ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat
Recent Posts
Increasing demand factors in residential
by gfretwell - 03/28/24 12:43 AM
Portable generator question
by Steve Miller - 03/19/24 08:50 PM
Do we need grounding?
by NORCAL - 03/19/24 05:11 PM
240V only in a home and NEC?
by dsk - 03/19/24 06:33 AM
Cordless Tools: The Obvious Question
by renosteinke - 03/14/24 08:05 PM
New in the Gallery:
This is a new one
This is a new one
by timmp, September 24
Few pics I found
Few pics I found
by timmp, August 15
Who's Online Now
1 members (Scott35), 293 guests, and 18 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Page 1 of 4 1 2 3 4
#130289 03/05/06 05:27 AM
Joined: Jan 2006
Posts: 19
S
Sparkee Offline OP
Member
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?

Stay up to Code with the Latest NEC:


>> 2023 NEC & Related Reference & Exam Prep
2023 NEC & Related Reference & Study Guides

Pass Your Exam the FIRST TIME with the Latest NEC & Exam Prep

>> 2020 NEC & Related Reference & Study Guides
 

Joined: Feb 2002
Posts: 182
B
Bob Offline
Member
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

Joined: Jul 2001
Posts: 599
J
JBD Offline
Member
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).]

Joined: Jan 2006
Posts: 19
S
Sparkee Offline OP
Member
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!

Joined: Jul 2001
Posts: 599
J
JBD Offline
Member
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).

Joined: Feb 2002
Posts: 182
B
Bob Offline
Member
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.

Joined: Jul 2001
Posts: 599
J
JBD Offline
Member
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".

Joined: Feb 2002
Posts: 182
B
Bob Offline
Member

Joined: Jul 2001
Posts: 599
J
JBD Offline
Member
Bob,

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

Joined: Feb 2002
Posts: 182
B
Bob Offline
Member
JBD
Check here. Maybe this is a better understood.
http://www.allaboutcircuits.com/vol_2/chpt_10/2.html

Page 1 of 4 1 2 3 4

Link Copied to Clipboard
Powered by UBB.threads™ PHP Forum Software 7.7.5