# 3 phase 4 wire system will make me mad!

Discussion in 'General Electronics Chat' started by s2pk, Jun 29, 2012.

1. ### s2pk Thread Starter Member

Jul 10, 2010
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let say we have cable ampacity of 100 Amps, and load connected to all three wires is 50Amps, so the neutral wire (return path) have to carry the current of all three live wires (150Amps). Now my question is how the neutral wire carries current greater than its ampacity???

2. ### gerty AAC Fanatic!

Aug 30, 2007
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Neutral is designed to carry UNBALANCED LOAD, if all 3 phases are carrying 50 amps like you say there should be no load on neutral.
I'm sure all the loads are not exactly the same, so there will be some current on neutral.

3. ### GetDeviceInfo Senior Member

Jun 7, 2009
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just restating the previous post;

the current in the phases add together at the common neutral point. If balanced, vectorally, the sum is zero, hence no flow in the neutral.

4. ### s2pk Thread Starter Member

Jul 10, 2010
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I am not satisfied with your answer plz elaborate, I found different currents on all phases and neutral wire showed the sum of currents on all phases. So if current on R, Y, B is say 98, 99, 100 amps respectively it means neutral will show reading of 98+99+100. How neutral wire can carry current greater than its ampacity ???

5. ### BillB3857 Senior Member

Feb 28, 2009
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You actually measured the sum of the individual phase currents on the neutral???????? Show us a diagram of what you have connected and how it is connected.

Jun 7, 2009
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Start here;

7. ### s2pk Thread Starter Member

Jul 10, 2010
77
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I will post the screenshot tomorrow

8. ### cork_ie Member

Oct 8, 2011
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If three men have three ropes and one end of each rope is connected to a small circular ring. If they pull the ropes in three different directions separated by 120 degrees , the power (rate of doing work) required to hold the ring at a fixed point will be the net of the power supplied by each of the three men, not the three added together. If each man was pulling equally there would be zero net pulling force.
While current flow does not initially appear to be analogous to work, energy (and power) they are similar whether mechanical or electrical.

9. ### takao21203 Distinguished Member

Apr 28, 2012
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yes the ring itself would not move, but there is still tension between the individual ropes. Isn't it true in a balanced AC circuit (for 3 phase) the N wire can be omitted altogether?

10. ### strantor AAC Fanatic!

Oct 3, 2010
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1,989
I have seen 3 phase heaters and motors do not have a neutral. They have a green earth ground but not a neutral.