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Greg:

In the code books,3 phase power is for commercial and industrial setups.

3 phase power is also in "Delta" and "Wye" configuration, but I am not going to elaborate on this.

Main transmission of power is also done 3 phase.

Distribution transmission isdone both single and 3 phase power.

For residential systems and light commercial where you do not have 3 phase motors etc, single phase power is used. It is generally transformed from7200volts or so to240 volt.

The low side winding of the transformer has a centre tap whichiscalled"neutral" and it is bonded to the earth - ground.

Thus from one side of the windingto centre(neutral) is 120 volts, other side of the windingto centre(neutral) is 120 volts and across the complete winding is 240 volts.

Generally, one side of the winding is called a "leg".

Hope this helps.