# 3-phase explanation

I have been reading about single phase and three phase power. I'm completely confused now.

Can someone please give me a working example of when I would use three phase power instead of single phase (some sort of common situation I could relate to) and why, instead of single phase?

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Have you read this first? – clabacchio May 16 '12 at 13:02

Single phase power will have periods with zero power (the power is zero when either the voltage or current crosses zero)

For most practical user situations, this does not matter because the load has enough energy storage to 'ride through' the nulls in power. The energy storage may be the spinning inertia of a mechanical system or the capacitors in an electronic system.

When the power (current) level starts increasing, three-phase power is preferred because you can reduce the number of conductors and this provides a cost benefit. This reduction becomes significant for distribution systems:

Consider a 3kW load with a line to neutral voltage of 100Vac. The single phase current is 30A and the three phase current is 10A. This means that for a single phase system, one needs two wires that carry 30A each. For a balanced three-phase system you only need three wires each carrying 10A. The neutral wire isn't needed since it carry's zero current. If you select the wire size to have the same current density (A/sq area), the single phase system uses twice the amount of copper that the three-phase system uses.

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No, three phase power does not have fluctuations with a steady load. That's the point. Power transfer with three phase is constant. Do the math. – Olin Lathrop May 16 '12 at 13:04
brain fart. corrected – madrivereric May 16 '12 at 13:12
Thank you all for excellent answers. I have marked this one as correct because the example at the end was the complete lay mens terms I needed, but thanks to all, they are great, +1 to all. – javano Aug 7 '12 at 19:23

Single phase is a phase related to the neutral. In Belgium that's 230V. But when you have three phases you can use them in relationship with the other phases, and then the voltage between phases is 400V (230V $\times$ $\sqrt{3}$).
Even at three phase you can use the 230V. You'll have 3 times the power available. That's called a star configuration. In delta you'll have 3 $\times$ 400V, that's 5.2 $\times$ the power at the same current.

Note that in the delta configuration the neutral isn't shown. In a perfectly balanced load the voltages (and their currents) add to a total of zero, so there won't flow any current through the neutral.

In a single phase the return current for each phase goes through the neutral.

You would typically use three phases when you need a lot of power, or to run synchronous motors.

One application is where I wired a 3-phase water boiler at 19A per phase. That's 22.8kW. If you would do that with a single phase you'd need to wire for 100A.
Until a few decades ago every house here would have three-phase power, and I remember my parents had a three-phase washing machine. Today you get just one phase, three-phase is optional, which you have to pay for. Like Olin says phases are staggered over neighboring houses so that the power of the three phases is more or less balanced. So I would have the "R" phase, my left neighbor the "S" and the other neighbor the "T" phase. I remember one time when there was a power-outage of just one phase in the whole neighborhood. Utilities don't like this because the generators are then unbalanced, and the neutral has to carry much more current than the nominal low value.