In a 3-wire single phase system, you have branch circuits going off of 2 different legs/buses. I read somewhere that if the total draw is balanced between the 2, then you will have 0V on the return (Neutral) conductor. I assume this 0V only refers to the neutral from your main panel back to the transformer, right?
Because it seems to me that on every branch circuit, the current flow in a circuit will be the same on the hot as on the neutral and that this is what GFCI checks for. So if electrons are moving through the neutral back toward the panel to complete a circuit, there has to be voltage pushing the electrons. Am I right about this?
If we close an AC branch circuit by flipping on a light, we will have 120V causing electrons to oscillate back and forth in the hot conductor (black), in the bulb filament (creating heat and light), and in the connected neutral conductor (white). Right? So if I were to test with a multimeter, wouldn’t I see 120V if I tested hot to ground, and 120V if I tested neutral to ground?
I’ve seen several references to 1.5-2V in the neutral conductor under load. Is this because it is downstream from the lightbulb?
Hi Robert, I think my question is more basic. We always talk about current flowing through hot and neutral to complete a circuit. The current flows because voltage is applied, right? So what do we mean when we say the voltage on the neutral is 0V, but yet current is flowing. Wouldn’t that require some voltage?
So Jim, that makes sense to me that I wouldn’t be able to get a reading going neutral to ground since they’re tied together. But is it still accurate to say there is voltage pushing current through that neutral. And what would that voltage reading be (if I could get a reading)?
“As I said earlier a voltage reading is between two points what two points are you measuring between?”
I’m actually less interested in measuring than I am in just verifying that a thing called voltage is causing electricity to flow from the illuminated bulb and through the neutral back to the panel.
If there’s 120V causing electrons to move and electricity to flow through the hot from the panel to the bulb, it seems to me the same 120V must be causing electrons to move and electricity to flow through the neutral and back to the panel to complete the circuit.
If this is true, then when we say there’s 0V on the neutral, it shouldn’t be understood to mean the electrons in the neutral aren’t being acted upon by the 120V that first caused electricity to flow from the panel to the bulb. Does this make sense?
If you have a 2-wire circuit the voltage is the same on both wires when it is measured between the two wires, that is how voltage is measured between two points. If you measured it to ground that would give you 120V from the hot leg and 0V from the neutral because the neutral is grounded but that is not relevant.
Looking at the photo in post #1, take a loop of wire and connected one end to the circuit breaker and the other end to the neutral bus, now cut it anywhere in the loop and turn on the breaker, the voltage between those two cut ends is 120 volts because there would be 120 volts in that loop with the meter connected.
New loop, both ends connected to a 2-pole CB, 240 volts. Cut the loop, turn on and measure, you get 240 volts because there is 240 volts in that the loop with the meter connected. Now measure each end of the cut loop to ground, you get 120 volts from each end, that measurement like the 0V to ground in the other example is irrelevant.
I guess my biggest hurdle was assuming that anywhere there is current there must be voltage to account for the current. So I guess I need to think of voltage as the pressure that starts the flow through the hot conductor, but flowing electricity (i.e. current) in the neutral conductor doesn’t have voltage. (I read on one forum where someone said to think of neutrals as water drains: water flows down the drain without being pushed.) I suppose in that scenario you could argue that gravity is the pushing force but that’s beside the point.
I think I’m getting a better grasp but let me know if I’m not.
Here’s a related question I’m trying to figure out: When we say neutrals only carry unbalanced current, are we just talking about a system where there is a split phase and two legs? I’m assuming the term “unbalanced” is referring to a larger load on one of the 120V legs than on the other. Is this correct?
Just my 2 cents but do not think of neutrals as water drains. They are always current carrying conductors and in a 2-wire circuit have the same amount of current flowing through them as the ungrounded conductor. They differ in only that they have zero potential to ground because in a grounded system we intentionally ground the neutral.
They also can kill you just as easily as an ungrounded conductor which is the reason that a few code cycles ago the NEC started requiring simultaneous disconnecting means for MWBC’s.
Yes that’s correct, in a 1Ø, 120/240 volt system the neutral will carry the difference or the imbalance between the 2 ungrounded conductors. If there is no imbalance then the neutral current will be zero.
For a 3Ø WYE system it’s a little different, for a 3-wire MWBC with two ungrounded conductors sharing a neutral if the ungrounded conductors are balanced (equal current) the neutral will carry about the same amount of current also. That’s why when applying derating factors sometimes the neutral counts as a current carrying conductor and sometimes it does not.
Is some of the confusion here coming from talking about neutral conductors in circuits for lighting, receptacles, etc, and the neutral SE and associated connections between the panel and the meter or power pole? Taking multi wire branch circuits aside, I think it may be easier to consider the “wiring in the walls” as operating somewhat differently than the wiring from the PoCo.
On individual lighting circuits the neutral carries the same amount of current as the hot conductor. Its only the SE neutral that will see varying amounts of current flowing through it depending on the balance of loads between the two hot busses at the panel. If one side has a 50 Amp load and the other a 40 Amp load the SE neutral should be carrying the 10 amp difference. Correct?
Yes that’s correct and I would add that there are circuits with the same characteristics that exist past the panel in the actual branch circuit wiring. Although MWBC’s are still code compliant they’re use is now almost non-existent in newer wiring. You will still see plenty of them in older houses and in those two circuits the neutral is carrying the current difference between them.
Also if you think of a 4-wire electric dryer circuit that will function similarly to the service entrance you’ve mentioned because the dryer has components that operate at both 120 and 240 volts. So when the dryer is running you will still see that the two ungrounded conductors have a different amount of current flowing through them. The current difference will be present in the neutral conductor run with the branch circuit to the dryer. If the black conductor has 17 amps and the red conductors has 19 amps then the neutral will have 2 amps of current flow.