Guides · Choosing a generator
Single-Phase or Three-Phase: Matching the Generator to the Supply
Most South African homes have a single-phase supply: one live conductor at 230 V to neutral. Most businesses, factories and larger properties have three-phase: three live conductors at 230 V each to neutral and 400 V between any two. A three-phase generator can feed single-phase circuits if they are spread evenly across its phases, but a single-phase generator cannot run a three-phase board or three-phase motors.
Technical guide · Last reviewed 16 October 2026
How single-phase and three-phase power differ
Alternating current in South Africa goes through 50 complete cycles a second (50 Hz), reversing direction twice in each. A single-phase supply carries one of these waves on a live conductor, with the neutral as the return. A three-phase supply carries three waves on three live conductors, each a third of a cycle (120°) behind the one before. From any phase to neutral you measure 230 V; between two phases you measure about 400 V, because the waves are out of step (230 V × √3 ≈ 400 V).
Because the three waves are staggered, the total power delivered to a balanced load stays steady through each cycle instead of pulsing, and in a motor the three phases together create a magnetic field that rotates. That is why three-phase motors start on their own and run smoothly, and why three-phase delivers more power for the copper in the cable. A single-phase supply produces no rotating field, so single-phase motors need a start winding or capacitor to get turning.
Single phase and three phase compared
| Aspect | Single phase | Three phase |
|---|---|---|
| Voltage | Single phase230 V live to neutral | Three phase230 V each phase to neutral; 400 V between phases |
| Capacity per phase | Single phaseThe full rating on one phase | Three phaseAbout a third of the rating on each phase |
| Motors | Single phaseSingle-phase motors only | Three phaseThree-phase motors, which start and run more smoothly |
| Typical generators | Single phasePortable units and smaller home standby sets | Three phaseMost sets from small commercial sizes upwards |
| Changeover | Single phaseSwitches live and neutral | Three phaseSwitches three phases and neutral |
| Main concern | Single phaseTotal load and starting surges | Three phaseLoad balance across phases and phase rotation |
Who has which
Which buildings are single-phase or three-phase
Most homes
Suburban houses, townhouses and flats are usually single-phase. Larger homes with heavy pumps, a workshop or several air conditioners are sometimes three-phase.
Smallholdings and farms
Often three-phase, to run borehole, irrigation and workshop motors. A single-phase generator there may only cover the house.
Shops and offices: Commercial generator services
A small shop may be single-phase; most offices, restaurants and multi-tenant buildings are three-phase, with tenants spread across the phases.
Factories and large plant: Industrial generator services
Three-phase supplies, with motors, compressors, lifts and large UPS systems that need all three phases from the generator.
Balancing single-phase loads on a three-phase set
A three-phase generator’s rating assumes the load is shared fairly evenly, so each phase can carry only about a third of the total. A 30 kVA set, for example, supplies about 43 A per phase. If a building’s kitchen, geysers and lights all sit on one phase drawing 60 A, that phase trips or sags while the other two idle, even though the total looks comfortable.
Uneven loading also unbalances the voltages: the heavily loaded phase sags, the lightly loaded ones rise, more current flows in the neutral, and three-phase motors run hot. Alternator makers limit how much unbalance their machines tolerate.
The fix is usually to move circuits between phases, not to buy a bigger set. A technician measures each phase’s current under real load, ideally during an outage with the whole building on the generator, and the board is then rebalanced by a qualified electrician or technician, spreading big single-phase loads such as geysers and stoves across the phases.
Phase rotation
Phase rotation, or phase sequence, is the order in which the three phases peak: L1, then L2, then L3. A three-phase motor turns in the direction set by that order. If the generator’s sequence differs from the mains, every three-phase motor runs backwards when the generator takes over: pumps deliver little or nothing, fans blow the wrong way, and some compressors, such as scroll types, can be damaged.
Rotation is checked with a phase-sequence meter when a set is installed, and again after any work on the alternator, the cabling or the changeover panel. Some equipment has a phase-sequence relay that refuses to start if the order is wrong, which can look like a fault in the equipment rather than the supply.
Why a single-phase set cannot run a three-phase board
A single-phase generator has one live output. It cannot produce the 400 V between phases or the rotating field that three-phase motors need, and a three-phase motor fed from one phase will not start and quickly overheats. Linking a board’s phases together so one supply feeds them all leaves 400 V equipment without a supply and creates a short circuit between phases if the mains returns. It must never be done.
What does work on a three-phase property is a single-phase set feeding a dedicated essential board, such as lights, plugs, IT and the alarm, through a changeover that switches those circuits only. Three-phase motors, lifts and large compressors wait for the mains.
Connecting a generator
What the phases mean for the changeover
A changeover, manual or automatic (ATS), usually switches every live conductor and the neutral between mains and generator: two poles on a single-phase supply, four on a three-phase one. It is interlocked so the two supplies can never be connected together, which protects the generator and anyone working on the municipal or Eskom network.
An ATS on a three-phase supply should watch all three mains phases. Load shedding cuts all three together, which a working panel always detects, but losing one phase, through a failed fuse or a damaged or stolen cable, leaves part of the building dark and three-phase motors at risk, and a panel that senses only one phase may not start the set. Changeover wiring is technician work; see changeover and ATS problems and generator installation.
- Single phase: live and neutral switched
- Three phase: three phases and neutral switched
- Interlocked, so mains and generator never meet
- Mains sensing on all three phases
Questions that decide which you need
- Is the supply to the building single-phase or three-phase?
- Does anything that must run on the generator have a three-phase motor, such as a borehole pump, lift, compressor or machine?
- Can the essential loads be grouped on one board, or must the whole building run?
- How much load sits on each phase today, measured under real use?
- Will you add three-phase equipment later?
Once you know the answers, the kVA sizing guide shows how to size the set. For a site check, contact us.
FAQ
Frequently asked questions
Is my home single-phase or three-phase?
Most homes are single-phase. A main switch with three linked levers, or a three-phase meter, suggests three-phase. Your electricity account may also say, and an electrician can confirm it.
Can a three-phase generator power single-phase appliances?
Yes. Each phase to neutral provides 230 V, so single-phase appliances are spread across the three phases. On a single-phase house, though, only one phase’s share of the rating is usable unless the alternator is reconnected for single-phase output, which some allow at a reduced rating. A technician can check what yours supports.
Why does one phase on our generator trip when the others are lightly loaded?
That phase is carrying more than its share. Moving some circuits to the other phases usually solves it. See shutting down or tripping.
Can I run a three-phase motor from a single-phase generator?
Not directly. A small three-phase motor can sometimes run from single phase through a suitable variable-speed drive, but that is a design decision for an electrician. Larger motors need a three-phase set.
What happens if only one mains phase fails?
Part of the building goes dark and three-phase motors can overheat. A changeover panel that senses all three phases starts the generator; one that senses a single phase may not. A technician can check how your panel senses the mains.
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