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#128499 - 04/21/03 11:19 PM neutral current
elecbob Offline
Member

Registered: 08/23/02
Posts: 142
Loc: WA
Can anyone point me the way to the formula for calculating the neutral current in a 3 phase wye circuit?thanks
bob

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#128500 - 04/22/03 04:04 AM Re: neutral current
BPHgravity Offline
Member

Registered: 04/08/03
Posts: 141
Loc: Port Charlotte, Florida
I do not know of any online versions of the calculation, but I will be happy to post it right here for you.

In = the square root of I^A+I^B+I^C-(IA*IB)-(IB*IC)-(IC*IA)

However, I have found that for field calculations, by determining the load of the greatest unbalance of any two phase loads will give you a very accurate value. EX: A=30amp, B=40amp, C=50amp. The greatest difference is 50-30= 20amp. If you perform the above calculation, the value comes to around 17.32amp. Not close enough for an exam question, but plenty clos enough for field calcs.
_________________________
Bryan P. Holland, ECO.
Secretary - IAEI Florida Chapter

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#128501 - 05/25/03 06:19 AM Re: neutral current
Ichabod Offline
Member

Registered: 05/24/03
Posts: 26
Loc: Statesboro, GA, USA
gravity, why does that work? I know it does because I used my HP-11C calculator and assumed that the phases were 120 degrees apart and got the same answer. The way I do it is to convert the polar quantities into rectangular and total them, then convert back to polar. Using the HP it only takes a few seconds once you learn how to do it, faster than your method. If the currents are not 120 degrees apart, then there is a lot of difference in doing it my way and your way. For an electrician, probably doesn't matter. I was a substation test engineer commissioning protective relays and had to know very accurately what the residual currents were. But I am intriqued by your method. Thought at first it was symmetrical components, but ruled that out.

Ichabod

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