In short. Between 15 and 50 kVA a generator stops being a backup appliance and becomes a small electrical system. The set now feeds a mix of lighting, IT, pumps, air-conditioning and refrigeration through an automatic changeover and a dedicated essential distribution board. Three answers decide the selection: what must keep running, what starts hardest, and what can stay disconnected. Get those right and balance single-phase loads across the phases, and a 415 V set in this band will serve for years.
Who buys in this range and why
A 15 to 50 kVA set is the standard choice for small offices, bank branches, clinics and nursing homes, restaurants, apartment blocks with one or two lifts, showrooms, schools, cold rooms, packaging units and workshops across Hyderabad and the districts. The owner wants the business to continue through a DISCOM outage, not merely to keep the lights on, so loads must be grouped, the changeover must be automatic, and the set must ride through motor starts while computers stay up.
The three questions that size the set
Running demand
List the loads that must stay on and total their running kW, then convert to kVA at the measured or assumed power factor. For a mixed commercial load 0.8 is reasonable; a cold room or workshop full of motors may be lower. The generator sizing page walks through the method and the margins, and a tidy load inventory makes the rest of the exercise quick.
Starting demand
Find the single hardest start among the essential loads. In this band it is usually a lift motor, a 5 to 10 HP water pump, a cold-room compressor or a packaged air-conditioner. A direct-on-line motor draws six to seven times its full-load current for a second or so, and a 20 kVA set that carries the running load comfortably may dip below the voltage at which contactors and computers hold in. The answer is either a larger set, a gentler starting method, or a rule that the pump does not start until the lift has arrived.
What can stay off
Every load that is not essential should be on the normal distribution board, not the essential board. Comfort air-conditioning, water heaters, decorative lighting and non-critical machines are the usual candidates. Each 1 kVA removed from the essential list is 1 kVA of starting margin gained for free.
Current, phase and balance
I = (kVA × 1000) / (√3 × 415)
In this expression I is the rated line current in amperes for a three-phase set at 415 V, and kVA is the generator rating. A 30 kVA set therefore delivers about 41.7 A per phase, and a 50 kVA set about 69.6 A (illustrative). The current calculator gives the figures for any rating. The per-phase number matters because most loads in this band are single-phase: a 30 kVA set can only supply about 10 kVA on any one phase. The electrician must distribute lights, sockets, split air-conditioners and small equipment so that no phase carries much more than the others. Manufacturers typically allow 10 to 20 percent unbalance between phases; beyond that the alternator heats unevenly, voltage on the lightly loaded phases rises, and single-phase equipment on those phases suffers.
| Premises | Typical essential loads | Hardest start | Usual band (illustrative) |
|---|---|---|---|
| Clinic or diagnostic centre | Lighting, computers, refrigerators, one air-conditioned room | Refrigeration compressor | 15 to 25 kVA |
| Bank branch or office | Lighting, IT, UPS, server room cooling | Server room air-conditioner | 20 to 30 kVA |
| Apartment block | Lift, pumps, common lighting, corridor fans | Lift motor or 7.5 HP pump | 25 to 40 kVA |
| Restaurant | Kitchen exhaust, refrigeration, lighting, billing | Walk-in cold room compressor | 25 to 40 kVA |
| Small workshop or packaging unit | Three or four machines, compressor, lighting | Air compressor on DOL | 40 to 50 kVA |
The changeover becomes part of the system
At this size the generator is almost always started and connected automatically. An AMF panel watches the DISCOM supply, starts the engine on failure, closes the generator contactor or breaker once voltage and frequency are stable, and returns the load with a short delay once the mains has been healthy for a settable time. The changeover must be interlocked so that mains and generator can never be connected together; in this band a four-pole contactor pair or a motorised changeover switch is normal, and the automatic transfer switch page compares the options. The essential distribution board sits downstream of the changeover and feeds only the loads that were agreed in the sizing exercise. Breaker, cable and panel incomer must all be rated for the set’s full current and the fault level at that point.
Fuel, runtime and placement
A set in this band consumes roughly 0.28 to 0.35 litres per kWh at 50 to 75 percent load, so a 40 kVA set carrying 24 kW uses about 7 to 8 litres per hour (illustrative). The baseframe tank on a canopied set usually holds enough for 8 to 12 hours; if outages in your area run longer, or if the set will cover a full shift, plan an external tank with a transfer pump. The set should sit on a level pad with clearance for the radiator discharge, the doors and the exhaust, away from windows and air-conditioning inlets, with an earth electrode for the frame and the neutral. Where the only option is a basement or a room, read the indoor installation guidance before committing, because ventilation and exhaust routing cost more than the set in some buildings.
Standby, prime and the small-load problem
Most sets in this band are bought for standby duty, meaning a few hundred hours a year during outages, and are rated accordingly. If the set will run every day for a shift, for example at a site with unreliable supply in a rural mandal, ask for the prime rating, which is typically about 10 percent below the standby figure, and service by hours. In either case avoid running for long periods below 30 percent load. A 50 kVA set keeping 5 kW of lights on at night accumulates carbon and smokes; the fix is a smaller set for night duty or a periodic load bank test.
Common mistakes
- Sizing from the DISCOM sanctioned load instead of the measured essential load. The set is either far too large and runs lightly loaded, or too small for the one motor nobody listed.
- Putting all the single-phase air-conditioners on the same phase because that wiring already existed. One phase overloads, the others run light, and voltage on them drifts high.
- Omitting the mechanical interlock on a contactor changeover. A welded contact parallels the set with the mains and destroys the alternator.
- Running a long, undersized cable from a canopy at the far end of the compound. Voltage drop at full load trips the UPS and contactors even though the generator is healthy.
- Leaving the AMF return delay at a few seconds. Every flicker on the mains triggers an unnecessary transfer and the engine stops before it has warmed through.
What to send us
Send your list of essential loads with the kW or HP of each motor and how it starts, the number of phases at the premises, the distance from the proposed generator location to the main distribution board, and photographs of the location and the existing panel. Mention whether outages in your area are usually short or run for several hours. If you already have a set, include a nameplate photograph. Use the generator sales enquiry form and we will propose a rating, a changeover arrangement and a cable size.
Questions engineers ask
Is a 25 kVA generator enough for a two-lift apartment block?
Often, if the lifts are modern geared or gearless machines with drives and only one starts at a time, and if the pumps are sequenced. Older DOL lift motors starting together can need 40 kVA or more. Check the lift motor nameplates and the starting method before deciding (illustrative).
Can I run single-phase loads from a three-phase generator?
Yes, provided they are spread across the three phases so the unbalance stays within the manufacturer’s limit, usually 10 to 20 percent. Each phase can carry only one third of the set’s rating.
Should the UPS be on the generator?
Yes, so that batteries recharge and the UPS output continues indefinitely. Keep the UPS below roughly 40 to 50 percent of the set’s rating because its input current is nonlinear and its battery charger adds demand after an outage, and widen the UPS input frequency window for generator supply.
How long does an AMF changeover take?
Typically 10 to 20 seconds from mains failure to generator supply, covering the start delay, cranking, warm-up and contactor closing. Loads that cannot tolerate that gap need a UPS in front of them.
Generators by rating
- Under 15 kVA generators
- 50–125 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 Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.