Why is it said that a typical 120V household circuit "reverses" polarity?

Question

Responders have been very helpful with a recent post of mine. But I'm sorry to say this part still perplexes me: In a 120VAC circuit, current flows from Line 1 through the load, and returns via Neutral. If it had been a 240V circuit, the return path would have been through Line 2.

When the current reverses, the flow now originates at Line 2, not Line 1, so how does the 120V circuit reverse? Is there a "push" back from Neutral to Line 1, similar to the "push" from Line 2? If so, what is the source of the reversed current flow?

I have always felt that the Neutral leg was not energized. Please, would someone tell me what I'm missing?

Answer 1

My previous answer was a bit unclear, and a bit misleading. So I'm going to take another stab at it.

As before, we'll start by looking at the secondary side of a single split-phase transformer. And again, the loads are represented by purple rectangles.

Then we'll spit the circuit up into individual circuits, so we can take a closer look at each circuit.

If we visualize the current flowing through the circuits in the first 60th of a second, the current will flow like this.

During the next 60th of a second, the current will flow the opposite way.

The interesting thing about this, however, is if you look at the current flow on the neutral. When you overlay the images, you'll see that the current flows both ways simultaneously on the neutral.

Because of science, the opposing forces cancel each other out. However, if one force is larger than the other, the current will not be completely cancelled out. So the "unbalanced" current will flow on the neutral, in whichever direction it's flowing during that cycle.

So technically, you could say the current is flowing both ways, and neither way (if they completely balance) at the same time.

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Extra Credit

Facts

• Neutral is always "0" volts.

• L1 and L2 cycle from positive volts, to negative volts (alternately).

• Energy always flows from points of higher potential, to points of lower potential.

Therefore...

When L1 is greater than "0" volts, current flows from L1 to N. When L1 is less than "0" volts, current flows from N to L1.

When L2 is greater than "0" volts, current flows from L2 to N. When L2 is less than "0" volts, current flows from N to L2.

When L1 is at a higher potential than L2, current flows from L1 to L2. When L1 is at a lower potential than L2, current flows from L2 to L1.

Answer 2

You have a "pure" sense of how electricity works, but are confused by the weird idioms of NEC electrical. That's understandable. Let me try to make it clearer. First, here's a simple application.

I'm sure you have an easy time understanding the above. Clear as a bell, right? Even if the lights have switches on them, it's pretty easy to understand. Now, transformers isolate each side, so your 120v supply has no potential with the pole line. Now suppose you want more. For whatever reason, you add another transformer like this:

OK, the above is still really easy, right? Circuits A, B, ground and pole line are still totally isolated from each other. If you stuck a voltmeter in lamp sockets from the two different circuits, you'd measure 0 volts and infinity ohms. Assuming everything is working properly, that is...

What if they weren't isolated? What if they were connected in a particular way?

Consider the effect on the landscape. Everything still works exactly the same as before. Only now, circuits A and B have a relationship. What's the voltage between the top of circuit A and bottom of circuit B? Well, this is AC - so it matters an awful lot how these transformers are phased. When transformer A is at the top of cycle (top wire + bottom wire -) if transformer B is the same, then the voltage across far sides of the transformers is 240V. Make sense? So now, you can use that 240V dryer. And that is the essence of how 120/240V works in a household setting. You've got it!

Now let's talk about safety. 240V is significantly more dangerous than 120v, and by bridging the transformers, we've created 240V potential. What's an easy and quick way to minimize that risk and keep most circuits at 120v? Try the below.

We've given the middle point a name - "neutral" - and to enforce that, we've tied it to actual earth ground by connecting it to a water pipe. Now each "leg" is 120V away from neutral - out of phase, of course, so we can get 240V by bridging them. Even the 240V service is no farther than 120V from earth ground.

Now, let's try to cut some costs. There's no reason to use two expensive transformers when you can just use one with a third winding. And now that the neutral wires are redundant, we can use one wire for that - mind you, that transformer might be up on a pole at the end of your street, so 3 wires instead of 4 is a big cost savings.
And notice what happens with current flow (ignoring the dryer). Starting at the bottom wire, two lamps worth of power flows through the lower lamps and through the upper lamps to the top wire, bypassing neutral entirely. Only that third upper lamp actually flows its current from neutral to the top wire. ("from" and "to" are interchangeable since this is AC). This is called an imbalance, and is no big deal. The difference flows down neutral. That lets us use 3 wires instead of 4 in places, particularly expensive places like transformer to service panel (breaker box).

Now replace the "lamps" with "circuit breakers and circuits" and you pretty much have a modern service panel (breaker box). We'll bring "earth ground" to every device and outlet (I'm not going to illustrate that) on a third wire that is green or bare... it's expensive but they found it keeps houses from burning down. And for labeling purposes, we'll make all the "neutral" wires white or gray (blue in the EU). "Hot" lines get to be any other color - commonly black and red. (brown in the EU).

Lastly, once past the circuit breaker in the panel, each circuit gets its own neutral, and this is done to assure that neutrals are not overloaded, and to allow the use of GFCI breakers. The notable exception being MWBCs (multi-wire branch circuits) which share a neutral just like the light bulbs above.

So, like I say, modern NEC electrical practice in the US is full of strange idioms that only make sense if you understand the history, safety and cost cutting motivations. I hope I cleared it up a bit.

Answer 3

In a 120v ac circuit, current flows from Line 1 through the load, and returns via Neutral. If it had been a 240v circuit, the return path would have been through Line 2.

Correct and incorrect. There is no way of labeling line 1 and line 2 other than just visually. We can't directly choose which side for the power to flow from. Power coming from line 2 can also go to line 1 for 240V or to neutral for 120V.

Just for clarity, this is only for a residential center tapped power system. The power comes off of the transformer at two ends, our lines 1 and 2.

When the current reverses, the flow now originates at Line 2, not Line 1, so how does the 120v circuit reverse? Is there a "push" back from Neutral to Line 1, similar to the "push" from Line 2? If so, what is the source of the reversed current flow?

Like I mentioned above, the neutral is just a point picked to subdivide up the 240V. Since there is a difference in potential between it and one of the outside points (the lines), the power flows towards it (overall) making the 120Vs.

On the left side you have line 1 as positive, it creates 240V trying to reach line 2 in the negative or either side hits 120V trying to reach the neutral at 0 potential. Same for the right side when it's reversed.

Regardless of if your power is positive or negative, the push is happening because it's trying to level out with neutral/ground. Now imagine this flipping 60 times per second (60 Hz is the frequency of American AC electricity). Regardless of the polarity, they're still trying to reach an even potential. However, 60 times per second is so fast that although this is what's going on; it is reversing so fast that the power is more so shaking back and forth rather than flowing.

To answer your question...

Think of the neutral as just a stopping point (at 0 potential) in a larger circuit. Say from neutral to L1 is POSITIVE 120V, and from neutral to L2 is NEGATIVE 120V, then when it reverses, neutral to L1 would be NEGATIVE 120V and neutral to L2 would be POSITIVE 120V. If there was no neutral, then the power would flow from the POSITIVE 120V all the way down to the NEGATIVE 120V, which would be 240V of difference in potential.