A single phase supply requires 2 wire one for forward and next for returning path. However a 3 phase supply only requires 3 wire why? How does the circuit complete with 3 phase supply?
closed as too broad by winny, Andy aka, PeterJ, Brian Drummond, Voltage Spike Aug 22 '17 at 16:04
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The return path for one phase is one or both of the other phases.
Loads can be connected "delta" - wired between the phases (L1 to L2, L2 to L3 and L3 to L1).
Alternatively, a star point can be created, and the loads star wired (L1 to S, L2 to S, L3 to S). In a 4-wire 3-phase system, the star point would be connected to neutral. However, if all three loads are identical, it turns out that the current in the neutral is zero, and so it can be left out entirely.
In a single phase system, current is balanced between the line and neutral conductors.
In a three phase system, current is balanced between the three conductors. This is possible due to the phase angle difference between the phases.