Three-phase current (Drehstrom) · Glossary

Three-phase current (Drehstrom) is an alternating current system with three sinusoidal voltages. They have the same frequency and are each shifted in time by 120 degrees relative to one another. In public low-voltage grids in Germany and Europe, the nominal voltage is set by DIN EN 60038 at 230/400 volts at 50 hertz. Between one line conductor and the neutral conductor there are nominally 230 volts, and between two line conductors 400 volts. The ratio corresponds to the factor square root of 3 (230 V × 1.732 ≈ 400 V). The three line conductors are usually called L1, L2 and L3 and are often called "phases" in everyday speech. Strictly speaking, they are conductors whose voltages have different phase angles.

Three-phase current is not a separate type of current alongside alternating current, but its three-phase form. A single-phase connection uses one line conductor against the neutral conductor, while a three-phase connection uses all three line conductors. What this means in everyday life is described in the entry Single-phase and three-phase. Two things are technically important: with three time-shifted voltages, electrical power can be transmitted more evenly, and in machines it can be converted into rotary motion particularly well. This is where three-phase current (Drehstrom, literally "rotating current") gets its name.

The 120-degree shift has concrete consequences. In a symmetrical system, the sum of the three voltages is zero at every instant. If all three line conductors are loaded equally, the same applies to the three currents. In the ideal case, no current then flows in the neutral conductor. In addition, the total instantaneous power remains constant over time and does not pulsate at twice the grid frequency as it does with single-phase alternating current. For this reason, three-phase current needs less conductor material for the same transmitted power than three separate single-phase circuits with six conductors, and generators, transformers and motors run more smoothly.

Power in the three-phase system

With a symmetrical load, the rule of thumb for active power is: P = square root of 3 × line-to-line voltage × conductor current × power factor (cos φ). An example: a connection with 16 amperes per line conductor at 400 volts delivers, at cos φ = 1, roughly 1.732 × 400 V × 16 A ≈ 11 kilowatts. This is the same result as three times 230 V × 16 A. A single line conductor with 16 amperes, by contrast, delivers about 3.7 kilowatts. For motors and other inductive loads, the power factor is less than 1. In that case reactive power additionally flows, which loads the lines without performing useful work.

Distinction from alternating current, heavy current and power

In everyday speech, three-phase current is often equated with "Starkstrom" (heavy current). This is imprecise. In electrical engineering and in older standard titles, "Starkstrom" refers to power supply installations, as opposed to "Schwachstrom" (low-voltage/weak current) in communications and signalling technology. In this sense, even a normal single-phase 230-volt socket belongs to heavy-current technology. Colloquially, however, "Starkstrom" usually means a three-phase 400-volt connection, for example for CEE sockets, workshop machines, cookers, instantaneous water heaters or wallboxes. Anyone referring to the type of connection should speak more precisely of single-phase alternating current and three-phase current (Drehstrom).

Three-phase current also does not automatically mean high power. The power that is actually usable depends on voltage, current, fusing, conductor cross-section, continuous load capacity and the connection conditions. A three-phase connection can provide high power because the load is distributed across three conductors. If it is fused small, however, its power also remains limited. The number of phases describes the electrical structure, not the power level alone.

Likewise, three-phase current must be distinguished from energy consumption. A three-phase device can draw a lot of power and still consume little energy if it runs only briefly. Power is measured in kilowatts, energy in kilowatt-hours. The grid must provide the power at the respective moment. For households, by contrast, billing is usually based on the amount of energy over time.

Why power grids are built as three-phase systems

Public power grids are historically and technically based on three-phase current. Generators in power plants, transformers, transmission lines and many industrial consumers operate in three phases. In the generator, the three voltages arise in three windings spatially offset by 120 degrees, past which a rotating magnetic field passes. In the motor, the process runs in reverse: the three offset currents in spatially offset windings create a rotating magnetic field, the rotating field (Drehfeld), and this drives the rotor.

This is why three-phase asynchronous motors are robust, efficient and simple in construction. They need no mechanical commutator with brushes, start up on their own and deliver a more even torque than single-phase motors. Pumps, compressors, conveyor systems, machine tools, fans and many larger drives are designed for three-phase operation. If two line conductors are swapped, the rotating field reverses and with it the direction of rotation of the motor. For this reason, attention is paid to a clockwise rotating field for three-phase sockets and machines.

Three-phase current also has advantages for the grid itself. Three-phase transformers manage with less core material than three single-phase transformers. High power can be transmitted over long distances at high voltage levels. In the distribution grid, many single-phase consumers can be distributed across the three line conductors so that the load remains as even as possible. If it is uneven, this is called phase imbalance. It leads to current in the neutral conductor, additional losses, voltage deviations between the conductors and a higher load on individual items of equipment.

Three-phase current at the house connection

In Germany, the house connection is usually three-phase. This does not mean that every socket supplies three-phase current. Normal household sockets are connected single-phase and use one line conductor, the neutral conductor and the protective conductor. Larger consumers, by contrast, are connected in three phases, such as electric cookers, instantaneous water heaters, wallboxes, larger heat pumps or commercial machines. In the house installation, the circuits are distributed as evenly as possible across L1, L2 and L3.

The technical connection rule VDE-AR-N 4100 limits the asymmetry in customer installations on the low-voltage grid. According to the VDE FNN guidance on this, devices with a rated power of up to 4.6 kVA may be connected single-phase, while larger ones in principle only three-phase. 4.6 kVA corresponds to 20 amperes at 230 volts. Additional conditions apply to single-phase generation systems, storage systems and charging equipment. Per line conductor and energy flow direction, the sum should not exceed 4.6 kVA, and in the entire customer installation at most 3 × 4.6 kVA = 13.8 kVA. If this is exceeded, a balancing device is required. The limit is intended to prevent many similar devices in a street from loading the same line conductor. In individual cases, the technical connection conditions (TAB) of the respective grid operator also apply.

The difference becomes particularly clear with electromobility. A wallbox can charge single-phase or three-phase. At 16 amperes per conductor, about 11 kilowatts are possible three-phase and about 3.7 kilowatts single-phase. Because of the phase imbalance limit, a maximum of 4.6 kVA is permissible single-phase. For notification, in Germany § 19 paragraph 2 of the Low-Voltage Connection Ordinance (Niederspannungsanschlussverordnung, NAV) applies: every charging facility for electric vehicles must be notified to the grid operator before commissioning. If the total rated power exceeds 12 kVA per electrical installation, the prior approval of the grid operator is additionally required, for example for a 22 kW wallbox. The grid operator must then respond within two months. Whether three-phase charging actually takes place also depends on the on-board charger in the vehicle.

The phase structure also plays a role in photovoltaics. An inverter turns the direct current of the solar modules into grid-synchronous alternating current. Small devices up to 4.6 kVA may feed in single-phase, while larger ones feed in three-phase and distribute the power across all three line conductors. To keep the voltage in the local grid within the permissible limits, inverters, grid connection point and cable lengths must be considered together. Even a correctly operating system can raise the voltage in a weak grid if many systems feed in at the same time.

Common misunderstandings

The three phases in the house are not three separate power grids. They belong to a common system with a fixed phase relationship. How heavily one line conductor is loaded affects the voltages of the others via the common neutral conductor and the transformer, especially in the low-voltage grid.

The term "phase" is also used imprecisely. Strictly speaking, it denotes the temporal position of a periodic quantity; in everyday speech it usually means the line conductor itself. This shorthand is practical but easily leads to errors when it comes to phase shift, reactive power, direction of rotation or phase imbalance.

A three-phase connection also does not mean that any number of additional loads can be connected without checking. Limits are set by the house connection fuse, the cable to the building, the local network transformer and the low-voltage lines. Grid operators therefore look not only at the individual device but at the simultaneity of many consumers in a grid section. Heat pumps, charging equipment and direct heating change this simultaneity because they draw high power over longer periods.

Finally, three-phase current is not a relic of large central power plants. A power system with many decentralised generators, battery storage systems and converters also needs a defined three-phase alternating current structure, as long as consumers, protection technology and distribution grids are designed for it. The three phases are then increasingly generated by power electronics. Grid-following inverters synchronise with the voltage, frequency and phase angle of the grid. The task thus shifts from rotating machines to electronics, while the three-phase basic structure remains.

Significance for grid operation and electrification

Three-phase current shows that power supply is more than quantities of energy. For grid operation, instantaneous values count: voltage, current, frequency, phase angle, the load on the individual conductors and the thermal limits of the equipment. An annual consumption in kilowatt-hours says little about whether a grid section can cope with another wallbox or heat pump. What is decisive is when which power flows on which line conductor.

With the electrification of heat, transport and industry, these questions are becoming more important. If several electric cars charge simultaneously in the evening, the load on the local network transformer rises. If many single-phase devices are distributed unfavourably, phase imbalance grows. If photovoltaic systems feed in strongly at midday and heat pumps run in the evening, the direction and level of power flows change over the course of the day. If control and load distribution are not sufficient, a grid reinforcement may become necessary.

Institutionally, the term therefore touches on connection rules, notification obligations, technical connection conditions, protection concepts and metering. In Germany, notification and approval obligations arise, among other things, from the NAV, and technical details from VDE-AR-N 4100 and the grid operators' TAB. Electrical contractors must design conductors, fuses and protective devices to suit the load. Modern meters usually record energy as a net figure across all three phases, while in the grid each line conductor is physically loaded individually. This difference between billing and grid physics explains some of the debate about self-consumption, feed-in and local bottlenecks.

Three-phase current is thus not merely a type of connection for powerful devices, but the three-phase basic form of the public alternating current grid. Anyone talking about house connections, phase imbalance, wallboxes, heat pumps, PV inverters or grid expansion is operating within this structure. The discussion only becomes precise when phase, voltage, power, energy consumption and grid load are kept apart.

Sources

Netzanschluss – Regelungen für Ladeeinrichtungen in der Niederspannung — Bundesnetzagentur; unbekannt. Abschnitt Regelungen für Ladeeinrichtungen in der Niederspannung
§ 19 NAV – Betrieb von elektrischen Anlagen, Verbrauchsgeräten und Ladeeinrichtungen, Eigenanlagen — buzer.de (Wiedergabe der Niederspannungsanschlussverordnung); unbekannt. § 19 Absatz 2 Sätze 1 bis 4
VDE FNN Hinweis: Anforderungen für den symmetrischen Anschluss und Betrieb nach VDE-AR-N 4100 — VDE Forum Netztechnik/Netzbetrieb (FNN); unbekannt. Abschnitte zum symmetrischen Anschluss, Grenzwerte 4,6 kVA und 13,8 kVA
VDE FNN Hinweis: Errichtung von mehreren Netzanschlüssen am Niederspannungsnetz in einem Gebäude und auf einem Grundstück — VDE FNN (bereitgestellt von AllgäuNetz); 2024-06-24 (Datum laut Dateiname). Abschnitt zu den Netzkenngrößen

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Edited by eCMS on 11.11.2026 19:31