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Old 03-30-2010, 11:14 PM   #1
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Discussion on MWBC


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Originally Posted by brric View Post
The currents on the neutral are cancelled by each other on a multiwire circuit. The reason you can share a neutral this way is that if

20 amps flow on load A and 20 amps flow on load B
there will be 0 current flow on the neutral white wire. This is also why the circuits must be on different phases other wise the neutral currents will become additive and could overload the neutral.
Based on this logic, is it then true that the neutral conductor in a perfectly balanced system carries no current? Lets say for example we have a typical residential panel. If both hot busbars have equal load demands, then are you saying the neutral conductor returning to the transformer from the panel has no current flowing through it?
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Old 03-31-2010, 06:13 AM   #2
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Originally Posted by secutanudu View Post
I was reading the instructions for the SquareD handle ties I plan to install today...this line has me confused:



Isn't that why I'm doing it? So they trip together? Or am I misunderstanding what it's saying?

Here's the installation manual: http://static.schneider-electric.us/...840-172-01.pdf
Doesn't this mean that adding the ties to the two breakers will not trip or break the common or neutral wire? That is the way I took it
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Old 03-31-2010, 06:15 AM   #3
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Discussion on MWBC


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Originally Posted by jlmran View Post
Based on this logic, is it then true that the neutral conductor in a perfectly balanced system carries no current? Lets say for example we have a typical residential panel. If both hot busbars have equal load demands, then are you saying the neutral conductor returning to the transformer from the panel has no current flowing through it?
That is correct. That is why reduced neutrals are sometimes used.
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Old 03-31-2010, 06:20 AM   #4
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Quote:
Originally Posted by jlmran View Post
Based on this logic, is it then true that the neutral conductor in a perfectly balanced system carries no current? Lets say for example we have a typical residential panel. If both hot busbars have equal load demands, then are you saying the neutral conductor returning to the transformer from the panel has no current flowing through it?
I had to prove this in my own mind before I could believe it, so to do this on my MWBC in my kitchen I took 2 electric irons of the same wattage, I plugged the first in and measured the voltage between the neutral and ground and that was a couple of volts. Then I plugged in the 2nd iron into a different outlet that I was sure was the other circuit (240 between the 2 outlets) and then measured the voltage between neutral and ground and that was 0 and I was convinced.
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Old 03-31-2010, 08:35 AM   #5
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Originally Posted by brric View Post
That is correct. That is why reduced neutrals are sometimes used.
So, in a perfectly balanced system, if no current is flowing through the neutral conductor, how do devices function? If no current is flowing in this scenario then theoretically one could disconnect the main neutral from the panel, which would allow the meter to stop turning, and one could save hundreds of dollars in electric bills, simply by operating a balanced system. Is this what you are saying?

Are you sure you mean to indicate 'no current flow' in the neutral, in a balanced system?
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Old 03-31-2010, 08:54 AM   #6
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Originally Posted by jlmran View Post
So, in a perfectly balanced system, if no current is flowing through the neutral conductor, how do devices function? If no current is flowing in this scenario then theoretically one could disconnect the main neutral from the panel, which would allow the meter to stop turning, and one could save hundreds of dollars in electric bills, simply by operating a balanced system. Is this what you are saying?

Are you sure you mean to indicate 'no current flow' in the neutral, in a balanced system?
Thats incorrect.

In a true balanced system there is no current on the neutral. Think of a 240 volt motor running with 2 hot wires and no neutral. Alternating current allows this to work. If you look further up in this tread you will find some great information and pictures. Oh and the meter still turns regardless of the neutral.
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Old 03-31-2010, 09:01 AM   #7
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Originally Posted by regularguy View Post
Thats incorrect.

In a true balanced system there is no current on the neutral. Think of a 240 volt motor running with 2 hot wires and no neutral. Alternating current allows this to work. If you look further up in this tread you will find some great information and pictures. Oh and the meter still turns regardless of the neutral.
You are correct. I misread the part about disconnecting the neutral. Thanks.
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Old 03-31-2010, 09:27 AM   #8
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Originally Posted by brric View Post
You are correct. I misread the part about disconnecting the neutral. Thanks.
So would the bulbs light up in this diagram?
Attached Files
File Type: pdf Light bulbs.pdf (44.8 KB, 145 views)
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Old 03-31-2010, 10:53 AM   #9
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If they're 120 volt lamps they'll blow as soon as they're energized. If they're 240 volt lamps they'll light more dimly than they shoud than if they were connected in parallel as they should be.
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Old 03-31-2010, 11:21 AM   #10
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In the light bulb diagram, what happens if a connection is made between the white conductor and the neutral bar? Assume 120V lamps. see attachment.
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File Type: pdf Light bulbs neutral.pdf (45.2 KB, 139 views)

Last edited by jlmran; 03-31-2010 at 11:37 AM.
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Old 03-31-2010, 11:22 AM   #11
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Quote:
Originally Posted by regularguy View Post
I had to prove this in my own mind before I could believe it, so to do this on my MWBC in my kitchen I took 2 electric irons of the same wattage, I plugged the first in and measured the voltage between the neutral and ground and that was a couple of volts. Then I plugged in the 2nd iron into a different outlet that I was sure was the other circuit (240 between the 2 outlets) and then measured the voltage between neutral and ground and that was 0 and I was convinced.
This test does not prove anything. The voltage between the neutral and ground does not correlate with whether there is or isn't current on the neutral. In an ideal system, the neutral is always at 0 volts, but it will have current flowing through it. The fact that you meausred 2 volts at one point and 0 volts at another point does not say anything about the current in the neutral.
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Old 03-31-2010, 01:19 PM   #12
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Originally Posted by DetroitEE View Post
In an ideal system, the neutral is always at 0 volts, but it will have current flowing through it.
Isn't current flow dependant upon voltage?
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Old 03-31-2010, 02:15 PM   #13
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Originally Posted by jlmran View Post
Isn't current flow dependant upon voltage?
Current is dependent upon potential difference (you have one wire at 120V and one wire at 0V, and a load in between to utilize the current that is being delivered to it).
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Old 03-31-2010, 02:44 PM   #14
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Quote:
Originally Posted by regularguy View Post
Thats incorrect.

In a true balanced system there is no current on the neutral. Think of a 240 volt motor running with 2 hot wires and no neutral. Alternating current allows this to work. If you look further up in this tread you will find some great information and pictures. Oh and the meter still turns regardless of the neutral.
Please discuss the attached diagram. Typical 240v service, and typical 120v, 100 watt lamps. Is this a balanced MWBC system? Which configuration will allow the lamps to light? Why or why not?
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Discussion on MWBC-mwbc-lamps.jpg  
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Old 03-31-2010, 02:58 PM   #15
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They will light in both instances but not correctly. The first is a series circuit. The second is a series-parallel circuit. Neither is a MWBC the way it is configured.
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