MWBC Question - Overloading Neutral Wire? - Electrical - Page 2 - DIY Chatroom Home Improvement Forum

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04-03-2010, 08:56 PM   #16
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Quote:
 Originally Posted by andrew79 if this were true then why does a 3 phase system work because as the above poster stated they're 120 degrees out of phase.
There's a difference between PHASE and CYCLE! In single phase power distribution, --by definition-- there are NO phases to talk about. The reason we have, --in single phase AC power-- 120/240v. is, that the position of the cycle (sine wave) is always 120 volts from Neutral and 240 volts from each Hot wire. That is different than Phase/s. Looking at drawings of Alternating Current Generators would greatly help in understanding this theory. !

Quote:
 Originally Posted by andrew79 if this were true then why does a 3 phase system work because as the above poster stated they're 120 degrees out of phase.
Please see post #6 by spark plug.!

04-03-2010, 10:55 PM   #17
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Quote:
 Originally Posted by spark plug Please see post #6 by spark plug.!
Your #6 post was, I assume was in response to the statement "there will be two legs that are 180 degrees out of phase relative to each other when the center tap is used as a neutral" !

The author of this statement should embarrassed that he has so publicly shown how misinformed he is!

You are correct in stating that the Sine Wave in a single phase application, cannot possibly be out of phase with itself!

04-04-2010, 07:05 AM   #18
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Quote:
 Originally Posted by spark plug But in 3 Phase power distribution, if any of the phases are 30% or more out of phase to each other, there will be no current flow at all!
Can someone explain the above?

No current flow in the neutral occurs only under "ideal" conditions, namely when the hot lines sharing that neutral have certain phase relationships and also the actual loads on each of the subcircuits (one hot line and neutral) fit certain relationships (like being equal in a "wye" 3 phase system).

Note that, given just two legs of a "wye" 3 phase system and the neutral, it is not possible to get the neutral current all the way to zero. A crude explanation can be made using arrows (vector arithmetic). Use an upward pointing arrow to stand for the current on the A phase (the length represents the amount of current). Use an arrow to the right and slightly downward (120 degrees from vertical up) for the B phase, an arrow to the left and slightly downward for the C lphase. The magic relationship where there is no neutral current is achieved if the arrows are of such lengths that stringing them together* base of the next arrow at the tip of the previous arrow can get the tip of the last arrow on the base of the first arrow. We can also do the single phase MWBC (120/240 volt circuit) with a straight down arrow for the B leg.

The neutral current is an arrow needed to get back to the base of the first arrow, if the last arrow did not quite reach.

Vector arithmetic shows quite dramatically the problem of connecting both the red and black of a MWBC to the same side of the line (incorrect wiring). Both the red arrow and the black arrow point the same way and the arrow (for neutral current) needed to get back to the base of the first arrow has to be extra long.

* You can draw an arrow anywhere on the paper but it has to point in the direction described.
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Last edited by AllanJ; 04-04-2010 at 08:26 AM.

04-04-2010, 12:58 PM   #19
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Quote:
 Originally Posted by spark plug Please see post #6 by spark plug.!
I think i figured out where i went wrong here. By your post i assumed that you ment if it was over 30%(each leg is 33.3% of from the next) then no power would flow period but if no current does that doesn't mean that there's no power. Obviously there wouldn't be any current flow in the neutral in a perfectly balanced system. Misunderstanding on my part .

Quote:
 Originally Posted by spark plug There's a difference between PHASE and CYCLE! In single phase power distribution, --by definition-- there are NO phases to talk about. The reason we have, --in single phase AC power-- 120/240v. is, that the position of the cycle (sine wave) is always 120 volts from Neutral and 240 volts from each Hot wire. That is different than Phase/s. Looking at drawings of Alternating Current Generators would greatly help in understanding this theory. !
I wasn't talking about residential power...i was speaking of 3 phase only. If you read my post further down you'll note that i refered to house power as two points off the same sine wave. my question was that in a house with a single phase circuit as your taking voltage from two points of the sine wave does one leg lead the other in regards to hitting peak voltage?

Quote:
 Originally Posted by Wildie Your #6 post was, I assume was in response to the statement "there will be two legs that are 180 degrees out of phase relative to each other when the center tap is used as a neutral" ! The author of this statement should embarrassed that he has so publicly shown how misinformed he is! You are correct in stating that the Sine Wave in a single phase application, cannot possibly be out of phase with itself!

If you take into account the fact that when one leg is at 120V+ the other is at 120V-....and this gives you your 240V then the two legs are indeed 180 degrees out of phase "with respect to each other" as they are 180 degrees apart on the sine wave. It's the wording that's confusing. In regards to electricity they aren't a different phase but as per a dictionary definition they actually are out of phase with each other.

 04-04-2010, 05:04 PM #20 Member   Join Date: May 2009 Location: Brooklyn, New York (NYC) Posts: 1,124 Rewards Points: 500 Andrew 79 (Poster #20) You're right about that statement. (that in 3Ph. one phase leads the other. Hence the other lags behind.) That is why (in 3ph.) the calculations are different. Any Two Hot legs, in a "Y" configuration will not yield double the voltage of any hot and Neutral. It'll only give 1.73 x the base voltage. i.e. 120x1.73 =208 (Sq. Rt. of Three)!
04-04-2010, 11:55 PM   #21
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Quote:
 Originally Posted by spark plug Andrew 79 (Poster #20) You're right about that statement. (that in 3Ph. one phase leads the other. Hence the other lags behind.) That is why (in 3ph.) the calculations are different. Any Two Hot legs, in a "Y" configuration will not yield double the voltage of any hot and Neutral. It'll only give 1.73 x the base voltage. i.e. 120x1.73 =208 (Sq. Rt. of Three)!
that i'm well aware of....trade school will drill that into your head. But my question is this....take an actual representation of a sine wave. If the entire sine wave occurs at one time then both 120+ and 120- happen at the same time, but if the sine wave actually forms as electricity flows the the 120v- portion would happen a split second after the positive part of the cycle no?

04-05-2010, 08:11 AM   #22
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Quote:
 Originally Posted by andrew79 ... split second after ...
Yes.

Single phase. Let's say that at some time the voltage hits +120#. Then 1/120 second later it goes to -120. Another 1/120 second later it hits +120 again.

Now if you got the 120 volts from a center tapped transformer secondary and the center tap was used as a neutral, then when the voltage from one leg to neutral hit +120, at that same moment the voltage from the other leg to neutral hits -120. (black lead of oscilloscope* on the neutral for both measurements)

* Device that displays the waveform (sine wave, etc.) on a screen.
# It actually peaks out a little more, something like 170, as seen on the oscilloscope. The area under the sine wave blip would equal the area of a rectangular block 120 "volts" high and 1/120 "second" wide, or had the waveform been a square wave instead of a sine wave, the voltage would peak out at 120.
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Last edited by AllanJ; 04-05-2010 at 08:48 AM.

 The Following User Says Thank You to AllanJ For This Useful Post: andrew79 (04-05-2010)
04-05-2010, 10:45 AM   #23
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Quote:
 Originally Posted by AllanJ Yes. Single phase. Let's say that at some time the voltage hits +120#. Then 1/120 second later it goes to -120. Another 1/120 second later it hits +120 again. Now if you got the 120 volts from a center tapped transformer secondary and the center tap was used as a neutral, then when the voltage from one leg to neutral hit +120, at that same moment the voltage from the other leg to neutral hits -120. (black lead of oscilloscope* on the neutral for both measurements) * Device that displays the waveform (sine wave, etc.) on a screen. # It actually peaks out a little more, something like 170, as seen on the oscilloscope. The area under the sine wave blip would equal the area of a rectangular block 120 "volts" high and 1/120 "second" wide, or had the waveform been a square wave instead of a sine wave, the voltage would peak out at 120.
thanks for explaining it that way. Makes sense now to me. I take it both legs are hooked up to the scope and the neutral is the reference point? I went to college for electronic engineering tech before i got into the trade but all we ever used the scope on was DC.
The 170v also makes sense...that's the actual peak voltage entering a house. The rms is 120V....mathematically it comes out to 171V.

04-06-2010, 09:16 PM   #24
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Quote:
 Originally Posted by andrew79 that i'm well aware of....trade school will drill that into your head. But my question is this....take an actual representation of a sine wave. If the entire sine wave occurs at one time then both 120+ and 120- happen at the same time, but if the sine wave actually forms as electricity flows the the 120v- portion would happen a split second after the positive part of the cycle no?
My take on it is similar (but from a different point of the equation). That's why (due to the time lapse and phases being off) the voltage does not double when using both phases. It only increases by 1.73 times.
(Btw. I checked out different power distribution systems in various countries. Even in Canada, where everything is essentially the same as here, the Canadian Electrical Code allows 347v. (which is 600v*1.73) to be used in offices for lighting.!

 04-07-2010, 12:25 AM #25 Master Electrician   Join Date: Mar 2010 Location: Toronto Ontario Posts: 1,165 Rewards Points: 500 yup...347v is the standard for lighting in Canada for anything other than residential. There are still alot of 120V applications and quite a few buildings in downtown toronto that still run at 277V/440V as i'm sure happens in alot of the major cities in Canada. MWBC are the bane of my existance . I've gotten to a point where i'll throw a voltmeter on the neutral even if i think i've got all three circuits off just to make sure some dummy hasn't changed up the number sequence on me. Too many times have i seen someone get hurt off a circuit that they thought they had all three legs off on.
04-07-2010, 04:43 PM   #26
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Quote:
 Originally Posted by andrew79 yup...347v is the standard for lighting in Canada for anything other than residential. There are still alot of 120V applications and quite a few buildings in downtown toronto that still run at 277V/440V as i'm sure happens in alot of the major cities in Canada. MWBC are the bane of my existance . I've gotten to a point where i'll throw a voltmeter on the neutral even if i think i've got all three circuits off just to make sure some dummy hasn't changed up the number sequence on me. Too many times have i seen someone get hurt off a circuit that they thought they had all three legs off on.
You're right about that. Double (and triple) checking for live circuits. I think I related this story here on DIY. Where I was working at an auto body shop. I shut this particular breaker and I was working along singingly. All of a sudden I get zapped! When I checked the breaker panel I saw that the breaker was in the ON position. Someone, an employee of the shop, out of concern for the public good observed a breaker that was "Tripped". So he flipped it back ON.! Lesson to be learned. Never "Assume" a circuit is OFF. Tag out and Lock out are very important safety tools. Here, in the US, there's an agency of the Federal Government called OSHA. Some people think that they're a nuisance. Or out to collect revenue! But they saved countless lives already and should continue to do so.!

07-04-2010, 04:08 PM   #27
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Quote:
 Originally Posted by Wildie Something else for you to think about! How can you have 2 hots, out of phase when You only have single phase service in most residential applications. It ain't possible! In single phase residential service, we have have 'poles' that usually supply 220 volts A/C! The wave form of these poles is always in phase and never change!
Something for YOU to think about!

In the US and CANADA, residential service panels are fed with (2) 120V feeds derived from a CENTER TAPPED transformer. The center tap is the NEUTRAL and the other two legs (poles) are HOT and 180 degrees OUT OF PHASE with regard to each other. That is, when one leg is at -170V the other leg is at +170V

http://www.beananimal.com/articles/e...he-reefer.aspx

Sure both poles are derived from the same sine wave, but both poles have a complete sine wave that is exactly 180 degrees opposed from the pole on the other side of the neutral. Play all the word games you want, the two sine waves derived from the transformer are 180 degrees out of phase.

You stuffed your foot in your mouth and have tried to play word games to make it look as if others were the ones who erred.

Last edited by TheVillageIdiot; 07-04-2010 at 04:17 PM.

 07-04-2010, 04:39 PM #28 I=E/R     Join Date: May 2010 Location: Minnesota Posts: 2,052 Rewards Points: 1,000 So now we understand MWBC??? Let me think a bit... And BTW, 240*sin(120°)=?????? Last edited by a7ecorsair; 07-04-2010 at 04:55 PM.
07-04-2010, 05:00 PM   #29
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Quote:
 Originally Posted by a7ecorsair So now we understand MWBC??? Let me think a bit... And BTW, 240*sin(120°)=??????
I assume (based on your avatar) that you were an A7 pilot or support person? If so, thank you for your service. If you on the other hand you built or worked for LTV (or one of the technology providers), thank you for supporting our nations finest.

07-04-2010, 06:27 PM   #30
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Quote:
 Originally Posted by spark plug My take on it is similar (but from a different point of the equation). That's why (due to the time lapse and phases being off) the voltage does not double when using both phases. It only increases by 1.73 times. (Btw. I checked out different power distribution systems in various countries. Even in Canada, where everything is essentially the same as here, the Canadian Electrical Code allows 347v. (which is 600v*1.73) to be used in offices for lighting.!
I don't understand where you are finding multiple phases, when we are only dealing with one phase. (one single phase)
I would be grateful if you would explain how we get multiple phases from a single phase source.

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