How much voltage is 3-phase

Three-phase current in simple terms

Three-phase alternating voltage in detail (3)




Despite the only 4 conductors, three full-fledged 400V ~ voltage sources are available. Strictly speaking, if the 400V ~ voltage sources were used exclusively, you would not need a neutral conductor at all, because 400V ~ is obtained precisely by using two 230V ~ lines as a voltage source.

This has been explained here: 400V AC voltage in detail.

You get the 3 x 400V ~ simply by staggering the 3 x 230V ~, so to speak for free.



If, as a consumer, you take three identical electromagnets and arrange them accordingly in space, you get an electromagnetic field with a sense of rotation over time. In this way, electric motors can be produced in a particularly simple manner. The exploitation of precisely this aspect is the global industry standard.

Three electromagnets is the minimum number needed to generate a sense of rotation.



If you swap which 2 of the 3 230V ~ lines, then the direction of rotation and thus the direction of rotation of the connected electric motor changes. This applies to both the 3 x 230V ~ and the 3 x 400V connection scheme.

This results in simple switching options for the direction of rotation in electric motors.



The so-called "star-delta" switchover offers a simple way of starting large electric motors gently. More on this on the following page.



The 4-wire system described offers a wide range of connection options.


The following applies in general to electrical power:


Power of the consumer = current x voltage


(This is discussed in the chapters Ohmic Resistance and Power Torque Energy)


This applies to a single voltage source.

A factor (cosine Φ) is still missing here for general validity, but it is practically always in the range [0.7 ... 1.0] and does not affect the following explanations.


Typical protection of line networks in the domestic and partly industrial area is 16 amperes.

With 230V ~ you can achieve loads of up to 16Ampere x 230V = 3680 watts.

The following table shows the maximum power that can be drawn from all conceivable connection options with a 16 amp fuse.


Voltage (s) used in the device
Calculation of the maximum power that can be drawn with 16A protectionResultExamples
1 x 230V ~ 16A x 230V3.7 KWMost of it in the household. Almost all fan heaters, kettles, etc. have a maximum of 2 KW.
Most welding machines up to 160 A.
1 x 400V ~ 16A x 400V6.4 KW

E.g. welding machines with 160 to 250A max. Most devices with outputs in the range of 4 to 6 KW. (no motors)

2 x 230V ~ 16A x 230V x 27.4 KW

Seldom occurs. Practically covered by 1 x 400V ~.

3 x 230V ~ 16A x 230V x 311.1 KW

Without using the direction of rotation: stove, weaker water heater.

With use of the direction of rotation: electric motors, e.g. in chippers from 3 KW.

2 x 400V ~ 16A x 400V x 2
12.8 KW

Seldom occurs. Practically covered by 3 x 230V ~.

3 x 400V ~ 16A x 400V x 319.2 KW

Domestic instantaneous water heaters. A 3 x 16 A fuse would be arithmetically sufficient for an 18KW instantaneous water heater in triangular operation (which should be classified as weak). In practice, protection is therefore much higher with correspondingly thicker cables.

In industry mostly only with the use of the direction of rotation: more powerful electric motors.


The author has already been contacted several times because there is allegedly an error in the table above. The following statements are the answer to the question that has been asked each time so far.

The essential thing that most stumble over is that the safeguards are fixed and the devices to be connected represent the variable.

1. For the sake of simplicity we consider ohmic loads and initially disregard safeguards, or the safeguards are arbitrarily high.

2. If the applied voltage is increased by the root (3) -fold with otherwise the same boundary conditions, the current also increases by the root (3) -fold.
A total of 3 times the power is then delivered.

3. Star connection: consumers see 230V
Delta connection: consumers see 400V.

4. So if you operate the same consumer once with a star and once with a delta connection, then the power drawn differs by three times.

The current in a delta connection is around the root (3) times higher (and of course the voltage as well) than in a star connection.

5. Point 4 with motors instead of ohmic is called star-delta connection, a frequently used start-up circuit for larger motors. The motors are started in star operation and after a short time they are switched to delta operation. The services can differ by a factor of 3, but do not have to, because other things also play a role here.

6. From now on we consider practical boundary conditions, as they always occur in domestic environments, specifically: safeguards.
In practice, the "otherwise the same conditions" are different than in the previous explanations:
The protection is given, and the load or the connected device is the variable.

7. With 1 x 230V / 16A fuse you can get 230V x 16A = 3.7 KW.

8. If you apply the load to 400V from point 7 onwards (apart from the fact that there are some practical hurdles here), the fuse will respond, or the load will be damaged, or both. So this case is irrelevant.

9. You have to take another consumer that draws "only" 16A of current at 400V.
Then the maximum power that can be drawn is 400V x 16 A = 6.4 KW, i.e. the root (3) times.

10. The same applies to 3 phases:
You have to use different consumers for 230V or 400V so that the available 16A can actually be taken.


Especially for delta connection:

The fuses are in the outer conductors. Each outer conductor serves 2 motor windings at the same time. 16A should flow through both windings. Since both 16A currents are out of phase by 120 degrees, the sum of both is again 16A!

-> Each of the outer conductors "sees" 2 winding currents of 16A each, phase-shifted by 120 degrees, which add up to 16A.

11. In electrician practice one always speaks (wrongly!) Of 400V or 0.4 kV as soon as one considers 3 outer conductors, that is, three-phase house connections always mean 400V connections, although it is only 230V to earth, and 400V devices for domestic use do not exist at all. 3P devices are always star connections in the home, even very powerful instantaneous water heaters.