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#130289 - 03/05/06 05:27 AM 2 phase neutral current?  
Sparkee  Offline
Member
Joined: Jan 2006
Posts: 19
Australia
Ok I understand single phase current in=current out and 3 phase which has a formula

But say you have a load of 1500w on one phase and 1700w on a 120/208 system...is the current 3200/120? Or is it the imbalance?


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#130290 - 03/06/06 06:35 PM Re: 2 phase neutral current?  
Bob  Offline
Member
Joined: Feb 2002
Posts: 182
Mobile, AL, USA
Sparkee
Its a little more complex than that.The currents are 120 degrees out of phase in the 2 phase wye system. In a single phase system with 2 hots and a neutral the currents are 180 degrees out of phase and substract in the neutral.
In order to caculate the neutral you must use vectors to arrive at the neutral current.
At 120 volts 1500 w = 12.5 amps at an angle of 0 degrees and 1700 w = 14.2 amps at 120 degrees. To add these together you use trig.
14.2 amps at 120° = 14.2 cos 120° + 14.2 sin 120°. Add the 2 loads together = (-7.1 + 12.5)angle 0 + (12.4)angle 90 which equals
5.4 (ang 0) + 12.5 (ang 90).
The get the magnitude the neutral current =
sqrt(5.4² + 12.5²) 13.5 amps.
Additional info is at http://www.electrician2.com/electa1/electa3htm.htm


#130291 - 03/07/06 10:25 AM Re: 2 phase neutral current?  
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
WI, USA
Two legs of a wye system is not 2 phase.

It is simply two legs of a three phase system, although some utilites call this a 120/208 network connection.

Use the normal 3 phase formulas to determine your neutral current, but of course one current will equal 0.

One common formula is:
In=SQRT((Ia*Ia+Ib*Ib+Ic*Ic)-(Ia*Ib+Ib*Ic+Ic*Ia))

so in your case:
In = SQRT(12.5*12.5+14.2*14.2)-(12.5*14.2))
In = SQRT(180.39)
In = 13.43A

[This message has been edited by JBD (edited 03-07-2006).]


#130292 - 03/07/06 09:06 PM Re: 2 phase neutral current?  
Sparkee  Offline
Member
Joined: Jan 2006
Posts: 19
Australia
thanks guys!

Sorry I wasn't clear on what system it was....it isn't a system we use here in Australia

So I take it use the 3 phase formula (minus the missing phase)?
both answers are very close thou!


#130293 - 03/07/06 09:40 PM Re: 2 phase neutral current?  
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
WI, USA
Sparkee, yes simply use the normal three phase formulas whith one current equal to zero.

Bob,
Specifically, when using only two "hot" wires of a wye, the currents in each conductor are not 120° out of phase with each other, however, the line to neutral currents are. You can not have a phase difference if you only have two wires (what goes out must come back).

When using only two "hot" wires of a single phase 3 wire system. the currents in each conductor are not 180° out of phase with each other, however the line to neutral currents are.

Neutral currents never subtract from each other, they always add (the trick is you may be adding a negative number).


#130294 - 03/08/06 06:14 PM Re: 2 phase neutral current?  
Bob  Offline
Member
Joined: Feb 2002
Posts: 182
Mobile, AL, USA
JBD
Quote
"Specifically, when using only two "hot" wires of a wye, the currents in each conductor are not 120° out of phase with each other, however, the line to neutral currents are. You can not have a phase difference if you only have two wires (what goes out must come back)."

Specifically, in a wye system such as this the line current and the phase current are the same and they are out of phase by 120 degrees. If that were not true my caculations would not have produced the same answer as your formula. An by the way, the voltages are also 120 out of phase.

JBD
Quote
"When using only two "hot" wires of a single phase 3 wire system. the currents in each conductor are not 180° out of phase with each other, however the line to neutral currents are.

Neutral currents never subtract from each other, they always add (the trick is you may be adding a negative number)."

The voltage vector relationship depends on the way the secondary windings are wound in relationship to each other.
This is where the (+) and (-) polarity markings really become important. This notation is often used to reference the phasings of multiple AC voltage sources, so it is clear whether they are aiding ("boosting") each other or opposing ("bucking") each other.

To mathematically calculate voltage between "hot" wires, you must subtract voltages, because the polarity marks show them to be opposed to each other:
If we mark the two sources' common connection point (the neutral wire) with the same polarity mark (-), we must express their relative phase shifts as being 180o apart. Otherwise, we'd be denoting two voltage sources in direct opposition with each other, which would give 0 volts between the two "hot" conductors.


#130295 - 03/08/06 08:29 PM Re: 2 phase neutral current?  
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
WI, USA
Bob,

Take a single 208V load (say a baseboard heater with no neutral connection), it is fed with a blue wire from phase A and a black wire from phase B. Are you telling me that the "incoming" current in the blue wire is 120° out of phase with the "outgoing" current in the black wire?


I beg to differ about connecting both "-" terminals of two transformers together to create a 3 wire circuit(I wish I was good at posting drawings).

A 3 wire single phase circuit is really two single 2 wire circuits connected together. One circuit is connected across transformer windings X1 and X2, the other circuit is X3 and X4. The ANSI convention for a 3 wire single phase transformer connection is to tie X2 and X3 together to create a neutral point. Also it is common in the industry to say the "line to line voltage from X1 to X4 is equal to the voltage from X1 to X2 PLUS the voltage from X3 to X4".


#130296 - 03/08/06 11:18 PM Re: 2 phase neutral current?  
Bob  Offline
Member
Joined: Feb 2002
Posts: 182
Mobile, AL, USA


#130297 - 03/09/06 12:13 AM Re: 2 phase neutral current?  
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
WI, USA
Bob,

What is your point? The last post in your link, by Infinity, agrees with my analysis.


#130298 - 03/09/06 04:53 PM Re: 2 phase neutral current?  
Bob  Offline
Member
Joined: Feb 2002
Posts: 182
Mobile, AL, USA
JBD
Check here. Maybe this is a better understood.
http://www.allaboutcircuits.com/vol_2/chpt_10/2.html


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