# Neutral Wire

• posted

Hello:

I know the neutral wire carries current but no voltage (no potential difference). If voltage is needed to move things along then how does the current get back to the transformer with no voltage on the neutral?

Thanks.

Terry

• posted

Erm, because there is a voltage difference (hopefully a fairly small one) between one end of the neutral wire and the other, when there is current flowing through it.

It is just that the resistance of the neutral wire is so low that very little voltage difference is needed to get that current to flow.

Oh, similarly, there is very little voltage difference between the two ends of the live wire, even when carrying full load current. Again, because the resistance of the wire is kept very low, so very little voltage difference is needed.

-- Sue

• posted

It happens that Terry formulated :

We may have different definitions of "neutral cable" but I have always known a neutral to carry the returning current. If you measured the potential difference anywhere along the neutral back to the active you will read the line voltage as long as the ciruit is open. If closed you will read the voltage drop between the two points.

The neutral on a transformer is labled 0 voltage, but if you measure the voltage between the neutral tap and any other tap you will read the secondary output voltage of those 2 taps.

Another way to see it is to look at a light switch. If the switch is open then there isn't any voltage on the neutral side and line voltage on the active side, so the potential difference in voltage is equal to the line voltage. Close the switch and you loose the difference in potential because both sides now carry line voltage. You know this because there is no longer a measurable difference of potential on each side of the switch.

• posted

That is certainly an interesting way to see things.

However, there has to be a potential difference "on each side of the switch" when it is closed and passing current - or no current would be flowing.

The difference is indeed measurable - given appropriate instrumentation.

-- Sue

• posted

The neutral current carrying wire has just as much voltage as the hot wire does. In fact the voltage is exactly equal but opposite in polarity at any instant. It is that voltage that drives the current into the load.

Your statement is one of relativity. Relative to ground, the neutral wire has little or no voltage. But ground is not what is driving the current into the load. Relative to the hot wire, the neutral has full voltage. Reverse the clip leads on the meter you measure with and notice that the voltage is exactly the same. This implies that it is the voltage from hot to neutral or from neutral to hot that drives the load. It doesn't matter where ground is as far as driving a load is concerned.

Also notice that many circuits have ground in the center point between two "hots." What is grounded and where neutral lies is a matter of convenience and safety and has nothing to do with current into the load.

• posted

You measure voltage as potential difference with earth being the reference point. Hot to earth 120v - neutral to earth 0v. Correct? Terry

• posted

Rarely. There is usually some voltage difference between earth and neutral, although small. Not at the point where they are bonded to each other (if they are), of course.

You measure voltage as potential difference between two points where the reference point is the point that you wish to use as reference. I have seen a circuit diagram where the reference point was sat at several hundred volts, compared to "earth". The internal supply rail was the common point for much of the circuitry, so it made sense to use that as the reference point*.

*Unfortunately I hadn't studied the circuit diagram enough to determine that, before touching a "15v" test point with the power on. A lesson that has never had to be repeated.

-- Sue

• posted

convenience

You can but earth or ground is not a true reference point but rather a "safety" point. Ground is not supposed to carry current, So, to measure a voltage driving a motor, for example, you do not measure it referenced to something that doesn't carry current; i.e.ground. You measure motor voltage across the motor leads. It is immaterial that one or the other of the motor leads may or may not be connected to ground somewhere. In fact, in most AC power circuits you will get an error of a volt or two if you try to use ground as a reference. This is because there are likely to be undefined stray currents flowing around in the ground paths. These cause confusion and errors.

• posted

No. There will be voltage present on both sides of the switch, however it will be the same voltage. so the difference of potential between the two sides will be 0 volts. The same would happen if you shorted the taps on a transformer. You have to remember that you can only measure the difference in voltage between two points. If the voltage is equal then your meter will show 0 volts (assuming the polarty is the same)

• posted

Total rubbish. The switch will have a finite resistance between its terminals in the on position. If current is flowing, there will be a voltage drop across the switch. Thus the voltage from each switch terminal to neutral will not be the same, when current it flowing. The voltage difference may, or may not, be significant - but it will be present. It will, usually, be very, very small compared to the supply voltage.

The same would happen if you "shorted" the taps on a transformer, using a link. There would be a voltage difference between one end of the link and the other. It may be in the order of uV, but it will be present if current is flowing through the link. There is no such thing as a Zero ohm resistor...the nearest that you can get is a resistor with a resistance that is effectively zero, for the application.

When it comes to a switch - you will be measuring the voltage between two points - the two terminals of the switch. The meter may indeed show zero with current flowing. That is because the meter isn't sensitive enough to register the voltage drop. You will get 0v across a 12v battery - if you measure it with a 1Mv FSD meter. That does not mean that there is actually zero volts between the battery terminals.

-- Sue

• posted

Sorry, just not making sense to me. In a home you have 220v and a neutral. The neutral is bonded to an earth ground.

If everything is wired correctly: I grab a 120 hot line while holding onto a piece of copper, that is driven into the ground, and I get zapped. There is potential difference.

I grab a neutral line and touch the copper rod and I don't get zapped. The same reason I can touch the electrical breaker panel and not get zapped, no potential difference. If a neutral wire carries current and voltage why don't I get shocked when I touch the panel?

Now, I know there is current on the neutral wire as it travels back to the transformer but the isn't any measurable voltage. If the neutral was at the same potential difference as the hot wire we'd get zapped every time we touched a water pipe. The neutral is not a hot wire.

When you say that "You measure voltage as potential difference between two points where the reference point is the point that you wish to use as reference" doesn't make sense to me. That's like saying I can connect one lead of my VOM on a 120v hot wire and the other end in a hot dog because it's a point of reference that I choose. The earth is the constant in the equation. It is the reference point. A brief Google search say's it better than I:

"Another usage of the term "voltage" is in specifying how many volts are across an electrical device (such as a resistor). In this case, the "voltage," or, more accurately, the "voltage across the device," is really the first voltage taken, relative to ground, on one terminal of the device minus a second voltage taken, relative to ground, on the other terminal of the device."

One lead of a VOM meter stuck into the ground and the other on a hot wire = voltage. One lead of a VOM stuck into the ground and the other on a neutral wire = 0 voltage - no potential difference. Voltage is the measurement of potential difference.

Please do not take this as an insult as I mean no disrespect. I'm trying to get to the end zone.

Terry

• posted

You're splitting hairs...all conductive materials have resistance...but that has nothing to do with the question that was asked...

• posted

The difference is that I am being a pedantic engineer. :)

The voltage difference between the terminals of a closed switch only matters to an engineer - no one else gives a damn and treats it as being zero - which, for most intents and purposes, it is. But not /all/ intents and purposes - so an engineer has to be aware that it is not zero.

When you measure the voltage between two points, you do so for a purpose. That purpose generally defines which points you choose.

If you were interested in the voltage distribution along a hotdog, you would indeed measure the voltage between one reference point (say where the hot wire is touching the hotdog) and the point of interest (say at every 1cm from that end to the other end of the hotdog, where the neutral was connected). That would give you a graph of voltage v distance - which would be a straight line if the hotdog was fully and uniformly filled but produce something far more interesting if it had voids or, say, a steel bolt somewhere inside. The purpose of the measurement could be to find hotdogs with bolts inside. That determines what points are used for testing.

As another poster has said, for a motor, you would probably measure between the motor terminals. Why you do so is because you want to know what voltage is reaching the motor. Because how the motor operates will be determined by the voltage across its terminals - not the voltage on the cables coming into the building. So you measure at the motor - not at the building panel.

Another key thing is "potential difference" doesn't necessarily equal lots of volts. It can be one millionth of a volt. Certainly not enough to zap you or to register on most equipment. But even a millionth of a volt could be absolutely vital and could need to be measured.

-- Sue

• posted

The question that was asked was how current flows along a neutral wire, as there is no voltage. The answer is that there *is* voltage. There is a potential difference between one end of the neutral wire and the other end of the neutral wire - when current is flowing. That difference isn't a lot and can, for most intent and purpose be considered zero - apart from when you are trying to understand how current can be flowing, if there is "zero" voltage.

It is because all conductive materials have resistance that there is always a voltage drop when current is flowing. Hence the "split hair" is the answer to the question..the voltage isn't in fact, zero.

-- Sue

• posted

You have likely already gotten more information than you want or need. The basic ideas are that: 1)Voltage does not exist at any point or on any wire. It only exists between two points or two wires. 2)Where there is esssentially zero resistance (for your purposes) such as from one end of a wire to another, current flows with essentially zero (for your purposes) voltage. The confusion may come in depending on whether or not you are concerned about very small voltages and very small resistances. Sometimes you need to know about them and take them into account, but for many purposes they can be ignored. For household electrician type understanding, voltages are generally measured to the neutral conductor, which is "almost" at zero voltage to ground.

Also understand that current flow must be in a continuous loop. It doesn't start somehere, go for a distance, and then stop. It either flows completely around or it doesn't. So understanding the difference between voltage and current, rather than thinking they both somehow just mean "electricity" helps.

A big problem in learning is that term usage is not always precise. The terms "power", "energy", "voltage", and "current" have definite different meanings but what does it mean when the experienced electrician tells his helper "Turn on the juice"???

Get the basic ideas clear first, understand 120 and 240 volts, amperes of current, and ohms of resistance. Then you can get just as deep as you want into millivolts, microvolts, milliohms, microohms, milliamperes, and microamperes.

Don Young

• posted

Of course theres voltage present...that is what I wrote. You keep saying that there is a potential difference from one end of the neutral to the other and I'm trying to tell you there isn't. Potential difference is the voltage between two points. there isn't any difference in voltage anywhere along a single conductor aside from the voltage drop due to the conductors own resistance.

None of this has anything at all to do with the origional question.

I just assumed that the OP knew ohms law and that you can not have current without voltage. If the connection from active to neutral is open then there is no voltage on the neutral and there will also be no current. However you will have a potential difference between the neutral and the active.

If the connection is closed then you will have voltage and current limited by the resistance and or inductance of the load. But if you measure the voltage of the circuit you will not be measuring the potential difference, you will be measuring the voltage drop across the load of the circuit. There is no potential difference on a complete circuit regardless if there is a voltage present or not.

my last post on this BTW.

• posted

I do know Ohms law however did not apply it. Sorry for being a little thick. When you apply Ohms law it falls into place.

Thanks to all for the time.

Terry

• posted

| I know the neutral wire carries current but no voltage (no potential | difference). If voltage is needed to move things along then how does | the current get back to the transformer with no voltage on the | neutral?

Voltage is a potential difference between TWO conductors (or two points). Considering one wire alone does not leave you with a way to see any voltage. You can see 120 volts between two wires where one of them is neutral and one of them is a phase wire. But as far as a light connected between them is concerned, they are equal.

The term neutral comes from there being two or three hot wires with phase angles such that if each has an equal current, the neutral balances out and has no current.

The neutral is almost always the grounded conductor. So relative to ground it will appear to have little or no voltage. There are some electrical systems that are ungrounded. Two wires in such a system would have their voltage between them. It would not be on a single wire. A low voltage lighting system is generally ungrounded and its 12 or 24 volts will be just like that, two equal wires with a voltage between them.

You would get a small amount of voltage between two points of a wire that is carrying a current. That voltage is figured by Ohm's law using what the current is and what the wire resistance it (usually very low). This is called "voltage drop". You could see some small voltage on a grounded neutral as a result of this when current is flowing on the neutral (which happens when the system load is unbalanced over its hot conductors).

• posted

Terry When you say there is "no voltage" on the neutral, you are measuring it with respect to the earth. However, if you measure between the hot and neutral, you will see full voltage of the system. That is what drives current from the supply through the hot wire, through the load, and through the neutral back to the supply. The supply voltage is what drives current, and you need a complete circuit (conductive path) from one side to the other in order for that current to flow. That conductive path includes the neutral wire.

Ignore the ongoing discussions about voltage drops in the wire, etc. They are correct, but not necessary for you to understand the answer to your question. Assume the wires are perfect conductors with zero rresistance, and the system resistance is the load. Current will still flow as I described.

• posted

| I grab a neutral line and touch the copper rod and I don't get zapped. | The same reason I can touch the electrical breaker panel and not get | zapped, no potential difference. If a neutral wire carries current | and voltage why don't I get shocked when I touch the panel?

How much "shock" you get from the neutral wire depends on what other things you are also touching at the same time:

1. A hot wire - you'll get zapped at a potential of 120 volts because that is the difference between the neutral and that hot wire.

1. The ground wire - you'll get very little potential (millivolts or less) since there is a (nearly zero ohms) connection between the neutral and the ground wire.

2. The earth itself - you'll get a little more this way that when touching the ground wire, but not much more.

1. The surrounding air - you'll get almost no potential this way, but it will still be above zero.

Note that for all cases, some current will flow. But in the latter 3 cases there will be very little current for the very little voltage.

If you were to touch the hot wire while only touching air (e.g. not neutral or ground), you would get the 120 volt potential, but you would be in series with a very high impedance (the air), and so the current would be very low. You might feel it (a tingle in the wire contact). Just don't try it because a mistake of accidentally touching something else (like the panel frame) could kill you.

Compare all that to the guy shown in this video:

In this case, he's approaching a very high voltage transmission wire in a a helicopter. And this is NOT a neutral wire ... this is a ***HOT*** wire.

Notice that the first action done is to connect the helicopter to the wire. That is because even though the helicopter is only touching air, at this voltage, it could let enough current through to kill a person if it passes through them. This connection prevents the man from being the conductor.

Once he is actually on the wire itself, his own body could still pose some danger. But he is wearing a special conductive outfit to pass most of the current around his body. The current he gets is small, but its definitely more than you would get in scenario #4 above. He has to take precautions to make sure the current doesn't get above the level that is safe for his own body.

| Now, I know there is current on the neutral wire as it travels back to | the transformer but the isn't any measurable voltage. If the neutral | was at the same potential difference as the hot wire we'd get zapped | every time we touched a water pipe. The neutral is not a hot wire.

Voltage is the difference between two points. If you measure the voltage between two points on a wire that is conducting current, then what you get as the voltage is the current times the resistance (Ohms law). Since the resistance in the wire is very low, the voltage will be low. The closer the points, the lower the resistance and the lower the voltage.

Note that if you could just raise the resistance at will in a section of the wire (like you had already installed an adjustable resistor), note that making that change also affects the circuit involved. Raising the resistance will reduce the current. The worst case, an infinite resistance, will just give you the full system voltage at points on each side of that resistance, which is 120 volts. So you can't just put a million ohms in a 1 amp current and expect to get a million volts, since the current will not stay at 1 amp when you do that.

| When you say that "You measure voltage as potential difference between | two points where the reference point is the point that you wish to use | as reference" doesn't make sense to me. That's like saying I can | connect one lead of my VOM on a 120v hot wire and the other end in a | hot dog because it's a point of reference that I choose. The earth is | the constant in the equation. It is the reference point. A brief | Google search say's it better than I:

The hot dog is as valid a point of reference as is a hamburger. Both would have a VERY high impedance (the air, for example) return path to the source of power. Choosing that point of reference doesn't prove anything.

Other reference points mean more. Meaningful reference points could include:

1. The grounding wire (the uninsulated or green insulated one).
2. The electrode stuck in the ground.
3. The frame of the panel.
4. Any other wire in the panel.

| "Another usage of the term "voltage" is in specifying how many volts | are across an electrical device (such as a resistor). In this case, | the "voltage," or, more accurately, the "voltage across the device," | is really the first voltage taken, relative to ground, on one terminal | of the device minus a second voltage taken, relative to ground, on the | other terminal of the device."

Voltage is always between two points. They can be two points on the same wire. Since the resistance between such two points is almost zero, you will get almost zero volts. If you have some significant resistance AND still also some significant current, then there will be significant voltage as well. Amps times Ohms gives volts. If you have 1 amp flowing through

12 ohms, it took 12 volts to push it through, and you'd measure 12 volts at points on each side of the resistor. Don't do this unless the resistor is rated for at least 12 WATTS because that current through that resistance is _dissipated_ power. It will make the resistor hot. It will burn up a typical 1/8 watt resistor in that circumstance.

| One lead of a VOM meter stuck into the ground and the other on a hot | wire = voltage. One lead of a VOM stuck into the ground and the other | on a neutral wire = 0 voltage - no potential difference. Voltage is | the measurement of potential difference.

Right. Although the voltage between the neutral and the earth itself may be a little more than 0. It might be 0.025 volts, for example. Nothing to worry about.

| Please do not take this as an insult as I mean no disrespect. I'm | trying to get to the end zone.

No problem. Keep asking if you still don't understand. Maybe someone will hit upon the explanation that gives you the touchdown pass.

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