In short. A single-phase generator system supplies 230 V at 50 Hz through one phase conductor and a neutral, matching the DISCOM connection of homes, shops, clinics and farms in India. It is the correct choice where every load is single-phase and total demand is small, typically below about 15 to 25 kVA. Above that, or where any three-phase motor exists, a three-phase set is cheaper per kVA, and feeding single-phase load from it needs care with phase balance.
Where single-phase supply belongs
Most DISCOM domestic and small commercial connections in Telangana and Andhra Pradesh are single-phase up to a sanctioned load of a few kilowatts, after which the utility insists on a three-phase service. A generator should normally mirror the connection it backs up: a single-phase house, a pharmacy, a mobile tower shelter, a borewell with a single-phase submersible pump or a small clinic with lighting, fans, refrigerators and a few air conditioners is a single-phase application. The loads are plugged into 230 V sockets, the distribution board has single-pole breakers and a neutral, and a single-phase set connects to it through a simple two-pole changeover switch. Larger single-phase sets exist, but above about 25 kVA the alternator becomes disproportionately large and expensive and the range of models narrows sharply. The smaller petrol and diesel sets in the under 15 kVA range cover most of this market.
Current, and why it climbs so fast
The defining characteristic of single-phase supply is that all the current flows through one conductor and returns through the neutral, so for a given power the current is much higher than on a three-phase system at 415 V.
I = (kVA × 1000) ÷ V
Here I is the current in amperes, kVA is the apparent power and V is the phase-to-neutral voltage, 230 V in India. A 10 kVA single-phase set therefore delivers about 43.5 A at full load, and a 25 kVA set about 109 A (illustrative). The same 25 kVA as a three-phase set at 415 V would deliver only 35 A per phase. That difference is why cables, changeover switches and breakers for a single-phase set are heavier than a three-phase user expects, and why voltage drop on a long run to a farm pump is often the real limit on how far the set can be from the load. The current calculator and the cable sizing guide give the figures for a specific installation.
Motor starting on a small set
The single-phase loads that cause trouble are motors: submersible pumps, air conditioner compressors and refrigerators. A single-phase induction motor draws five to seven times its running current for the half second or so it takes to reach speed, and a small alternator has very little reserve to supply it. The result is a deep voltage dip, flickering lights and in bad cases a stalled motor and a tripped set. As a working rule, the generator should be able to supply the starting kVA of the largest motor plus the running load of everything else that is on at the moment it starts. A 2 hp (1.5 kW) submersible pump may need 8 to 10 kVA at the instant of starting, which is why a 5 kVA set that comfortably runs the house cannot start the borewell (illustrative). Soft starters and inverter-driven air conditioners reduce the problem substantially. The motor starting page covers the calculation and the alternatives.
| Typical single-phase load | Running demand | Starting demand | Implication for set size |
|---|---|---|---|
| LED lighting and fans, whole house | 0.5 to 1.5 kW | Same as running | Negligible |
| 1.5 ton split air conditioner, fixed speed | About 1.8 kW | 8 to 12 kVA for a fraction of a second | Usually the sizing event |
| 1.5 ton inverter air conditioner | 0.5 to 1.8 kW | Close to running | Allows a smaller set |
| 1 hp single-phase pump | About 1 kW | 5 to 6 kVA | Start it before other loads |
| Domestic refrigerator | 0.15 to 0.3 kW | 1 to 2 kVA | Several restarting together add up |
Using a three-phase generator for single-phase loads
When the demand is above about 15 kVA, a three-phase set is often the better buy even if all the loads are single-phase, but it cannot simply be connected to one phase. A three-phase alternator is rated per phase: a 30 kVA set can deliver only about 10 kVA from each phase, and loading one phase to 30 kVA would overheat that winding and the neutral while the other two idle. The loads must be split across the three phases as evenly as the circuits allow, which means the distribution board is wired as three-phase with the single-phase circuits distributed among the phases. Manufacturers state a maximum permissible unbalance, often in the region of 10 to 20 percent between phases, and some alternators can be reconnected at the factory or by a qualified technician for dedicated single-phase output at a reduced rating. The three-phase generator systems page explains the balance and neutral questions in more depth.
Installation points that are often skipped
Small sets are often installed by the house electrician rather than a generator specialist, and the same faults recur. The changeover switch must break both the phase and the neutral so that the generator neutral and the DISCOM neutral are never connected together, and so that the set can never back-feed the street supply. The set needs its own earth electrode bonded to the frame and to the neutral in the way the manufacturer specifies. The exhaust must discharge outdoors away from windows, since carbon monoxide from a small set running in a stairwell or a covered yard is a recurring cause of fatalities. Fuel should be stored in a closed container away from the hot exhaust. For a set used only in outages, battery, oil level and fuel freshness need a monthly check like any larger standby machine.
Common mistakes
- Sizing the set on the sum of running loads and ignoring the air conditioner compressor; the set stalls every time the compressor cycles on.
- Connecting a three-phase set to a single-phase board through one phase only; that winding overheats at a third of the nameplate rating.
- Using a single-pole changeover that leaves the neutrals joined; the generator can back-feed the DISCOM line and endanger a lineman.
- Running a long undersized cable to a farm pump; the voltage at the pump falls so far under starting current that the motor never reaches speed.
- Running the set in a closed room or under a staircase for noise reasons; exhaust gases enter the living space.
What to send us
Send a list of the appliances or equipment to be run together, with the rating of any pumps, air conditioners or compressors and whether they are inverter type, a photograph of the DISCOM meter and the main distribution board, and the distance from the proposed generator position to the board. Mention whether a three-phase connection is planned in future. Use the generator sales enquiry and we will recommend a single-phase or three-phase set, the size and the changeover arrangement.
Questions engineers ask
What size single-phase generator runs a house with two air conditioners?
Two fixed-speed 1.5 ton air conditioners with lighting, fans and a refrigerator usually call for a 7.5 to 10 kVA single-phase set, with the sizing driven by compressor starting rather than running load. With inverter air conditioners a 5 to 6 kVA set is often enough, but the exact figure depends on the appliance ratings.
Can I convert a three-phase generator to single-phase?
Some alternators can be reconnected for single-phase output, typically delivering about two thirds of the three-phase rating, but this must be done according to the manufacturer’s connection diagram by a qualified technician. Many smaller alternators cannot be reconnected and must instead have the single-phase loads balanced across all three phases.
Why does the voltage drop when the pump starts on my generator?
A single-phase motor draws several times its running current while accelerating, and a small alternator cannot hold its voltage against that surge. The dip is worse if the cable to the pump is long or thin. A larger set, a shorter or heavier cable, or a soft starter on the pump are the remedies.
Does a single-phase generator need a neutral earth?
Yes. The generator frame must be connected to an earth electrode and the neutral earthed as the manufacturer specifies, so that an earth fault on the generator supply trips the protection and the enclosure cannot become live. The earthing of the DISCOM supply does not protect the installation when it is running on the generator.
Is a single-phase or three-phase generator cheaper for 20 kVA?
At 20 kVA a three-phase set is usually cheaper to buy and far easier to source, because single-phase alternators above about 15 kVA are specialist items. The saving is worthwhile only if the single-phase loads can be distributed across the three phases reasonably evenly.
Generators by rating
- Under 15 kVA generators
- 15–50 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.