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Old 03-31-2010, 04:19 PM   #16
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Originally Posted by jlmran View Post
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?
If the two light bulbs are of equal wattage (i.e. equal resistance), they will light up exactly the same if the neutral is present or not. Additionally, no current will flow through the neutral if it is present.

Let me try to explain it like this...

POCO provides your home with 240v comming out of a transformer... that's 240v between L1 and L2. All voltages are relative, and in this case the voltage of L2 relative to L1 is 240v.

You also get a third wire from the center of the transformer. We call this the neutral. Since the neutral is half way between L1 and L2, there is 120v between netural and either L. But you have to think of it this way... the voltage between neutral and L1 is 120v while the voltage between neutral and L2 is -120v. Since this is all alternating current, the devices don't care which is negative and which is positive (becasue in reality, every 1/30th of a second, the actual polarity is reveresed).

Because there is 240v between L1 and L2, and because the resistance of the two light bulbs are equal, the voltage drop between them will be equal... therefore there will be 120v accross the two light bulbs and they will pull the exact same current as if the neutral were there or not.

Now here is the reason the current though the neutral is zero when an MWBC is perfectly ballanced...

During the cycling of the alternating current, when the current is flowing OUT of L1, and IN to the neutral, the current is also flowing IN to L2 and OUT of neutral. So what happens when everything is balanced is that ALL the current simply flows OUT of L1 and IN to L2, with all of it bypassing the neutral.

So lets try to think of it this way... voltage at a single point is just a number on a meaningless scale. The only thing that matters is the voltage potential between two points. So I'm going to change the reference point of my scale. I'm going to say L1 is zero, neutral is 120v, and L2 is 240v. Now this also means that I've just defined local earth ground as 120v. But you still are not going to shock yourself when you touch a neutral and ground because both are going to be at a potential of 120v and therefore no current is flow (for current to flow, you must have a voltage difference).

So now, with our two lightbulb that have the exact same resistance, the electrical equation "Voltage Difference = Current * Resistance", the voltage difference across the two light bulbs is going to be equal. So even without a neutral wire in place, the voltage at L2 is 240, the voltage at L1 is zero, and the voltage have way between them (between the two lightbulbs) is 120v. But we've already said that the voltage of neutral (i.e. ground) is also 120v. So if we connect a 120v neutral to a 120v point in the circuit, because the voltage difference between the two points is zero, no current is going to flow.
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Old 03-31-2010, 07:00 PM   #17
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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?
the one with the neutral will work fine no matter what wattage the lights are that are installed. It's not a smart way to do it but it'll work.
the one with no neutral will work if both lights are of equal wattage ,
but lets say you've got a 100 w bulb and a 40w bulb, at this point simple ohms law tells us that the circuit will draw .583 amps. This means that the 40W bulb will get 68.61volts and turn on very dim and the 100w bulb will get 171.53 volts and turn on very bright.....for about 2 seconds before it blows. then both lights go out because you've now got an open on a series circuit.

Last edited by andrew79; 03-31-2010 at 07:07 PM. Reason: more info
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Old 03-31-2010, 07:21 PM   #18
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Commonly used circuit breaker never break the neutral wire. It means that handle ties do not necessarily trip both breakers as does the common trip of a 2 pole circuit breaker.

Sorry, sure got this in the wrong location.

Last edited by brric; 03-31-2010 at 08:19 PM.
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Old 03-31-2010, 07:52 PM   #19
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....This means that the 40W bulb will get 68.61volts and turn on very dim and the 100w bulb will get 171.53 volts and turn on very bright.....for about 2 seconds before it blows. then both lights go out because you've now got an open on a series circuit.
I think you have it backwards: The 40 watt bulb will operate much brighter than normal, and then blow. The 100 watt bulb will survive this experiment.
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Old 03-31-2010, 08:41 PM   #20
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I think you have it backwards: The 40 watt bulb will operate much brighter than normal, and then blow. The 100 watt bulb will survive this experiment.
take your total load which is 140W and divide your votage into it(240V). This will give you the current running through the whole system being as it's series circuit. Now take the wattage of each light individually and divide it by the current to get the voltage dropped across each light. The larger load takes up more of the voltage.
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Old 03-31-2010, 08:47 PM   #21
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the 40 watt bulb survives the experiment.
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Old 03-31-2010, 08:48 PM   #22
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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?
The bottom diagram (with the neutral) represents a multiwire branch circuit.
For a neutral wire sized for the breaker on one of the hot wires, the two hot wires must be connected to opposite sides of the 120/240 volt service.
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Old 03-31-2010, 08:54 PM   #23
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[quote=AllanJ;422421]The bottom diagram (with the neutral) represents a multiwire branch circuit.
For a neutral wire sized for the breaker on one of the hot wires, the two hot wires must be connected to opposite sides of the 120/240 volt service.[/qxxxxxxxxxxxxxxxxxxx

Last edited by brric; 03-31-2010 at 09:04 PM.
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Old 03-31-2010, 09:00 PM   #24
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That is not a MWBC. The neutral needs to be connected independantly of each lamp. The two lamps would not be connected to each other in a MWBC.
the lamps aren't connecting to each other...that's the neutral. Each lamp has it's own feed and they're sharing a neutral. Assuming that each line is connected to opposite "phases" and the bulbs are exactly the same wattage then there will be no current comming back on that neutral. It's a diagram for discussion "in theory"....hopefully no one would ever actually do this.
the one problem that you will have with this circuit is that if you shut one leg off your essentially backfeeding it with the other. So if you happen to lose the neutral or happen to provide a better path to ground it could cause some serious hurt.
3 phase systems are a MWBC....shared neutral. The second diagram in essence is infact a multiwire branch circuit. Although i know you can use a shared neutral with 3 phase i'm not sure what the code states on attempting it in a house. I'm fairly sure it's not legal.

Last edited by andrew79; 03-31-2010 at 09:10 PM.
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Old 03-31-2010, 09:08 PM   #25
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the lamps aren't connecting to each other...that's the neutral. Each lamp has it's own feed and they're sharing a neutral. Assuming that each line is connected to opposite "phases" and the bulbs are exactly the same wattage then there will be no current comming back on that neutral. It's a diagram for discussion "in theory"....hopefully no one would ever actually do this.
the one problem that you will have with this circuit is that if you shut one leg off your essentially backfeeding it with the other. So if you happen to lose the neutral or happen to provide a better path to ground it could cause some serious hurt.
If no current flows back through neutral, then why would losing the neutral create a problem?

No one ever do this? There have been a substantial amount of posts on this website about MWBC. Someone is doing it.
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Old 03-31-2010, 09:09 PM   #26
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Another way of thinking about things.

The sum of the currents entering and leaving any point in the system, where entering is represented by a positive number and leaving is represented by a negative number, is zero.

Consider the T junction of the white lines in the diagram with the neutral. The current entering from the left lamp minus the current leaving and returning via the neutral minus the current leaving and going to the second lamp equals zero.

If the current coming from the first lamp is equal to the current going to the second lamp (via the white T junction as shown), which would be true if the lamps had equal resistance, then the current returning vai the neutral must be zero.
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Old 03-31-2010, 09:09 PM   #27
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the lamps aren't connecting to each other...that's the neutral. Each lamp has it's own feed and they're sharing a neutral. Assuming that each line is connected to opposite "phases" and the bulbs are exactly the same wattage then there will be no current comming back on that neutral. It's a diagram for discussion "in theory"....hopefully no one would ever actually do this.
the one problem that you will have with this circuit is that if you shut one leg off your essentially backfeeding it with the other. So if you happen to lose the neutral or happen to provide a better path to ground it could cause some serious hurt.
You are right. I stand corrected. It is also a perfect illustration of how 240 volt will flow through 120 volt devices if a service neutral is lost.
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Old 03-31-2010, 09:12 PM   #28
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It is also a perfect illustration of how 240 volt will flow through 120 volt devices if a service neutral is lost.
Agreed...but this statement is true regardless of MWBC presence or not...due to a common neutral bar. Correct?
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Old 03-31-2010, 09:12 PM   #29
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If no current flows back through neutral, then why would losing the neutral create a problem?

No one ever do this? There have been a substantial amount of posts on this website about MWBC. Someone is doing it.
Losing the neutral leaves a 240 volt circuit from line 1 through the lamps to line 2. We use MWBC's all the time.
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Old 03-31-2010, 09:14 PM   #30
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Agreed...but this statement is true regardless of MWBC presence or not...due to a common neutral bar. Correct?
Yes that is correct.
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