Why is three-phase offset by 120 degrees?
When there's 120° between phases the sum of the voltages at any time will be zero.  



This means that with a balanced load no current flows in the return line (neutral).  

  

Also, if each phase is 230V with respect to the neutral (star operation), then there will be 230V \$\times\$ \$\sqrt{3}\$ = 400V between any two phases (triangle or delta operation), and they're also equally spaced, i.e. at 120° angles.  

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