How does the neutral wire prevent MWBCs from operating at 240v?

[jselesk2]

I am aware that if the neutral wire becomes open on an MWBC, this will result in a 240v circuit across the two phases of the circuit. I am curious as to why this does not happen under normal operation of a MWBC in which the two phases share a neutral wire. Why does all the current coming from both phases end up traveling down the neutral and not from phase to phase? Thank you.

 

[roger]

It wouldn't be 240 across both (actually it wouldn't be 240 on either but that is nit picking) it would be high on one side and low on the other, see the illustrations below.

Here is a true neutral

And a neutral carrying the unbalanced current.

Roger

 

[480sparky]

Transformers that supply 120/240v circuits have 240v windings with a center tap. When the neutral is connected, it connects to the center tap of the transformer, creating two 120v circuits and the multi-wire operates as a 120/240v circuit. Without that connection, no 120v circuit is created by the transformer. So only 240 volts can be supplied by the transformer.

 

[romex jockey]

I am aware that if the neutral wire becomes open on an MWBC, this will result in a 240v circuit across the two phases of the circuit. I am curious as to why this does not happen under normal operation of a MWBC in which the two phases share a neutral wire. Why does all the current coming from both phases end up traveling down the neutral and not from phase to phase? Thank you.

If i'm reading you right it's because a 120V load uses one side of the cycle , not both

~RJ~

 

[JFletcher]

The resistance on the neutral is much much much lower then the resistance in either 120 volt load. if the resistance starts rising, like due to a poor connection, or becomes infinite, due to a cut or lifted wire, then those 120 volt loads are effectively wired in series instead of parallel, and will see 240 volts across the pair of appliances....

 

[petersonra]

you still have 240v across the two high legs. the neutral has no effect on that relationship.

 

[ActionDave]

I am aware that if the neutral wire becomes open on an MWBC, this will result in a 240v circuit across the two phases of the circuit. I am curious as to why this does not happen under normal operation of a MWBC in which the two phases share a neutral wire. Why does all the current coming from both phases end up traveling down the neutral and not from phase to phase? Thank you.

I know what you are asking, I asked the same question when I first joined here. I have a little better understanding now than I did then, but for me it's still just easier to accept that it works because that's the way it works.

 

[LarryFine]

If I may, read this: http://forums.mikeholt.com/showthread.php?t=189279

 

[Jamesco]

Here is nice video that may help you better understand.
https://www.youtube.com/watch?v=eVamt9IdQd8

 

[ActionDave]

Roger's diagrams, Larry's written explanation, and the video Jamesco posted all do a good job of telling us what the neutral does but none explain why. How do the electrons on an unbalanced MWBC coming from phase A know they are supposed to turn right and head down the neutral path, and the electrons from phase B know they are supposed to turn left and head down the same neutral? Why don't they go wherever they want on whatever copper wire they like the best?

 

[Besoeker]

Roger's diagrams, Larry's written explanation, and the video Jamesco posted all do a good job of telling us what the neutral does but none explain why. How do the electrons on an unbalanced MWBC coming from phase A know they are supposed to turn right and head down the neutral path, and the electrons from phase B know they are supposed to turn left and head down the same neutral? Why don't they go wherever they want on whatever copper wire they like the best?

Common misconception about electrons and ac.

 

[ActionDave]

Common misconception about electrons and ac.

I'm a commoner.

 

[LarryFine]

How do the electrons on an unbalanced MWBC coming from phase A know they are supposed to turn right and head down the neutral path, and the electrons from phase B know they are supposed to turn left and head down the same neutral? Why don't they go wherever they want on whatever copper wire they like the best?

It's all because of voltage applied and resultant current. The neutral carries whatever current it must to keep the load end neutral point at the same relative voltage as the supply end of it, the same way the line conductors do the same thing: keep the load end at the same potential as the supply end.

Current through the resistance of the wire causes that effort to slightly fail, and we call the result voltage drop. It happens on the neutral as well, but only to the extent of that caused by the difference between the loads on each line. So, the direct answer to your question is "the relative impedances".

 

[mbrooke]

Roger's diagrams, Larry's written explanation, and the video Jamesco posted all do a good job of telling us what the neutral does but none explain why. How do the electrons on an unbalanced MWBC coming from phase A know they are supposed to turn right and head down the neutral path, and the electrons from phase B know they are supposed to turn left and head down the same neutral? Why don't they go wherever they want on whatever copper wire they like the best?

Phase angle. The electrons go where ever forces (magnitude and angle) push them.

 

[JFletcher]

If you think of electricity as a living thing, it is lazy and resourceful. The neutral is the path of the least resistance back to the source so it will take that path versus going through another piece of equipment with high resistance.

If you remove the neutral on a multiwire branch circuit or a service, electricity is still going to find a way back to the source it will just have to go through the other phase and the equipment attached to it. Unless both pieces of equipment have almost the same resistance, one will see a higher voltage than the other, and if it's bad enough, that piece of equipment will let out the Magic Smoke.

In Lay terms that's about the best way I can explain why the neutral works.

( and yes, I'm aware that electricity takes all paths back to the source)

 

[Besoeker]

I'm a commoner.

And I'm sure you have abundant capacity to learn...............

 

[kwired]

If you think of electricity as a living thing, it is lazy and resourceful. The neutral is the path of the least resistance back to the source so it will take that path versus going through another piece of equipment with high resistance.

But only the unbalanced current is what returns on the neutral.

Now complicate it with the 120 degree phase angle of a three phase wye system. Still similar concepts but it doesn't balance out the same way, especially when only two lines and the neutral are what is involved.

 

[wwhitney]

Roger's diagrams, Larry's written explanation, and the video Jamesco posted all do a good job of telling us what the neutral does but none explain why. How do the electrons on an unbalanced MWBC coming from phase A know they are supposed to turn right and head down the neutral path, and the electrons from phase B know they are supposed to turn left and head down the same neutral? Why don't they go wherever they want on whatever copper wire they like the best?

The electrons do go wherever they want--it's a question of determining the rules that express what they want. What electrons want is to reduce their energy, and voltage is a measure of the energy per electron. So electrons want to move from higher voltage to lower voltage. [The analogous way of looking at gravity is to say that objects want to reduce their potential energy, that is why they move from a higher altitude to a lower altitude, i.e. fall.]

When we wire up the MWBC, we want the electrons to give up the same energy (per electron) in each of the two loads. The neutral connection, as a source or sink of electrons at the midpoint energy, ensures they do this. If the electrons started to give up more or less energy (per electron) in one load, the energy imbalance would cause more electrons to flow through the neutral connection one way or the other to counteract that imbalance.

Cheers, Wayne

 

[Andy Delle]

If i'm reading you right it's because a 120V load uses one side of the cycle , not both

~RJ~

No, both the side of the sinewave are used. It's just that they are 180 degrees out of phase. So at any given instant the neutral only sees the current on either the positive or negative sinewave, not both.

 

[GoldDigger]

No, both the sin and cosine are used. It's just that they are 180 degrees out of phase. So at any given instant the neutral only sees the current on either the sine or cosine load, not both.

The sine and cosine functions are 90 degrees out of phase, not 180.
The two lines in 120/240 3-wire single phase are +sin(omega*t) and -sin(omega*t) respectively.

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