When current flows in a wire there is resistance due to the
flow, when there is no current flowing there is no resistance.
With no resistance you read the full line voltage... with
resistance you read the lower voltage..
said another way, current flow creates the resistance. the
more current flowing the more resistance. reducing the size
of the conductor increases such resistance. If the conductor
is large enough and the current flow is low enough there is no
measurable resistance...but there will always be some
resistance with current flow.
I don't care which way you say it, that explanation is just plain wrong.
Wire has resistance. Period. It is a function of the wire size, length,
material, and temperature. It doesn't vary with current (before the purists
jump on this, yes, it will change very slightly with the temperature which
is a function of current, but that is insignificant for purposes of
clarifying his statement).
When current flows through the wire resistance, it develops a voltage drop
across it, which depends on the current by Ohm's law. Therefore, the voltage
at the load is lower than at the supply. The more current, the lower the
load end voltage.
Wire no matter how fine, even 1 angstrom, microscopic in
diameter has no resistance what so ever if there is no flow.
Sorry, thats just how it is. Resistance is a function of
how many electrons you are trying to push through the wire
(and the temperature of the wire... super cooled wire has very
It is a function of the wire size, length,
You have right but you have your language confused... and you
have missed on the issue the original poster was asking
about..which is why does the voltage drop with a load applied.
Indicating that he is getting a higher voltage across the
feeders with no load, which will virtually always be the
case.... than with a load applied, which voltage then is
reduced by virtue of the resistance, which ohly shows up with
You are making an incorrect assumption that resistance stands
separate from current flow. Ohms law which you cite is clear
on that... drop the volts and amps (current) to zero....and
resistance drops to zero.
its in the algebra also.
I think that is a superb question... you can also check the
resistance of a motor winding with a hand held ohm meter using
only 9 volts DC current... you can get an ohm reading of say
3 ohms on the run winding, using just the 9 volt meter...and
that 3 ohms when plugged into Ohms law formula's along with
the voltage supplied will give you the accurate measureable
amp draw.... so the resistance appears to be completely
independent of the current through the circuit in that case as
you are saying.
So I am not in disagreement with you at all.... I think there
are some semantics issues though..
and like you said you can purchase resistors with resistance
coded on them.
Using ohms law however you can notice that the resistance in
any *circuit is a function of voltage and amperage... if you
input zero for both voltage and amperage, the formula will
produce zero for resistance...because of course there is no
flow to resist... I think it is this sort of application we
are discussing when it can be seen that voltage drops a
amperage on the line increases beyond the rated current
carrying capacity of the feeders
same with house current at a 15 amp rated receptical for
instance...it might read 110v with a test meter plugged into
the receptical, and a lot less with a 15 amp electric heater
plugged into it. you can see this effect in an inadequately
wired house, as more lights are turned on, others dim...
voltage drop induced by increasing resistance in the
feeders.... that resistance a direct result of current flow.
I have a problem with this-- A.C. motor, D.C. motor it doesn't
matter, once it starts spinning you have inductance resisting A.C.
or pulsing D.C. So I doubt your static D.C. measuement would
be an accurate representation to give you a resistance to calculate
the current draw of a motor.
Yes, you say there is no resistance without current flow,
I say if a tree falls in the woods and no one is present, it still
If you'll notice, ohms law is very linear, all the way to zero but not
There's just somthing funny about doing calculations with zero.
You don't need to go "beyond rated current carrying capacity
of the feeders" to see the voltage drop caused by the resistance
of the wire.
The voltage will drop as more current is drawn even in an
adequately wired house, it's just a matter of degree.
The resistance doesn't increase because of increased current flow,
it's the voltage drop that increases.
Those would be locked rotor numbers based on the static
resistance of the motor windings alone... once it got going
then there would be impedance I believe as you assert.... and
the running amps would be much lower.
I think the issue is not that the actual resistance NUMBER
goes up, but that when the amperage rises, multiplied by the
static resistance the total wattage of resistance in the wire
increases, it gets hotter.
As the wattage lost to heat in the wire goes up, with
resistance stable, the voltage in the wire must go down.
This algorithm may or may not be entirely relevant to that.
that has to be predominantly correct... so what happens then
when you reduce the load or increase the load, will not the
wattage lost in the wire change as the current in the wire
changes...and that wattage is derived from volts x amps...so
as amps rise in the wire of constant resistance, voltage must
drop..... is that not the basis of ohms law as applied to
both loads and losses in any circuit?
agreed... the resistance number remains exactly the
same..so resistance does not incease as I suggested.... the
heat loss in the wire increases, and for that to happen
voltage must drop as amperage increases... I was attributing
that correctly to resistance in the wire, but resistance had
not increased just the wattage lost to the resistance had
Ben Miller has it right. Resistance is a property of the material.
Please note that, for a pure ohmic resistance, ignoring thermal effects we
have R=V/I. You are saying that for I= 0 then R= 0. However for a non-zero
voltage V the relationship implies that R must be infinite if I is 0. This
is contradictory to your statement. If the voltage is also 0 then the
statement becomes R=0/0 which is NOT necessarily 0 and could be any value
(and of no interest) as 0/0 is undefined.
Now you give an example of a motor which measures 3 ohms with a hand held
ohmmeter - which does NOT have a 9VDC current-but a 9VDC voltage source
behind an internal resistance- the current is a few ma.
The actual current drawn in the case of a motor is not determined by the
resistance of 3 ohms but by the mechanical load on the motor as reflected to
the electrical side- the motor presents a generated or back emf in series
with the winding resistance- it is an active load. The ohmmeter result does
not allow determination of the current by Ohm's Law because, with an active
load, such as a motor, Ohm's Law is useless garbage. Ohm's Law, strictly
speaking, is true for a motor only when the shaft is locked so it cannot
turn. In such a situation, the motor is useless. I suggest that you review
Ohm's Law definition and applicability.
Your last paragraph has poor math as 0/0 is not defined. It could be 0 or
could be infinite or anywhere in between. Secondly, the last statement in
this paragraph appears somewhat garbled. Any current, whether above or below
the rating of the conductor will cause a voltage drop between source and
load. The larger the current, the higher the drop - not because the
resistance increases but because there is a V=IR drop in the conductor even
for R constant- leading us back to the original statements given correctly
by Ben and others.
Resistance is a physical constant of the wire. You can look up "resistance
per foot" in reference tables for a given size wire at a given temperature.
You can have wire-wound resistors sitting on your bench, with the resistance
clearly marked on them. You can measure the resistance of a wire with 1 mA
flowing or 10A flowing, and it is the same!
I didn't miss it. I explained it. The resistance is there, in the wire.
Voltage drop occurs when current flows.
I must have slept through that class. As I have explained several times,
resistance DOES stand alone.
Not clear at all. Could you cite some reference material that explains your
That is surely the case. there are many examples.. on
the other hand resistance according to Ohms law is zero when
voltage and amperage are zero.... and we can see voltage
dropping on an overloaded parallel lighting circuit as
fixtures are added and the wattage starts to exceed the
current carrying capability of the wire... that was the OP's
original question... then I spun it to resistance...perhaps
incorrectly... as the extra light bulbs were added...
... then the resistance would have dropped in the *load
circuits (the combined light bulbs), causing more amps to flow
in the supply circuit... more amps than the size of wire would
permit without a voltage drop...that is also in the NEC
tables, voltage drop with increasing amperage on a certain
the only thing that will cause the voltage to drop as I see it
(and I could be missing something here) is resistance in the
wire... that is also seen in too small house feeders...low
voltage at the house under full load etc.
"more amps than the size of wire would permit without a voltage drop..."
Wire has X amount of resistance per length and the current times that
equals the voltage drop.
EX. A fifty foot run of #12 wire with 10 amps flowing.
50 feet must be multiplied by two ( the current must return to the source)
So 100 feet of #12 wire has about .16 ohms of resistance.
.16 ohms x 10amps = 1.6 volts
That 50 foot run would have a voltage drop of 1.6 volts with a 10 amp load.
Okay. Thanks to everybody who answered my question. I have a much
clearer picture of what is going on. I'm going to start taking a space
heater around with me so I can measure the length of the runs to the
outlet I'm at just for the fun of it :)
All he needs to do is measure the voltage at the receptacle without the
heater, and that is the voltage at the panel. Then plug in the heater and
note the value under load. The difference is the voltage drop.
It's not quite that easy! You need to know the resistance of the space
heater, since it will not draw the same current at which it was rated,
with any voltage drop. It's resistance is its rated voltage squared
divided by its rated wattage. This will be a constant, (in spite of
what Phil Scott says), neglecting the slight difference in temperature
at the lower voltage. Then, the resistance of the wiring to the outlet
is simply the difference in the loaded voltage and the unloaded voltage,
divided by the loaded voltage, all times the heater resistance.
In the USA, almost all outlets are wired with #12, (20A), or #14, (15A)
wire. Copper wire of these sizes has a resistance per thousand feet of
1.62 or 2.57 ohms. You can calculate the run lengths from that.
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