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.
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.
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
The difference is indeed measurable - given appropriate instrumentation.
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)
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
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.
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.
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
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.
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.
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.
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.
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.
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"
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
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.
Benjamin D Miller, PE
Because the neutral wire and the panel, as well as the water piping in the
building, are all electrically bonded together. The resistance between these
objects is virtually zero, unlike your body which can be measured in the
tens of thousand of ohms and up. The current is taking the path of least
resistance through all of the metal instead of through your body. If
however, the was no bonding between the neutral and ground, then it would
act just as a hot wire now does and zap you.
| 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.
2. 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.
3. The earth itself - you'll get a little more this way that when touching
the ground wire, but not much more.
4. 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
| 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.
|WARNING: Due to extreme spam, googlegroups.com is blocked. Due to ignorance |
| by the abuse department, bellsouth.net is blocked. If you post to |
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
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