Question Regarding Current Imbalances and Neutral Wire

[flintstrike37]

I hope this is an ok place to ask this question. I found the following thread from 2007 after googling a question about the neutral wire:

Help Understanding Neutral Conductor

Essentially I had the same question as the original poster. All the explanations were extraordinarily helpful, particularly the one from LarryFine on the first page. I was actually drawing the circuit he was describing and was completely understanding the explanation until I got to one part:

If we happen to add matching wattages of 1.5-volt bulbs to the two 1.5-volt halves of our 3-volt system, no current will flow through the wire to the center tap. However, if we have an imbalanced set of loads, the difference current will flow on the neutral.

For example, let's say we have 2 amps flowing through the upper wire, and three amps flowing on the lower wire. That's a difference of one amp, and that will flow on the neutral conductor to the center tap. However, nothing will flow into the earth.

So, I understand in a 3 wire system you have 240v between the two hot wires because they are out of phase with one another (polarity is opposite), and that the neutral wire is connected to the center tap of the transformer and therefore provides 120v between the hot wires, and also that the neutral is considered the grounded conductor. But, I still do not understand why the "imbalance" of current between the two hot wires flows back through the neutral wire. If I put a one 2 amp load on each of my hot circuits, why is there no current traveling back through the neutral wire? I though the current always followed the return path of the circuit. Along the same lines, if I put one 2 amp load on the "A" hot wire and one 4 amp load on the "B" hot wire, why do I only have 2 amps returning on the neutral wire?

 

[ggunn]

I hope this is an ok place to ask this question. I found the following thread from 2007 after googling a question about the neutral wire:

Help Understanding Neutral Conductor

Essentially I had the same question as the original poster. All the explanations were extraordinarily helpful, particularly the one from LarryFine on the first page. I was actually drawing the circuit he was describing and was completely understanding the explanation until I got to one part:



So, I understand in a 3 wire system you have 240v between the two hot wires because they are out of phase with one another (polarity is opposite), and that the neutral wire is connected to the center tap of the transformer and therefore provides 120v between the hot wires, and also that the neutral is considered the grounded conductor. But, I still do not understand why the "imbalance" of current between the two hot wires flows back through the neutral wire. If I put a one 2 amp load on each of my hot circuits, why is there no current traveling back through the neutral wire? I though the current always followed the return path of the circuit. Along the same lines, if I put one 2 amp load on the "A" hot wire and one 4 amp load on the "B" hot wire, why do I only have 2 amps returning on the neutral wire?

Consider a 120V load, line 1 to neutral. The current travels up the line to the load, through the load and back along the neutral, right? Well, half the time, yes; the rest of the time the current is flowing the other way.

Now think of the same size load line 2 to neutral. It's the same thing, except when the current is flowing line 1 to neutral in the first circuit it is flowing neutral to line 2 in the other.

Now tie the two neutrals together. At any point in time you happen to pick, the neutral current flowing one direction from the first circuit exactly cancels the neutral current flowing the other direction from the second circuit. All the current flows in the line conductors and there is no current flow in the neutral. The loads are balanced.

Now make the loads different sizes (an unbalanced load). The current through the neutral from the lesser load is completely canceled by an equal portion of the current from the greater load, and the rest of the current from the greater load is carried back to the source on the neutral.

 

[flintstrike37]

Thank you for the reply. Your explanation is helpful. Although it seems quite logical now, it had not occurred to me that the neutral wire is simultaneously serving as the return path of the current on line 1, while it is serving as delivery path of the current on line 2. Forgive my lack of understanding, but I still have a question. I get that that when the amperage is equal they cancel each other and when they are not equal the remaining current on the greater load is carrier on the neutral. . . but *what* exactly is causing it to "cancel" out. Is it because the currents are 180 degrees out of phase from one another, or am I way off base on that?

 

[flintstrike37]

Thank you for the reply. Your explanation is helpful. Although it seems quite logical now, it had not occurred to me that the neutral wire is simultaneously serving as the return path of the current on line 1, while it is serving as delivery path of the current on line 2. Forgive my lack of understanding, but I still have a question. I get that that when the amperage is equal they cancel each other and when they are not equal the remaining current of the greater load is carried on the neutral. . . but *what* exactly is causing it to "cancel" out. Is it because the currents are 180 degrees out of phase from one another, or am I way off base on that?

 

[kingpb]

In the simplest term, yes.

 

[kwired]

Thank you for the reply. Your explanation is helpful. Although it seems quite logical now, it had not occurred to me that the neutral wire is simultaneously serving as the return path of the current on line 1, while it is serving as delivery path of the current on line 2. Forgive my lack of understanding, but I still have a question. I get that that when the amperage is equal they cancel each other and when they are not equal the remaining current of the greater load is carried on the neutral. . . but *what* exactly is causing it to "cancel" out. Is it because the currents are 180 degrees out of phase from one another, or am I way off base on that?

I would say yes, you have an equal magnitude simultaneously going opposite directions so the net is zero.

I don't know that I dare get more complicated, but if you have a neutral on a three phase wye system and equal phase to neutral on each phase they don't cancel each other with just two phases and a neutral but do with all three phases and a neutral. But the voltages and currents of each coil are 120 degrees apart instead of 180 in this particular system.

But at same time you can simplify things by using two batteries and connecting them in series and then connecting loads from end to end, end 1 to center, or end 2 to center and you will still get similar results, the "neutral" will only carry imbalance current but we have DC current instead of AC. I guess we still in a way have a 180 degree differential as one end is + and the other is -, it is just continuously that polarity where in an AC circuit polarity is cycling, but at same rate on each end so they are always same differential to one another.

 

[ggunn]

Thank you for the reply. Your explanation is helpful. Although it seems quite logical now, it had not occurred to me that the neutral wire is simultaneously serving as the return path of the current on line 1, while it is serving as delivery path of the current on line 2. Forgive my lack of understanding, but I still have a question. I get that that when the amperage is equal they cancel each other and when they are not equal the remaining current on the greater load is carrier on the neutral. . . but *what* exactly is causing it to "cancel" out. Is it because the currents are 180 degrees out of phase from one another, or am I way off base on that?

No, you are not off base. That's the explanation in the simplest terms, and of course there can be complicating factors, but essentially you've got it.