Multi wire branch circuit

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The electron man

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Question regarding mwbc if i have 2 equal loads the neutral will have zero current flow and the circuit will operate as a 240v circuit, my question is if the neutral is not being used and you have 2 separate loads how is the loop created to complete circuit
 
Both poles of the hot wires are connected to opposite legs so 240 volts between them. The resistances are the same on both legs. As the ac wave cycles 1 leg is at it's peak voltage while the other leg is low so the current is balanced on each leg and the neutral is not used.

This is in theory only and with different wire lengths the circuit can't ever be truly 100% balanced so the neutral always carries something.
 
The electron man said:
Question regarding mwbc if i have 2 equal loads the neutral will have zero current flow and the circuit will operate as a 240v circuit, my question is if the neutral is not being used and you have 2 separate loads how is the loop created to complete circuit
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Only the long wire labeled N in your drawing, from the neutral bar on the left to the 3-way N junction, has 0 current. The other two shorter N wires to each load each carry current.

So the loop is black - load 1 - short N1 wire - short N2 wire - load 2 - red.

Cheers, Wayne
 
wwhitney said:
Only the long wire labeled N in your drawing, from the neutral bar on the left to the 3-way N junction, has 0 current. The other two shorter N wires to each load each carry current.

So the loop is black - load 1 - short N1 wire - short N2 wire - load 2 - red.

Cheers, Wayne
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So were dose the current go once it reaches the j box
 
The electron man said:
So were dose the current go once it reaches the j box
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It comes in N1 and out N2 (or vice versa), where N1 and N2 are the short N wires on the right.

In other words, if you are thinking that at the same point in time that current is flowing towards the junction box in both N1 and N2, that is not correct for this configuration.

If you look at post #3, current is flowing towards the loads in the black conductor, and away from the loads in the red conductor. In the opposite half of the cycle (1/120 seconds later), current will be flowing away from the loads in the black conductor, and towards the loads in the red conductor.

Cheers, Wayne
 
wwhitney said:
It comes in N1 and out N2 (or vice versa), where N1 and N2 are the short N wires on the right.

In other words, if you are thinking that at the same point in time that current is flowing towards the junction box in both N1 and N2, that is not correct for this configuration.

If you look at post #3, current is flowing towards the loads in the black conductor, and away from the loads in the red conductor. In the opposite half of the cycle (1/120 seconds later), current will be flowing away from the loads in the black conductor, and towards the loads in the red conductor.

Cheers, Wayne
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You just gave me the aha moment now i see how the loop is completed through the short neutrals you the man
 
A useful concept to add is the 'principal of superposition', which shows up in lots of places.

Basically if you have 2 valid circuits, you can add them up and still have a valid circuit. @roger 's diagram in post 3 shows this. The black circuit alone is valid with 10A on the neutral. The red circuit alone is valid with 10A of the opposite polarity on the neutral. Combine both circuits so the neutral is shared, and you have a valid circuit with the sum current on the shared conductor
 
winnie said:
Basically if you have 2 valid circuits, you can add them up and still have a valid circuit.
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I think what tripped up the OP is this:

If you are thinking of a 120V 2-wire circuit, it is common to think of current going out on the ungrounded conductor and returning on the neutral. With AC, that is of course only true half the time each cycle; the other half the cycle current is taking the reverse route.

If you have a configuration as in the OP, and you assume that simultaneously current is going out on both ungrounded conductors and returning on N, you don't get 0 current in the common N. And if the common N were removed, you would get current piling up at the junction box. That would be the case if you don't have an MWBC, and both ungrounded conductors are supplied from the same leg.

So you need to recognize that with an MWBC, when current is going out on the black conductor, that is the half-cycle when current is returning on the red conductor. And vice versa.

Cheers, Wayne
 
OP, read this post:

Understanding the Neutral Conductor

Dennis, here's a really basic lesson, and we'll start with direct current, because the fact that we use alternating current is not relevant to the basics: Picture a battery, like you'd find two of in a flashlight. You have a terminal at each end, with a difference in potential of 1.5 volts...
forums.mikeholt.com
 
Think of it as a 240 circuit with the neutral added, rather than a 120/240v circuit with it removed.

In other words, how do you think line-to-line loads work without a neutral.
 
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The wire nut in the yellow connecting the two neutrals together is the common point allowing the current to return via line1/line2 and complete the circuit.

Just picture two bulbs wired in series from a two pole breaker and that’s what it is. The neutral would only carry the unbalanced (which in this case there would be none)
 
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