# Voltage Drop Under Load

• posted

I've noticed that when 120V 20A AC circuits are heavily loaded, the voltage at their outlets drop somewhat. I thought that in a parallel AC circuit, voltage remained constant but current changed. What is going on? Can someone point to a detailed discussion of this phenomenon (either in print or on the Web)? Thanks in advance.

-- Jack

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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.

Phil Scott

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• posted

I had a discussion (again) with my father the other evening. This one has baffled me for years.

The mains on the street supply the house. They make available 50Hz, 60 Amp,

240VAC. This is 240x60=14400 Watts (14.4 kW) !!!

That is enough to lift my 1.6 tonne car at a velocity of almost one m/s. That's a vertical lift of aprox 4 kmph. Not all that fast, but faster than I could lift the thing!

And when I turn on my light switch, my little 60 Watt bulb glows nicely, at a rate not many more times greater than that my car hifi consumes.

Why is this so?

• posted

I had a discussion (again) with my father the other evening. This one has baffled me for years.

The mains on the street supply the house. They make available 50Hz, 60 Amp,

240VAC. This is 240x60=14400 Watts (14.4 kW) !!!

That is enough to lift my 1.6 tonne car at a velocity of almost one m/s. That's a vertical lift of aprox 4 kmph. Not all that fast, but faster than I could lift the thing! With some imagination we could rig say 10 houses and drive to the moon at the legal speed limit!!

And when I turn on my light switch, my little 60 Watt bulb glows nicely, at a rate not many more times greater than that my car hifi consumes.

Why is this so?

• posted

Maybe the water analogy is one way to address this question.

You house have water mains with maybe 50 to 100 psi pressure (voltage). The mains are capable of supplying hundreds of gallons (amperes) a minute (gallons per minute = watts).

If you turn your faucet on full open you may get several gallons per minute out of it. Yet you can open the faucet just a bit and get a few drops a minute.

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As the impedance of the supplying transformer approaches the impedance of the load the voltage is divided between them according to ohms law. That is why many plants that have 208/3 phase and only REALLY have about 200 volts at the equipment. Just plain overloaded.....Ross

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The wire supplying the loaded outlet has resistance which causes the drop. All outlets further down the line with experience the same drop.

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Try my slide show on voltage drop at:

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Phil, You do talk a lot of DRIVEL.

The resistance does NOT varey with the current. The voltage drop due to the current times the reistance increases with incrase in current (simple Math).

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"Billy H" wrote in message news:43f8d16a\$0\$82638\$ snipped-for-privacy@ptn-nntp-reader03.plus.net...

The simple analogy is that of a reservoir full of water.

When you fill a glass from the reservoir you only get what you need not the millions of gallons that are available.

Similarly each electric device only uses the current and hence the power that it is designed to use and not all that happens to be available at any point in the supply system.

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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.

Ben Miller

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Thanks Ben You probably saved me from the Temp Coefficient flames which I left out because as you say it is insignificant to this discussion.

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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 little resistance).

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 current flow.

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.

Clear enough...no?

its in the algebra also.

Phil Scott

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If resistance is a function of "how many electrons you are trying to push through the wire", I must ask, how many electrons do I have to push through a 100 ohm resistor to make it 100 ohms?

Mike

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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 theory?

Ben Miller

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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.

Phil Scott

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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 makes sound.

If you'll notice, ohms law is very linear, all the way to zero but not zero. 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. Mike

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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 size wire.

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.

Phil Scott

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Phil said, "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 resistance 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.
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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.