In short. A 125 to 250 kVA generator usually carries its running load with ease and is judged by how it survives the way load arrives. Process motors, chillers, compressors and lift groups create block loads that pull voltage and frequency down together, and the sensitive equipment already connected sets the limit. The strongest answer is often not more kVA but a sequenced restart, a gentler starting method and an alternator chosen for transient duty. Decide that before the capacity.
What is different at this capacity
At 125 to 250 kVA the installation is typically a hospital, a hotel, an IT office, a mid-sized factory, a cold storage, a pharmaceutical unit or a large residential complex. The set feeds a switchboard rather than a single board, several motors can start within seconds of each other, and there is almost always a UPS, a PLC or a building management system watching voltage and frequency. The question is no longer whether the set can carry 180 kVA; it is whether it can accept a 45 kW pump starting while 120 kVA is already connected and keep every contactor closed while it does so.
The questions that settle the transient
- What is already online when the largest motor starts, because the dip comes from the sum, not from the motor alone.
- How the motor is started: direct-on-line, star-delta, soft starter or variable-frequency drive.
- What voltage dip the connected equipment tolerates, which is usually set by contactors, UPS input windows and PLC power supplies.
- Whether starts can be sequenced by timers, by the building management system or by a load management controller.
- What happens if the motor does not start, which decides how much margin the design must carry.
The motor starting page sets out the calculation; what follows is how to use its result to choose between options.
More kVA or better control
When the screening shows an unacceptable dip there are five responses, and they are not equal in cost or in side effects.
| Option | What it changes | Running cost effect | Side effects to check |
|---|---|---|---|
| Larger generator set | Reduces both voltage and frequency dip | Higher fuel per kWh at light load, higher capital | Light-load running, larger cables and breaker |
| Oversized alternator on the same engine | Reduces voltage dip only | Small | Frequency dip unchanged, longer canopy |
| Star-delta or soft starter | Cuts starting kVA to a third or half | None | Reduced starting torque, so the driven load must start unloaded |
| Variable-frequency drive | Starting current near running current | Often saves energy on pumps and fans | Harmonic current, alternator heating, derating |
| Load sequencing | Starts motors one at a time after transfer | None | Restart takes longer, needs timers or a controller |
In practice the cheapest combination is usually a sequenced restart plus a reduced-voltage starter on the one or two largest motors, with the set then sized for running load, the largest single step and a sensible growth allowance. A load management controller does the sequencing automatically and can shed deferrable loads if the set approaches its limit.
How much step the set can take
Available step kVA ≈ (transient capability of the set) minus (kVA already connected)
The transient capability is the kVA the set can supply momentarily within the agreed voltage and frequency limits, and it is read from the manufacturer’s motor-starting curve for the alternator and the load-acceptance data for the engine; the kVA already connected is the running load at the instant of the start. A 200 kVA set with an alternator that supports about 400 kVA of motor-starting load at a 25 percent dip, carrying 110 kVA, has roughly 290 kVA available for a step (illustrative). A 45 kW pump started direct-on-line demands around 330 kVA and exceeds that; on a star-delta starter it demands about 110 kVA and passes with margin. The engine must be checked separately, because ISO 8528-5 defines load acceptance in kW steps and a turbocharged engine in this class typically accepts a first step in the region of 50 to 60 percent of rated kW within class G2 limits. The load step screening tool performs both checks.
Frequency, governors and the engine
Voltage dip is an alternator property and frequency dip is an engine property. When a block load lands, engine speed falls until the governor adds fuel and the turbocharger spins up; an electronic governor on a common-rail engine recovers in a second or two, a mechanical governor on an older engine takes longer and overshoots more. Frequency dips matter because UPS systems, synchronous clocks and some drives trip on frequency as well as voltage, and because repeated deep dips are hard on the engine. The frequency dip page covers the ISO 8528 performance classes and how to specify them.
Installation at this scale
A 250 kVA set delivers about 348 A at 415 V, so the generator breaker is a 400 A MCCB or a small air circuit breaker, the cable run is two parallel runs per phase or a single large conductor, and the changeover is a four-pole ATS in that class rather than a contactor pair. Fuel consumption at 75 percent load is around 40 to 45 litres per hour, so an external tank sized for 12 to 24 hours is normal (illustrative). Canopies in this band are 4.5 to 5.5 metres long and weigh several tonnes, which affects crane access and slab design. The exhaust will carry a CPCB IV+ aftertreatment unit and its heat, and the radiator discharge needs an unobstructed path of 3 metres or more. These sets are also the smallest at which paralleling two units is sometimes cheaper than one large set, particularly where the night load is small; the 250 to 500 kVA page discusses that trade.
Common mistakes
- Testing the motor start with nothing else connected and declaring it acceptable. In service the dip is far deeper because 100 kVA of other load is on the bus, and the UPS transfers to battery.
- Choosing a variable-frequency drive for the largest pump and ignoring the alternator derating for harmonic load. The set runs hot in summer and the AVR becomes unstable at light load.
- Reconnecting the switchboard in one block after transfer. The engine stalls or trips on underfrequency with every mains failure.
- Buying 250 kVA for a 120 kVA problem to be safe. The set spends its life at 30 percent load, wet-stacks, and costs more per unit than the right set with a soft starter.
- Specifying the starter after the generator is ordered. The set turns out too small for DOL or too large for the drive that was eventually fitted.
What to send us
Send a load schedule showing the running load in kVA, every motor above 5 kW with its starting method and whether it can be delayed, the UPS rating and its input tolerance if known, and the single-line diagram or a photograph of the main switchboard. Tell us which equipment has tripped during outages in the past, if any. Submit it through the application engineering enquiry form and we will return a step-load check with the options and their costs.
Questions engineers ask
Can a 200 kVA generator start a 50 HP pump?
A 37 kW pump started direct-on-line needs roughly 270 kVA, which a 200 kVA set can usually manage only if nothing else is connected and a 30 percent dip is acceptable. With a star-delta or soft starter the start falls to 90 to 130 kVA and the set handles it with other load connected (illustrative).
What voltage dip should I design for?
Many specifications use 15 to 20 percent for sites with contactors, PLCs and UPS equipment, and up to 30 percent where only motors and lighting are connected. Confirm against the most sensitive device’s hold-in voltage rather than adopting a general figure.
Is it better to buy one 250 kVA set or two 125 kVA sets?
One set is cheaper and simpler. Two sets give redundancy and let one unit carry the night load efficiently, at the cost of synchronising controls and more switchgear. Choose two when the load varies widely or when an outage of the generator itself is unacceptable.
How long should a sequenced restart take?
Typically 30 to 90 seconds from transfer to the last motor, with the largest or most critical loads first. Longer sequences are fine for comfort loads but not for fire pumps, lifts with passengers or process cooling.
Generators by rating
- Under 15 kVA generators
- 15–50 kVA generators
- 50–125 kVA generators
- 250–500 kVA generators
- 500–1000 kVA generators
- 1–2 MVA generators
- 2 MVA and above generators
Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 19 October 2026.