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

[Besoeker]

Over time- however at any one point in time there is polarity. This matters when hooking up/paralleling transformer windings, for example.

Where it is needed, and that's not often, we'd use the dot notation to show starts and finishes of windings.
Usually the diagram suffices. A hexaphase is exactly what it says on the box to paraphrase a UK TV ad....

 

[LarryFine]

Isn't it usually derived from a centre-tapped 240V winding giving 120-0-120 ?

It can be that, or it can be two separate 120v windings that can be connected in series or in parallel. I series, you can, but don't have to, connect a wire to the central junction point.

I'd have called that a single supply in the same way I'd consider three-phase star (wye) as one supply, one source.

Not that that I think it matters all that much as long as we can clearly communicate what system we are discussing.

Agreed on both points.

 

[jaggedben]

It can be that, or it can be two separate 120v windings that can be connected in series or in parallel. I series, you can, but don't have to, connect a wire to the central junction point.

Once again, the word 'parallel' just doesn't belong in this discussion about an MWBC. That would be something entirely different.

 

[LarryFine]

I agree that a MWBC requires that the two windings of a dual-wound secondary must be connected in series, but they could be connected in parallel if you only need a single voltage, which of course can not provide a MWBC.

For example, a utility transformer's output can be configured for 120/240v, like the one that supplies your home, or for 120v like one of the three that supplies a 208Y/120v 3ph service, by rearranging internal jumper bars.

Thus, the word 'parallel' can certainly be in the discussion, but it also certainly does not apply to a MWBC.

 

[SG-1]

Thus, the word 'parallel' can certainly be in the discussion, but it also certainly does not apply to a MWBC.

I need this explained:

In a series circuit when you switch one load off, they are all off.

In a parallel circuit when one load is switched off, the others continue to operate normally.

A series circuit component has no more that one point in common the source.

A parallel circuit component has two points in common with the source.

Which of the above statements is false & causing my confusion ?

 

[Jamesco]

Over time- however at any one point in time there is polarity. This matters when hooking up/paralleling transformer windings, for example.

Not just paralleling. Though reversing one winding with respect to the other can be like two Bull Moose head butting one another.

If winding polarity is not observed when connecting two windings in series, say in the case of 120/240 output, it is possible to connect the two windings in series and have 120V from each of the outer most leads of the series windings to the center connection neutral, but zero volt nominal from one outer most lead to the other outer most lead.


.

 

[SG-1]

Not just paralleling. Though reversing one winding with respect to the other can be like two Bull Moose head butting one another.

If winding polarity is not observed when connecting two windings in series, say in the case of 120/240 output, it is possible to connect the two windings in series and have 120V from each of the outer most leads of the series windings to the center connection neutral, but zero volt nominal from one outer most lead to the other outer most lead.


.

I have actually done that experiment with a small 2kva distribution transformer. The neutral current adds in the neutral conductor.

 

[gar]

180412-2140 EDT

A MWBC is a parallel circuit. You have two voltage sources, each with a high internal impedance, connected in parallel, and connected to a very low impedance load. That load is the common neutral wire to the center tap.

Se SG-1's description of how to identify a series circuit.

.

 

[jselesk2]

Can anyone give a correct mathematical proof that answers my question? All these different analogies really aren't helping my understanding.

 

[LarryFine]

I need this explained:

In a series circuit when you switch one load off, they are all off.

True, in that any opening in the circuit breaks the single current pathway. Example: old-fashioned Xmas light string; one bulb blows, all lights off.

In a parallel circuit when one load is switched off, the others continue to operate normally.

True. Example: every circuit in your house.

A series circuit component has no more that one point in common the source.

I don't understand that, but the two ends of the string of loads are supplied from the two terminals of the power source.

A parallel circuit component has two points in common with the source.

True, which applies to each paralleled branch of the circuit just like the single series circuit in the previous answer; there are merely multiple circuits.

Which of the above statements is false & causing my confusion ?

You'd have to express your confusion one more time.

 

[LarryFine]

If winding polarity is not observed when connecting two windings in series, say in the case of 120/240 output, it is possible to connect the two windings in series and have 120V from each of the outer most leads of the series windings to the center connection neutral, but zero volt nominal from one outer most lead to the other outer most lead.

True, just as if you placed one battery incorrectly in a 2-battery device.

 

[LarryFine]

I have actually done that experiment with a small 2kva distribution transformer. The neutral current adds in the neutral conductor.

Right. You just can't supply any line-to-line loads. This is what happens when you place more than one breaker on the same phase on a MWBC.

 

[LarryFine]

Can anyone give a correct mathematical proof that answers my question? All these different analogies really aren't helping my understanding.

Please read this, if you haven't already, and then ask a specific question: http://forums.mikeholt.com/showthread.php?t=189279

 

[jaggedben]

I agree that a MWBC requires that the two windings of a dual-wound secondary must be connected in series, but they could be connected in parallel if you only need a single voltage, which of course can not provide a MWBC.

Right. Parallel does not provide an MWBC, and thus parallel circuits are off topic with respect to the OP's question. You could play baseball in Australia on a Tuesday. That's also off topic.

 

[jaggedben]

I need this explained:

In a series circuit when you switch one load off, they are all off.

In a parallel circuit when one load is switched off, the others continue to operate normally.

A series circuit component has no more that one point in common the source.

A parallel circuit component has two points in common with the source.

Which of the above statements is false & causing my confusion ?

The red statement is not false, per se, but you're asking too much of it. It's not a definition. There are plenty of other arrangements where it is true. For example, loads connected to two completely different sources with absolutely no connection whatsoever (e.g. two flashlights) still satisfy the red statement, but are not a parallel circuit. Likewise, the red statement is true of loads on the different legs of an MWBC, but that doesn't mean it's a parallel circuit. Same with circuit arrangements than have a common return that isn't a neutral.

Now consider your blue statement, which is essentially correct. Do two loads on the two different parts of a 3 wire MWBC have two points in common with any one source? No, they don't. They only have one point in common with each other, which is the neutral. Black does not connect to red if you don't want a short circuit. Thus if the blue statement is true, an MWBC is not a parallel circuit.

To be paralleled, both (or all) of each conductor have to be joined together. i.e. black to black and white to white, and red to red if there is one (and so on). If you're just joining the neutral, it isn't the same arrangement. (I'm not too keen on the convention of referring to a parallel circuit in the singular. What you have, really, is paralleled circuits, i.e. two or more circuits that are combined into a single one somewhere. And BTW, you can parallel loads to the same source, or sources to the same load, or multiple sources and loads together on a complicated distribution system.)

To remind you of why it's not just semantics: in post #25 you said "We know that, in a parallel circuit the voltage across each load is the same voltage as the source it is in parallel with." That is true in a parallel circuit because any two corresponding sets of conductors that you would be measuring voltage on go back to the same two points. But the green statement would no longer be true if you go around saying that an MWBC is included in the definition of a parallel circuit. To repeat, the voltage across a load on black and white is not the same as the voltage across a load on red and white, because they aren't paralleled to the same two points and thus not paralleled to the same source.

 

[jaggedben]

180412-2140 EDT

A MWBC is a parallel circuit. You have two voltage sources, each with a high internal impedance, connected in parallel, and connected to a very low impedance load. That load is the common neutral wire to the center tap.

Se SG-1's description of how to identify a series circuit.

.

:lol::lol::lol: I guess you're right. But to repeat, being a parallel circuit does not explain why the voltage from black to white and red to white are the same, because they're not.

I fear I'll have to take back my posts above, but it will just confuse people instead of helping.

 

[SG-1]

Here are two MESH analysis of a balanced MWBC that I did last year. I wired the circuit both ways using 4 ammeters to confirm the analysis. The lower transformer polarity is swapped in one resulting in double current through the neutral.

I chose the loop direction to assume the current was exiting the polarity terminal of the transformer ( marked with "x" ) in each analysis.

I will do an unbalanced analysis & post it later.

 

[SG-1]

The red statement is not false, per se, but you're asking too much of it. It's not a definition. There are plenty of other arrangements where it is true. For example, loads connected to two completely different sources with absolutely no connection whatsoever (e.g. two flashlights) still satisfy the red statement, but are not a parallel circuit. Likewise, the red statement is true of loads on the different legs of an MWBC, but that doesn't mean it's a parallel circuit. Same with circuit arrangements than have a common return that isn't a neutral. No, the statement is not a definition, it is a condition. A condition that the circuit must meet. The two flashlights are not wired into the same circuit, so they do not meet the condition.
Now consider your blue statement, which is essentially correct. Do two loads on the two different parts of a 3 wire MWBC have two points in common with any one source? No, they don't. They only have one point in common with each other, which is the neutral. Black does not connect to red if you don't want a short circuit. Thus if the blue statement is true, an MWBC is not a parallel circuit Both ends of each load is connected to both ends of the 120V transformer coil. They have two points in common, parallel. The series branch of the circuit has one point of each load connected across both transformer coils (240V) Black & Red.

To be paralleled, both (or all) of each conductor have to be joined together. i.e. black to black and white to white, and red to red if there is one (and so on). If you're just joining the neutral, it isn't the same arrangement. (I'm not too keen on the convention of referring to a parallel circuit in the singular. What you have, really, is paralleled circuits, i.e. two or more circuits that are combined into a single one somewhere. And BTW, you can parallel loads to the same source, or sources to the same load, or multiple sources and loads together on a complicated distribution system.)

To remind you of why it's not just semantics: in post #25 you said "We know that, in a parallel circuit the voltage across each load is the same voltage as the source it is in parallel with." That is true in a parallel circuit because any two corresponding sets of conductors that you would be measuring voltage on go back to the same two points. But the green statement would no longer be true if you go around saying that an MWBC is included in the definition of a parallel circuit. To repeat, the voltage across a load on black and white is not the same as the voltage across a load on red and white, because they aren't paralleled to the same two points and thus not paralleled to the same source.

Now I see where we parted ways.
The MWBC has two 120V sources. One load or set of loads is wired to one source & the other set of loads or load is wired to the other source, no short circuit.

The problem is that a MWBC has both parallel & series paths. The series path is why things burn when the neutral opens & disables the parallel path.

A MWBC could be described as two paralleled circuits as you said in the above paragraph. All loads in a normal dwelling are wired in parallel, this must include all loads on a MWBC.

 

[SG-1]

True, in that any opening in the circuit breaks the single current pathway. Example: old-fashioned Xmas light string; one bulb blows, all lights off.

I don't understand that, but the two ends of the string of loads are supplied from the two terminals of the power source.




Saying the same thing a different way. We are on the same page.

 

[jaggedben]

Can anyone give a correct mathematical proof that answers my question? All these different analogies really aren't helping my understanding.

I think this still deserves a response with more robust math.

See https://www.youtube.com/watch?v=gisNQMMGPAQ

Check out the formula at 27 seconds. Note that R2 is playing the role of the neutral in an MWBC.

RT=5+ 1/(1/3+1/4) = 6.71 ohms

I've highlighted the neutral resistance there. Now let's see what happens as the neutral resistance drops to the negligible amount of a copper wire.

RT=5+ 1/(1/2+1/4) = 6.33 ohms
RT=5+ 1/(1/1+1/4) = 5.8 ohms
RT=5+ 1/(1/0.1+1/4) = 5.1 ohms
RT=5+ 1/(1/0.01+1/4) = 5.01 ohms

As the neutral resistance approaches zero, the total resistance of both parallel routes approaches the value of R1, which is 5. Do the rest of the math on the currents and voltages, and you'll see as R2 diminishes, the voltage drop across R1 approaches the total source voltage of 6V, and the current across R2 approaches the total current across R1. R3 matters less and less, and draws less and less current.