Line current and phase current in a 3 phase 3 wire system.

[Toyona10]

A short question:
Why and how is the line current equal to the phase current in the 3phase- 3wire system?

 

[Bobbywhy]

Hope this helps.

In transmission, loads are always balanced, hence creating its own star point (not needing a neutral to take the out of balance current). This means that there is only need for 3 wires - one for each phase (ABC). Hence not needing to run another wire

In distribution however, the neutral is needed as this is able to take out the out of balance current as no loads are balanced (excluding 3phase motors etc).
By having the forth wire (neutral) allows the connection of single phase houses as electricity needs a circuit to flow.

The transmission side of a transformer (the primary side) is usually delta connected meaning that line and phase voltages are equal whilst line and phase currents vary by root 3.

The distribution side however (the secondary side) is star connected meaning that their phase and line currents are equal whilst their line and phase voltages vary by root 3.

http://wiki.answers.com/Q/Why_trans...t_and_distribution_has_3_phase_4_wire_circuit

Some good explanations of 3 phase power can be found at:

http://www.allaboutcircuits.com/vol_2/chpt_10/5.html