under 70%, with a 10% overload allowed for an hour in twelve; it’s about 10% below the standby figure. Continuous (COP) is for a constant load for unlimited hours, lower again. A 500 kW standby set is roughly a 450 kW prime set.
How much does motor starting affect generator size?
Often it decides it. A motor started across the line draws roughly six times its full-load current for a second or two, and the generator’s alternator has to supply that starting kVA without letting the voltage dip more than the site tolerates (15 to 20% for most equipment, 30% at the outside). A 100 HP motor draws about 600 kVA starting; on a 250 kW (313 kVA) set that’s a dip in the 25 to 35% range, on a 500 kW set about 15%. Soft starters cut the starting draw to two or three times running current; drives to little more than running current.
Can a generator be too big?
Yes. A diesel set running for hours under about 30% of its rating runs cool, fouls its injectors and exhaust with unburned fuel (wet stacking) and shortens its life; it also costs more to buy and burns more fuel per kWh. If the normal load is small and the peak is large, the answer is often a smaller set with a load bank, two paralleled sets, or managing the peak with load shedding on the transfer switches.
Sources: ISO 8528-1 (rating definitions); NFPA 110 (emergency and standby power systems) and NFPA 70 Articles 700, 701, 702 and 445; IEEE 446 (the Orange Book); makers’ sizing tools (Caterpillar SpecSizer, Cummins GenSize, Kohler PowerCalc, Generac Industrial).