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ITFrequently Asked Questions · General
What do single-phase and three-phase power mean?
Single-phase power (230V) is traditionally used in homes and covers the basic needs of a residential installation. Three-phase power (400V), on the other hand, offers more power and stability for high energy demands, such as heat pumps, EV chargers and so on.
A single-phase supply has one phase and the neutral, with 230 V between them. A three-phase supply has three phases, offset in time from one another, plus the neutral: the voltage from each phase to neutral is again 230 V, while between any two phases it is 400 V. So in a home with a three-phase supply, ordinary sockets are still 230 V; the circuits are simply shared out across the three phases.
The advantage of three-phase power is that it carries more power with less current per conductor and drives large motors more smoothly. That is why it is used by industry, office buildings and homes with large loads.
This matters in practice for a UPS. Small UPS units for computers and small offices are single-phase. Larger ones, for server rooms or industrial equipment, may have a three-phase input and a single-phase or three-phase output.
A common mistake in three-phase installations is an uneven distribution of loads: one phase is overloaded and its breaker trips, while the others are almost idle. Before adding a large load, such as a UPS or an EV charger, ask an electrician to check the phase balance.