In short. In the 50 to 125 kVA band the running load is rarely the problem; the start of one lift, pump, compressor or chiller is. Converting kW to kVA gives the steady-state demand but says nothing about starting current, voltage dip or recovery. List the running load, find the largest motor and its starting method, check the transient against the alternator’s capability, and only then choose between a larger set, a softer start or a sequenced load.
Where this band sits
Sets of 50 to 125 kVA serve mid-sized commercial buildings, hospitals with a few wards, hotels, multi-storey apartments, schools, cold storages, printing and packaging units, small foundries and most light industry in the Telangana industrial estates. The loads are a mix of lifts, water and fire pumps, air compressors, packaged air-conditioning, refrigeration, UPS systems and lighting. Almost every one of those categories contains a motor, and the biggest of them decides the set.
Do not convert kW to kVA and stop
The relationship kVA = kW / PF gives the apparent power the alternator must carry continuously, and the kW to kVA calculator does that arithmetic instantly. It does not tell you whether a 100 kVA set can start a 30 kW compressor while 60 kVA of other load is connected, how far the voltage will fall when it tries, whether the contactors will hold and the UPS stay on line, or how quickly frequency recovers. Those are transient questions.
Starting kVA ≈ (motor kW / (η × PF)) × starting current multiple
Here motor kW is the shaft rating, η is the motor efficiency, PF is the running power factor and the starting current multiple is the ratio of locked-rotor to full-load current, typically 6 to 7 for a standard induction motor started direct-on-line. A 15 kW motor at η 0.9 and PF 0.85 draws about 19.6 kVA running and about 118 kVA at a multiple of 6 when started DOL (illustrative). A star-delta starter reduces the starting kVA to roughly one third, about 39 kVA, with starting torque reduced in the same proportion; a soft starter can be set between those values; a variable-frequency drive starts the motor at little more than running current. The starting kVA page covers the detail.
| Starting method | Starting kVA relative to DOL | Starting torque | Where it suits generator supply |
|---|---|---|---|
| Direct-on-line | 100 percent | Full | Small motors, or a set much larger than the motor |
| Star-delta | About 33 percent | About 33 percent | Pumps and fans that start unloaded |
| Soft starter | Adjustable, often 40 to 60 percent | Reduced in proportion to current squared | Compressors and conveyors needing a controlled ramp |
| Variable-frequency drive | Close to running kVA | Full, controllable | Any motor, but adds harmonic current |
What voltage dip the set will produce
When a motor starts, the alternator sees a large, low power factor current and its terminal voltage drops until the AVR forces excitation up. The size of the dip depends on the ratio of starting kVA to the alternator’s rating and on its transient reactance. As a screening rule, an alternator can supply a motor-starting kVA of roughly two to three times its rating with a voltage dip in the region of 20 to 30 percent, and about equal to its rating for a dip near 10 percent; the manufacturer’s motor-starting curve gives the exact figure. Contactors drop out around 70 to 80 percent of nominal voltage and UPS systems may transfer at 85 to 90 percent, so the acceptable dip is set by the most sensitive load already connected, not by the motor. The voltage dip page explains how to read the curves.
A worked selection (illustrative)
A hotel has 55 kVA of running essential load and a 22 kW packaged air-conditioner compressor that starts direct-on-line. Running kVA alone suggests a 75 kVA set. The compressor start demands about 170 kVA at a multiple of 6, and the 75 kVA set would dip well beyond 30 percent, dropping the lift contactors. The options are a 160 to 200 kVA set, which will then run at 30 percent load most of the time, or a soft starter on the compressor limiting its start to about 70 kVA, which a 100 kVA set with a well-chosen alternator can accept with a dip near 20 percent. The second option is cheaper to buy and to run.
Sequence, frequency and the engine
The alternator handles voltage; the engine handles frequency. A block load forces engine speed down until the governor adds fuel, and turbocharged engines in this band accept a single step of roughly 50 to 70 percent of rated kW within their ISO 8528 performance class. If the AMF panel connects the whole essential board at once after a mains failure, the engine sees every motor starting together. Staging the restart, with lifts first, pumps a few seconds later and air-conditioning last, often costs nothing more than timers in the distribution board and lets a smaller set do the job. See transient response for the engine side of the problem.
UPS, drives and the alternator
UPS rectifiers and variable-frequency drives draw non-sinusoidal current. On the DISCOM network that distortion is absorbed by a stiff source; on a 100 kVA generator with a few percent reactance it distorts the voltage waveform, heats the alternator and can confuse the AVR. The usual practice is to keep nonlinear load below about 40 to 50 percent of the set’s rating, or to specify an alternator one frame size larger with a low-reactance winding and a PMG-excited AVR. The UPS and nonlinear loads page sets out the checks.
Installation points that scale with the set
A 125 kVA set delivers about 174 A at 415 V, so the generator breaker is now an MCCB of 200 A or more, the cable to the panel is typically 95 to 120 sq mm copper or an aluminium equivalent depending on length, and the changeover is a contactor pair or a motorised switch in that class. The canopy is roughly 3.5 to 4 metres long and the radiator discharge needs 2 to 3 metres of clear space. Fuel consumption at 75 percent load is in the region of 20 to 25 litres per hour, so a 12 hour outage needs about 300 litres (illustrative).
Common mistakes
- Choosing the set from the kW to kVA conversion and a 25 percent margin. The largest compressor stalls or trips the lift contactors on every start.
- Specifying a VFD to solve starting and forgetting the harmonic current it adds. The alternator runs hot and the AVR hunts at light load.
- Letting the AMF panel reconnect the whole essential board in one step. The engine bogs down, frequency falls and the UPS goes to battery immediately after the generator has taken over.
- Buying a set two sizes up to be safe and running it at 25 percent load for years. Fuel efficiency is poor, carbon builds up and overhaul comes early.
What to send us
Send a load list that separates running load from the motors, with each motor’s kW or HP, its starting method and whether it can be sequenced, plus details of any UPS and drives with their ratings. Add the distance from the generator location to the main panel, photographs of the site and the existing switchboard, and the nameplate of any set being replaced. Send it through the generator sales enquiry form and we will return a transient check and a recommended rating.
Questions engineers ask
What size generator do I need for a 30 HP DOL motor plus 40 kVA of other load?
The 22 kW motor starts at roughly 170 kVA, so with 40 kVA already connected a 125 kVA set will dip too far for most contactors. Fit a star-delta or soft starter and a 100 to 125 kVA set becomes workable; keep DOL and you are looking at 200 kVA or more (illustrative).
Can one generator in this band run a lift, a fire pump and the building lighting?
Yes, if the fire pump start is checked separately because it must be able to start with everything else connected. Many Indian installations put a dedicated diesel engine on the fire pump, which removes the biggest start from the generator.
Why does my 100 kVA set trip the UPS when the compressor starts?
The UPS input voltage window is narrower than the dip the compressor produces, so the UPS transfers to battery and may alarm. Reduce the dip with a soft starter, widen the UPS input tolerance for generator mode, or move the compressor start later in the restart sequence.
Is an oversized alternator better than a larger engine?
For motor starting, yes. A larger alternator on the same engine reduces voltage dip without the fuel penalty of a bigger engine, because voltage dip is an alternator property and frequency dip is an engine property.
Generators by rating
- Under 15 kVA generators
- 15–50 kVA generators
- 125–250 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.