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 DIY Chatroom Home Improvement Forum Current through neutral wire?
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11-29-2007, 01:48 PM   #1
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## Current through neutral wire?

Hi, any one can help !!!
my problem is : 3 phase 4 wire system with balanced load VL 400 V and Z 59+j36.5, i get IP 3.329 A in each phase and the current through neutral will be ZERO , but IF u disconnect one phase the current will change in neutral wire : i use multisim and i simulate the circuit and when i disconnect the phase i get 3.329 A in neutral wire , but how can i get same value by calculation i try and i get 2.88 A something like that , so any one have any idea how to calculate the current after disconnecting one phase ? , i would realy be thanks full .

11-29-2007, 04:06 PM   #2

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Hi

I'm way to rusty with the phasor stuff but may be can get you steered in the right direction with another approach. So looking at the attached image....

The one equation is for an unbalanced system the other is a balanced system where In = 0. If I'm balanced and I close the switch in the neutral nothing happens no current flows and In =0 an Vn =0. If I remove one of the phases in the equation I am no longer balanced and current must flow in the neutral when I close the switch. If I remove V(cn) and in turn eliminate I(c) my equation becomes

I(a)+I(b) = I(n) so I'm not sure how you solve that using the phasor calculations but maybe this helps....
Attached Images

Last edited by Stubbie; 11-29-2007 at 06:30 PM.

 11-29-2007, 04:23 PM #3 Electrician   Join Date: Nov 2005 Location: Winnipeg, Manitoba Posts: 1,298 Rewards Points: 802 I can't answer your question but it doesn't sound like something a DIY would ask. Try out the sister site http://www.contractortalk.com and they may be able to help you out.

 11-29-2007, 04:47 PM #4 Licensed Electrical Cont.     Join Date: Feb 2004 Location: NY State Posts: 7,821 Rewards Points: 1,992 Or maybe he is a student looking for an answer to a test/exam question. The M O of asking a question like this as your fist post is pretty typical. __________________ Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.
11-29-2007, 07:23 PM   #5
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Quote:
 Originally Posted by Speedy Petey Or maybe he is a student looking for an answer to a test/exam question. The M O of asking a question like this as your fist post is pretty typical.
yeah thats right i am a student and this question is a tutorial we had it in the lap and i had aruge with my friends about it, so i deside to ask about it , in the end its knowlege, if it was an exsam why i should bother my self and other to get the answer, i have 7 brothers and all of them are engineers so any one can help out but as i say maybe my question will help be useful knowlege, thanks all any way.

11-29-2007, 11:28 PM   #6
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There are 2 ways to calculate this;

1] use phasor diagrams.
2] use polar or rectangular coordinates.

Also, bear in mind that in a multiphase system, the neutral current can only be zero Amps if;

1] the current in each phase is equal.
2] the Power Factor of each phase is equal.

Harmonics cannot be accounted for in this simple calculation.

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