# neutral sharing

matt wrote:

Sam ting (same thing's Chinese cousin) as what happens on the hot wire, only 180 degrees out of phase. What comes out must go back in. Kirchoff's law.
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matt wrote:

The current on one half cancels the current on the other half. Any difference in current flows on the neutral. Say 10 amps is on one half, and 3 amps is on the other - the net current on the neutral is 7 amps.
A DC example may help you: Say you have a 12 volt battery and 3 twelve volt bulbs, each drawing 1 amp:
+----------+---------+--------+ | + | | | Battery Bulb Bulb Bulb | - | | | +----------+---------+--------+
Now, you want to add 3 more bulbs that get their current from a different battery, but you want to save on wire. You can do this:
+----------+---------+--------+ | + | | | Battery Bulb Bulb Bulb | - | | | +----------+---------+--------+ | + | | | Battery Bulb Bulb Bulb | - | | | +----------+---------+--------+
Now to understand what happens to the current flow: First we will stipulate that current flow is from positive to negative. (Depending on who you ask, it is positive to negative, or negative to positive. It doesn't matter for this example, as long as we are consistent, thus the stipulation: we'll use positive to negative.) Arrows will show the direction:
+---->-----+---->----+---->----+ ^ + v v v Battery Bulb Bulb Bulb ^ - v v v +------<---+-----<---+-----<---+
+----->----+---->-----+--->----+ ^ + v v v Battery Bulb Bulb Bulb ^ - v v v +-----<----+----<----+----<----+
Now when we want to combine the two circuits to save wire, look at the arrows:
+---->-----+---->----+---->----+ ^ + v v v Battery Bulb Bulb Bulb ^ - v v v +----><----+----><---+----><---+ <===LOOK ^ + v v v Battery Bulb Bulb Bulb ^ - v v v +-----<----+----<----+----<----+
Notice that current can still go up from the batteries and down through the bulbs, but on the middle line it "wants" to go both left and right at the same time. It can't do that, so the net effect is there is no current on the middle line. The bulbs all light up just fine, because they get the current they need, in spite of the fact that there is no current on the middle line.
If that is hard to envision, think of the current as ping pong balls moving through pipes, and the bulbs as glass windows where you can see the ball pass by. Each arrow also represents a ping pong ball, going in a particular direction. Where two arrows meet like this >< there is no motion.
In your AC residential wiring, the principle is the same. The line where the two circuits are joined is named "neutral". In the residential circuit, current flowing on the neutral to light bulbs in the top half of the drawing opposes current flowing on the neutral to light the bottom half. Thus current flowing at the top of the diagram, then down through the top bulb and trying to go to the left on the neutral can't do it. However, it has another path to follow - it can go down through the bulb in the bottom half, and then go to the left on the bottom wire. So the bulbs light, and there is no current on the neutral. The currents from each half of the circuit cancel out.
Thas describes what happens when the current is equal in both havles of the circuit. If it is unequal, then the difference in current between the top half and the bottom half flows on the neutral. For example, say all three bulbs are turned on in the top half, but only one bulb is turned on in the bottom half:
+---->-----+---->----+---->----+ ^ + v v v Battery Bulb Bulb Bulb ^ - v v v +-----<----+-----<---+-----<---+ <===neutral ^ + v Battery Bulb ^ - v +-----<----+
Each bulb draws 1 amp, so the 3 bulbs on the top draw a total of three amps. The bulb on the bottom draws 1 amp. Thinking of it it terms of ping pong balls, for every 1 ping pong the lower bulb needs, the (combined) 3 upper bulbs put 3 ping pong balls on the neutral wire. The lower bulb takes one of them, leaving 2 on the neutral wire travelling left. So the current on the neutral is 2 amps, and each bulb is getting the 1 amp it needs. Thus the 3 amps of current in the upper bulbs cancels out the 1 amp of current in the lower bulb and the remaing 2 amps flows on the neutral.
Ed
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Good explanation. I almost jumped on you when I saw the arrows going both ways on the neutral wire, but you explained it well. Good job.

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A nice explanation of a multiwire setup, but 'matt' asked "if you have 6 outlets on the *same single pole circuit*..."
That's not what we've been talking about (nor the question that you answered). If all the outlets are on a one single pole circuit, the various load currents do not 'cancel' out in the neutral.
daestrom
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daestrom wrote:

Actually, Matt asked: "whats happening on that common neutral wire?" The subect is neutral *sharing*.
The *common* neutral exists on a multiwire branch, not a regular branch. Feel free to correct the op on "single pole" if you want.
As to "that's not what we've been talking about" - it sure looks to me like "we" have been discussing neutral sharing.
In any event, it was an attempt to help the op understand current on a common neutral.
Ed
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"if you have 6 outlets on the same single pole circuit and each outlet has an appliance plugged into it, all with different loads. whats happening on that common neutral wire?"
Now, see where he said, 'on the same single pole circuit'?? What part of that sounds like multi-wire??
The way he phrased the question, the 'commonn neutral wire' he's asking about is the one feeding the 6 outlets. That's a common wire back to the source, not a 'common neutral' for a multi-wire circuit.
Yes, most of 'us' were discussing 'multi-wire branch circuits'. But then 'matt' jumped in with a question about a single circuit connection. And some folks just went right on and answered him as if he *did* ask about multi-wire circuits. I was pointing out that if you read matt's question carefully, you'd have to give 'matt' a different answer.
daestrom
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daestrom wrote:

We see it differently, but thanks for your opinion.
Ed
Ed