DC-AC diagram - Image: Heurisko Ltd.

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Phase is a term used to describe alternating current. Phase explains the timing of the electron movements. Three-phase electricity is common worldwide because it is a cheaper and easier way to transmit electricity. Most homes only use single-phase electricity, however.

What does Phase mean?

Phase is a term used to describe one feature of alternating current. If wires are in phase it means that timing of the electron movement, back and forth, is the same. The electrons are 'in step' or 'in time' or 'in phase'.

To distribute AC power it is more efficient to use three circuits that are out of phase. This idea of three phase power was discovered by Nikola Tesla (1856 -1943). Much of its efficiency is because there is always voltage (electrons moving) in at least one wire.

Why not 1, 2 or 4 phases?

The international colour code for each phase of red, yellow and blue is used in this diagram. Notice how within each phase (colour) the electrons are moving back and forth but not in unison with the other phase (colour) - Image: Heurisko Ltd.

Three-phase power (and its multiples) removes the need for a neutral or 'return path'. This is because joining the three phases together results in no overall current flow.

Three-phase power is an arrangement that fits in very nicely with generator design. The 120° phase separation allows close to the optimum spacing and size of the copper conductors around the stator bore. The 3-phase generator is the cheapest form to make.

Multiple phase generators are made for specific purposes, usually military, but they are expensive.