Three-phase electric power is a common method of alternating current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads. A three-phase system is generally more economical than others because it uses less conductor material to transmit electric power than equivalent single-phase or two-phase systems at the same voltage. The three-phase system was introduced and patented by Nikola Tesla in the years from 1887 to 1888.
In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electric current. This delay between phases has the effect of giving constant power transfer over each cycle of the current and also makes it possible to produce a rotating magnetic field in an electric motor.
Three-phase systems may have a neutral wire. A neutral wire allows the three-phase system to use a higher voltage while still supporting lower-voltage single-phase appliances. In high-voltage distribution situations, it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).
Three-phase has properties that make it very desirable in electric power systems:
The phase currents tend to cancel out one another, summing 
to zero in the case of a linear balanced load. This makes it possible to eliminate or reduce the size of the neutral conductor; all the phase conductors carry the same current and so can be the same size, for a balanced load.
Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations.
Three-phase systems can produce a magnetic field that rotates in a specified direction, which simplifies the design of electric motors.
It is precisely for this reason that it is so often used in the Commercial and Industrial properties. Whether it’s your local garage, shop,factory or office block you will most often find a 3 phase 415 volt supply. Extra care must be taken when working with this system as an electric shock from 415 volts can quite often be fatal, where as 230/240 volts, without RCD protection, in the majority of cases will just give you a hard belt.
Three is the lowest phase order to exhibit all of these properties.
The three phases are typically indicated by colors which vary by country, but in the UK are as follows.
| UK until April 2006 | |||||
| UK since April 2006 |
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