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Glossary · Energy industry & energy transition

Single-phase and three-phase

Single-Phase Three-Phase Three-Phase Current Service connection wallbox heat pump unbalanced load charging power distribution grid

Single-phase and three-phase describe how an electrical device is connected to the low-voltage grid. A single-phase device uses one of the three line conductors and the neutral conductor. A three-phase device uses all three line conductors of a three-phase system. In the European household grid, the nominal voltage between one line conductor and the neutral conductor is 230 volts, and between two line conductors 400 volts. This is why small household appliances usually run on a single phase, while larger consumers and generators are often connected on three phases.

A kettle, a television, a computer or a single Schuko socket are generally single-phase. An electric cooker, many heat pumps, larger motors, three-phase wallboxes and most inverters of larger photovoltaic systems, on the other hand, use all three phases. The difference concerns not only the type of connection. It also affects the possible power, the load on individual cables, the fusing and the distribution grid.

Phase, voltage and power

In the low-voltage grid, a phase is a current-carrying line conductor (L1, L2, L3). A typical German service connection has three line conductors and one neutral conductor arriving. Depending on the grid type, the protective conductor function is split or earthed in the building. The alternating voltages of the three line conductors are offset from each other in time by one third of a cycle, i.e. 120 degrees. When all three phases are loaded equally, the currents in the neutral conductor largely cancel out. In this way, a large amount of power can be transmitted with comparatively low currents per conductor.

For active loads, power is approximately voltage times current. A single-phase device at 230 volts and 16 amperes draws about 3.7 kilowatts. This is why many single-phase charging solutions for electric cars have exactly this rating. For three-phase, one calculates three times 230 volts times the current per phase, or equivalently the square root of three times 400 volts times the current per phase. With 16 amperes per phase, this gives about 11 kilowatts; with 32 amperes per phase, about 22 kilowatts. However, this only applies if the service connection, cables, fusing, device and grid operator allow it.

The kilowatt figure alone therefore says little about the load on the grid. At 11 kilowatts three-phase, about 16 amperes flow on each phase. If one were to draw 7.4 kilowatts single-phase, i.e. 32 amperes on a single phase, that one phase would be loaded twice as heavily. The other phases, by contrast, would remain unused. Such single-phase operation is generally not permitted on the German public low-voltage grid without additional measures (see below).

Distinction from connected load, charging power and electricity consumption

Single-phase or three-phase describes the electrical method of connection, not electricity consumption. Consumption is a quantity of energy in kilowatt-hours. Power in kilowatts indicates how much energy is converted per unit of time. An example: an electric car charges 30 kilowatt-hours once single-phase at 3.7 kilowatts and once three-phase at 11 kilowatts. The amount of energy is similar in both cases, apart from slightly different charging losses. What differs are the charging time (about eight hours versus just under three), the current per phase and the load on the grid during charging.

The connected load is also not the same as the power actually used. A wallbox designed for 11 kilowatts often charges at less. This depends on the vehicle, the state of charge, the temperature, the charging settings and any load management. A three-phase heat pump does not continuously draw high power, and a single-phase device can briefly load one phase heavily. Three-phase also does not automatically mean more efficient in the sense of lower energy consumption. Efficiency depends on the device, the operating point and the application.

Phase imbalance as a practical grid issue

Phase imbalance arises when the three phases are loaded unequally. A certain unequal distribution is normal because many household appliances are single-phase. Electrical installers therefore distribute the circuits of a building as evenly as possible across L1, L2 and L3. It becomes critical when larger single-phase consumers or generators operate permanently on the same phase.

Phase imbalance has tangible physical consequences. On a heavily loaded phase, the voltage drops more. A balancing current flows in the neutral conductor, and the neutral point shift can also displace the voltages of the other phases. Local grid transformers and cables are loaded unevenly. The low-voltage grid is designed for usual household asymmetries, but not for arbitrary concentrations of single-phase high loads.

In Germany, the technical connection rule VDE-AR-N 4100 governs symmetry. Grid operators implement it through their technical connection conditions. The key parameters for installations on the public low-voltage grid:

  • Devices with a rated power above 4.6 kVA must in principle be connected three-phase. Devices up to 4.6 kVA may be connected single-phase, but should be distributed evenly across the line conductors.

  • For single-phase charging equipment, generation installations and storage systems, a total limit of 4.6 kVA per line conductor and direction of energy flow applies. For the entire customer installation, this gives a maximum of 3 × 4.6 kVA = 13.8 kVA.

  • If these limits are exceeded, a symmetry device is required that limits the imbalance at the grid connection point. It can also be built into the charging equipment. Individual grid operators generally require a symmetry device for charging equipment above 4.6 kVA.

For single-phase charging, the limit is therefore 4.6 kVA, i.e. about 20 amperes. In practice, 3.7 kilowatts at 16 amperes is usually common. Three-phase charging at 11 kilowatts distributes the power evenly and is therefore usually more favourable for the distribution grid than a powerful single-phase charging point.

Significance for wallboxes, heat pumps and photovoltaics

The difference is particularly visible with wallboxes. The wallbox provides three phases, but the on-board charger in the vehicle decides how many of them are used. Some vehicles charge three-phase, others only single- or two-phase. A car charging single-phase uses only one phase even at an 11-kilowatt wallbox and therefore usually at most 3.7 kilowatts. The wallbox rating alone thus does not determine the actual charging power. With several charging points, it can make sense to assign the phases in rotation so that single- and two-phase charging vehicles do not all end up on L1.

With heat pumps, the type of connection depends on the electrical design. Small units can be operated single-phase; larger ones usually run three-phase. Electric heating elements and auxiliary units often have their own connection conditions. For the grid, it is not only the seasonal performance factor that counts, but also when which power is drawn. Many heat pumps are easily manageable over the year, but can jointly contribute to peak load during cold hours.

Photovoltaic systems and battery storage systems also feed in single- or three-phase, or charge and discharge accordingly. Plug-in solar devices and small inverters are usually single-phase; larger rooftop systems generally use three-phase inverters. For phase imbalance, the question applies in mirror image: feed-in can also load the phases unequally. This is why the symmetry limits apply separately by direction of energy flow. In streets with many single-phase systems on the same phase, this becomes relevant for voltage maintenance in the distribution grid.

Connection rules and the role of the grid operator in Germany

Legally, the type of connection takes effect mainly through the Low-Voltage Connection Ordinance (Niederspannungsanschlussverordnung, NAV) and the technical connection rules. Under § 19 paragraph 2 NAV, every charging device for electric vehicles must be notified to the grid operator before commissioning, regardless of its power. According to the Bundesnetzagentur (Federal Network Agency), this also applies to charging at a single- or three-phase socket. If the total rated power of the charging devices exceeds 12 kVA per electrical installation, commissioning additionally requires the grid operator's prior consent. The grid operator must respond within two months. If it refuses, it must state the obstacle, possible remedies and the time required.

A single 11-kilowatt wallbox is therefore subject to notification but not to consent. Two such wallboxes at the same installation together exceed 12 kVA and require consent. According to the Bundesnetzagentur, insufficient grid capacity can delay the connection but is not a reason for definitive refusal. In addition, since 2024 the rules on controllable consumption devices under § 14a EnWG have applied to newly commissioned wallboxes, heat pumps, air conditioners and grid-drawing storage systems with more than 4.2 kW. They depend on power, not on the number of phases.

Why the distinction is becoming more important for the power system

In the past, household load consisted of many small single-phase consumers with limited power that ran at the same time more or less by chance. With electromobility, heat pumps, home storage and photovoltaics, devices in the range of several kilowatts are becoming commonplace. Bottlenecks arise less from the annual amount of energy than from simultaneity, power level and phase distribution. A local grid can transport enough energy over the year and still reach its limits in individual hours or on individual phases. When grid operators limit power, this is due to the physical limits of cables, fuses, transformers and voltage bands, not to a general electricity shortage.

Typical misinterpretations

  • Three-phase does not mean higher consumption: it enables higher or more evenly distributed power, but says nothing about the kilowatt-hours per day or year.

  • A three-phase service connection does not mean that every device uses all phases: socket circuits each sit on one phase, and an electric cooker distributes its hotplates across several phases.

  • A three-phase wallbox does not guarantee three-phase charging: what matters is the on-board charger in the vehicle.

  • It is not only wallboxes above 11 kW that must be registered: under the NAV, every charging device is subject to notification. The consent requirement begins at more than 12 kVA total power per installation.

Single-phase and three-phase therefore do not denote comfort or efficiency classes, but the distribution of electrical power in the three-phase grid. For the power system, it matters not only how much energy is used, but also on which phase, at what power and at what time it flows.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 19:42

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