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

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04-02-2010, 12:38 PM   #1
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I have a MWBC setup in the master bedroom suite. The two individual circuits both seem to have max possible loads at/above 15A (assuming all lights on, all electronics on, and vacuum running). Does this mean that the common neutral wire has a possible max load of 30A or more? If so, is this a problem, given it's 14/3 NM wiring? Or does the reverse-phasing minimize the neutral load?

 04-02-2010, 12:44 PM #2 Member   Join Date: Jan 2009 Location: Granbury, TX Posts: 325 Rewards Points: 252 ..er, never mind! If I'd though it over a couple more seconds, I'd have realized that my final comment was the right answer. The two hots are 180* out of phase. The neutral just caries the imbalance. If only one circuit is active, and maxed at 15A, the neutral would carry 15A. If both circuits were maxed at 15A, the neutral would only carry the difference between the two -- a minimal amount. Thus, the max load on the neutral would be the max load on ONE of the individual circuits. Sorry, just needed more time to think it through.

 04-02-2010, 12:50 PM #3 Member   Join Date: Jan 2009 Location: South of Boston, MA Posts: 17,248 Rewards Points: 2,000 Yup, as long as you are certain the hots are 180 out from each other you are OK

04-02-2010, 02:47 PM   #4
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Quote:
 Originally Posted by Itsdanf ..er, never mind! If I'd though it over a couple more seconds, I'd have realized that my final comment was the right answer. The two hots are 180* out of phase. The neutral just caries the imbalance. If only one circuit is active, and maxed at 15A, the neutral would carry 15A. If both circuits were maxed at 15A, the neutral would only carry the difference between the two -- a minimal amount. Thus, the max load on the neutral would be the max load on ONE of the individual circuits. Sorry, just needed more time to think it through.
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!

In the MBWC wiring scheme, when each pole is drawing maximum current, there is absolutely no current flowing in the neutral.
It only when the load on one pole or the other is reduced, will neutral current be present!
So if the current in one pole is dropped to 10 amps, then the neutral current will rise to 5 amps.
And if the load on one pole is removed, then we will have 15 amps of neutral current flowing!

 04-02-2010, 04:26 PM #5 Member   Join Date: Nov 2007 Location: Nashua, NH, USA Posts: 8,597 Rewards Points: 2,800 When a single phase (AC) source is put through a transformer or autotransformer with a center tap in the output winding, 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. If you take the two hot leads of the 220 volt feed without using a neutral, you have one 220 volt circuit. It is not meaningful to discuss in phase or out of phase with just one circuit. The three legs of a 3 phase system are 120 degrees out of phase. It is possible although rarely done to put each of the three phases through transformers with center tapped output windings, tie together the three center taps as a neutral, and end up with six feeds which can be arranged in order 60 degrees apart. __________________ Stick to your lawn watering schedule until it really starts to pour. After the storm you have only the same number of rest days you always had and then you need to start watering again. Last edited by AllanJ; 04-02-2010 at 04:36 PM.
04-02-2010, 04:39 PM   #6
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Quote:
 Originally Posted by AllanJ When a single phase (AC) source is put through a transformer or autotransformer with a center tap in the output winding, 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. If you take the two hot leads of the 220 volt feed without using a neutral, you have one 220 volt circuit. It is not meaningful to discuss in phase or out of phase with just one circuit. The three legs of a 3 phase system are 120 degrees out of phase. It is possible although rarely done to put each of the three phases through transformers with center tapped output windings, tie together the three center taps as a neutral, and end up with six feeds which can be arranged in order 60 degrees apart.
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!!

04-02-2010, 04:42 PM   #7
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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!!
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.

04-02-2010, 11:19 PM   #8
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Quote:
 Originally Posted by AllanJ When a single phase (AC) source is put through a transformer or autotransformer with a center tap in the output winding, 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. If you take the two hot leads of the 220 volt feed without using a neutral, you have one 220 volt circuit. It is not meaningful to discuss in phase or out of phase with just one circuit. The three legs of a 3 phase system are 120 degrees out of phase. It is possible although rarely done to put each of the three phases through transformers with center tapped output windings, tie together the three center taps as a neutral, and end up with six feeds which can be arranged in order 60 degrees apart.
Allan, so many electricians seem to fail to understand this very point. You are correct that in a 3 phase system, the voltage wave on each of the three conductors will be rising and falling at different times. As our power was originally generated by revolving alternators, it was reasonable to design these alternators in such a fashion that each pole of the alternator would have its voltage wave peak at 120 degrees after its predecessor.
3 phase means that the peak voltage of each wave will arrive at the load at a different moment in time.
In power distribution, in a community, one conductor that is carrying the first voltage will feed a certain street or area. The next with its wave arriving 120 degree's later, is sent somewhere else. The third conductor, that is 240 degrees behind is sent to another destination entirely.
The net result is that only one of the phases will arrive at a given home and this is why its said that residential service is single phase.
So, your buddy, two blocks over will have his power arrive a little later than you, but he'll not be aware of this!

We in the electrical business have incorrectly carried 3 phase terminology over into our residential lexicon and this has led to a great deal of confusion on how our single phase electrical actually functions.

From the various comments that I read, here in this thread, it is apparent that this confusion is quite common!

 04-02-2010, 11:40 PM #9 Member   Join Date: Jan 2009 Location: Granbury, TX Posts: 325 Rewards Points: 252 For what it's worth, I get the concept, even if the terminology gets confusing. I understand that, in my single-phase service, L1 and L2 are alternating in an exactly opposite cycle ("180 degrees out of phase relative to each other "). And I get what that implies for my MWBC circuits. Thanks for all the feedback. Also, for what it's worth, I get a headache when I think too much about 3-phase systems....
04-02-2010, 11:42 PM   #10
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Quote:
 Originally Posted by AllanJ When a single phase (AC) source is put through a transformer or autotransformer with a center tap in the output winding, 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. If you take the two hot leads of the 220 volt feed without using a neutral, you have one 220 volt circuit. It is not meaningful to discuss in phase or out of phase with just one circuit. The three legs of a 3 phase system are 120 degrees out of phase. It is possible although rarely done to put each of the three phases through transformers with center tapped output windings, tie together the three center taps as a neutral, and end up with six feeds which can be arranged in order 60 degrees apart.
What you describe here is common in industrial/commercial applications.
For instance, each of your transformer secondary windings are fed to a standard breaker panel. The center tap connected to the neutral bar, the full taps feeding the main breaker.
Everything that is fed from the distribution breakers will be single phase! Its single phase, because only one phase is sent from its transformer secondary.

 04-03-2010, 06:38 AM #11 Member   Join Date: Nov 2007 Location: Nashua, NH, USA Posts: 8,597 Rewards Points: 2,800 When wiring up and connecting and using 120 volt lights and equipment on a 120/240 volt service, we don't worry about the two legs' being 180 degrees out of phase (one half of the circuits getting power slightly delayed) except when wiring up a MWBC where we need the out of phase relationship to not have overloading of the neutral. __________________ Stick to your lawn watering schedule until it really starts to pour. After the storm you have only the same number of rest days you always had and then you need to start watering again.
04-03-2010, 09:10 AM   #12
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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.
Times 2

04-03-2010, 11:12 AM   #13
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Quote:
 Originally Posted by AllanJ When wiring up and connecting and using 120 volt lights and equipment on a 120/240 volt service, we don't worry about the two legs' being 180 degrees out of phase (one half of the circuits getting power slightly delayed) except when wiring up a MWBC where we need the out of phase relationship to not have overloading of the neutral.
there should never be a case in a mwbc where the neutral gets overloaded. max load per leg is per breaker size which your neutral should be sized for. Any load on the other leg is subtracted from the total of the first leg so it's impossible to overdrive it. Unless of course(and i've seen this alot) someone shared a neutral between two hots comming of the same point of the sine wave.
Quote:
 Originally Posted by jlmran Times 2

I agree...alot of people get confused in respect to the fact that with three phase your dealing with three different sine waves. With houses your dealing with one sine wave but the voltage is just being taken at two different points of it. So in a house i guess instead of saying that the two legs are 180 out of phase(with respect to each other) Maybe it should be referred to as one leg "leading" the other by 180 degrees?

 04-03-2010, 11:40 AM #14 Member   Join Date: Feb 2010 Location: Oklahoma Posts: 992 Rewards Points: 506 My "times 2" post meant I am agreeing with you when you ask sparkplug what is meant by his/her following statement: 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!! 120 degrees is 33.333333% of 360 degrees, so in a typical 3-phase system, each pole is separated by greater than 30% of the allowed separation.
 04-03-2010, 05:57 PM #15 Master Electrician   Join Date: Mar 2010 Location: Toronto Ontario Posts: 1,165 Rewards Points: 500 i had to reread that....i thought it said 30 degrees not 30%...my appologies to the original poster.

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