Why does voltmeter read 240V?

I'm having trouble understanding a concept. I know that if you stick a voltmeter into two hots of different circuits in your house, the meter reads
240V. How can a meter get a reading if you aren't completing a circuit? I understand testing a regular 110V line, and I understand why it doesn't matter which way you probe the outlet when testing for 110V. When you test a 110V line a neutral is involved, thus completing a circuit. Does the answer to my question have something to do with the fact that the two hots are 180degrees out of phase with each other? I can picture the 2 sine waves, but I still don't understand why probing 2 hots can produce a meter reading. Thanks for any pointers or answers. Brian
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Hot Circuit 1 || +-------------------+----------------+ || ( 1 | | || ( --- 110VAC | >----+ || ( ----- socket | ) || ( Neutral | ----- 220VAC ) || +-------------------+ ----- socket ) || ( 2 | | >----+ || ( ----- 110VAC | || ( --- socket | || ( Hot Circuit 2 | | || +-------------------+----------------+ 3 Transformer on pole outside
Between transformer terminals 1 and 3, there is 220 VAC. Between terminals 1 and 2, or 3 and 2, there is 110 VAC.
Note that a voltmeter placed on the hot side of two sockets that are on the same circuit will have essentially zero volts. Only sockets that are on different circuits will show 220 VAC between the hot terminals.
(I'm using the convention of calling it 110/220 VAC. In fact on most single phase systems in the US it will nominally be 118/236 VAC, but the actual voltage can range significantly higher or lower. It is also referred to as 115/230 and 120/240, but its all the same... a single phase, "residential", distribution system.)
--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@barrow.com
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I've modified the drawing. If we remove the middle tap/leg, and just use the two outside legs, do you get a complete 220V circuit with the top wire carrying the 220V and the bottom wire being the return/neutral?
Hot Circuit 1 || +-------------------+----------------+ || ( | || ( | >----+ || ( | ) || ( ---- LOAD ) || ( ---- here ) || ( | >----+ || ( | || ( | || ( | || +-------------------+----------------+ 3 Transformer on pole outside
I think I have this licked. My only other confusion is if we go back to the original drawing posted here:
Hot Circuit 1 || +-------------------+----------------+ || ( 1 | | || ( --- 110VAC | >----+ || ( ----- socket | ) || ( Neutral | ----- 220VAC ) || +-------------------+ ----- socket ) || ( 2 | | >----+ || ( ----- 110VAC | || ( --- socket | || ( Hot Circuit 2 | | || +-------------------+----------------+ 3
If one person is using the #3 leg as a return/neutral on the 220V circuit, how can someone else draw 110V on the #3 leg?
Please excuse my dumbed down referral to the neutral as the return. It helps me understand things somewhat.
Thanks.

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The normal distribution system is *not* arranged that way. The #3 terminal is never considered "neutral". There is no point where there is 220V between a neutral leg and a hot leg.

I should have included an earth ground connected to the neutral leg too! That might help clarify it slightly.

Well, it isn't really correct to say that that leg is a neutral, because it isn't. It is never any less a "hot" leg than is the other leg.
But yes, there can be current flowing from point 3 to point 1, other current flowing from point 3 to 2 and 1 to 2. The last two are out of phase though.
Hence... consider what amp meters in each leg would show if there were 10 Amps flowing to a load on each socket. The 220VAC socket would cause legs 1 and 3 to both show 10 Amps. There would be 10 Amps from 1 to 2 which would show up in leg 1 as additive to the 10 Amps from the 220VAC load, so that leg would show 20 Amps. The same would be true of the meter in leg 3. The neutral leg however would have 10 Amps from leg 1 and 10 Amps from leg 2, but they would be out of phase... so the meter would read 0 current.
If the load on the lower 110VAC socket is removed, there would be only 10 Amps in leg 3, all going to the 220VAC load. The load in leg 1 would remain the same at 20 Amps. But now the meter in leg 2 would show 10 Amps (which is half of the current in leg 1).
Think about that... when you pull the plug on that one socket, the current in the neutral goes from 0 to 10 Amps!

Well... the problem is that while the neutral leg shown in that diagram is in fact what could be called a "return" for both of the hot legs if and when there is a load connected to one or both of the two 110VAC sockets, there really isn't a "return leg" for the 220VAC circuit as such. It's actually a balanced circuit (the 110VAC circuits are "single ended"), with a differential voltage applied (that 180 degree out of phase business you mentioned at one point). But that's getting to far off into transmission line theory, and is not what most people think about when the look at AC power distribution.
(Of course my background is not in power, but in signal transmission... :-)
Oh, when you think this makes sense, get one of the people here with a power background to explain one or more of the various 3 phase systems...
--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@barrow.com
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Let's see if I'm any closer. The neutral leg isn't always neutral? You said if we pulled the lower socket, then the neutral shows a 10amp reading, which means it's carrying a load. That would be opposite of neutral right? :) I've saw an amp meter in action just the other day when my friend was testing my air condition. To be honest, up until this point in my life, I thought the current stopped at the load, and didn't show anything on the neutral side, just like voltage. I've always seen amps tested on the hot leg. Can you test amps on any circuit by using the neutral leg?
Ok, I modified the drawings again to see if this makes sense. All I did was add pointer arrows indicating which direction the current would flow during a period of 1/60th second. The first drawing would be time=0, and the next drawing would be 1/60th a second later. Can I say that since leg 1 and 3 are 180degrees out of phase, that the current is flowing in opposite direction? (when I say opposite, I mean current is leaving the transformer in one leg, and returning on the other leg. With a load connected, it appears they are making a circle) Please say yes. That's the only way I can make sense of this. Please correct me if my drawings are way off base. You also referred to the two 110VAC legs as being "single ended" That term is greek to me. Maybe I should look that up to help myself here.
Hot Circuit 1 (current flow --> ) || +-------------------+----------------+ || ( 1 | | || ( --- 110VAC | >----+ || ( ----- socket | ) || ( Neutral | ----- 220VAC ) || +-------------------+ ----- socket ) || ( 2 | | >----+ || ( ----- 110VAC | || ( --- socket | || ( Hot Circuit 2 | | || +-------------------+----------------+ 3 (<-- current flow)
Hot Circuit 1 (current flow <-- ) || +-------------------+----------------+ || ( 1 | | || ( --- 110VAC | >----+ || ( ----- socket | ) || ( Neutral | ----- 220VAC ) || +-------------------+ ----- socket ) || ( 2 | | >----+ || ( ----- 110VAC | || ( --- socket | || ( Hot Circuit 2 | | || +-------------------+----------------+ 3 (--> current flow)
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I think your last statement is the key to your problem. The neutral is not the return for EVERYone.
Any simple cct like this consists of 2 wires with the load between them. either one end of the transformer and the centre -- 110 volts or both ends -- 220 volts. With a 110 volt load and a 220 volt load there will be more load on one half of the transformer but that is no problem.
The confusion comes from the antique Edison wiring system used in North America (only). There the so-called neutral is just the centre tap of a 240 volt transformer which is usually grounded so that no part of the household wiring is more than 120 volts above ground for supposed safety reasons.
Most of the rest of the developed world use a 3 phase distribution where the neutral is the centre point of a Y connection and there is 230 volts between the neutral and any phase and 400 volts between any 2 phases. Generally the neutral is grounded at each building entry as well as at the distribution transformers.
--
John G

Wot's Your Real Problem?
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The hot lines are 3.... line 1, 2 and 3 depending on the transformer, and what voltage, lets say 220 in this case.... all of the lines will read 110 to ground because the ground is null and line is ocilating at 60 times second with the voltage...so you get read as you know... so you call that a circuit though your meter and you are correct. the lines will read 220 though between each other,
One of the lines on some kinds of utility pole transformers is called a *stinger and it will read much higher voltage to ground... usually twice with others read but not aways.
for three phase power, in commercial locations the stinger is used but never to ground...the 110 circuits come off of the other two lines to neutral as you know (and neutral is grounded at the utility pole)
now to answer your question... these lines are called 'phases' for a reason.. each phase coil is physically located a different section of the generator (divide 360 by 3 and get the phase angle) be it a nuclear plant or hydro electric or steam etc.
The occilation of power in each line is TIMED differently by these physical means and are called 'out of phase' with each other..
when you are reading between these two you are reading the difference in voltage as it averages out between the two lines because they are out of phase with each other,,,at the instant one is pushing max voltage in one direction, the other line voltage is going in the oposite direction.
Electrons will want to flow between these two in order to equalize..... its this pressure that is driving your meter...and useful in running motors etc.
Phil Scott
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Thanks Phil. I didn't read your response until after my last post showing what you seem to be describing. When #1 leg is "pushing" the #3 leg is pulling, so they help each other there when passed through a load.
I use a table saw on a 240V receptacle. My dad is an electrician, but he has a hard time explaining this stuff to me. He's been used to working with this stuff as second nature, he doesn't think of what's going on behind the sceen. He tought me long ago that if you pull two hots from differnt poles in the breaker panel, you get 220/240V. So I wired up my own 240V receptacle by paralleling off of two existing 110V receptacles from different poles. Works like a charm. I just don't turn on anything else when the table saw is running.
So theoretically, if we had a 240V light bulb, we could power it using the two hot legs coming into the house, and hook up the neutral wire to the light casing somewhere in case one of the 110V wires came loose and contacted the metal case. That about right?

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Phil, so you're saying that in the following diagram, the 3 legs coming out of the transformer each have the ability to be hot?
Hot Circuit 1 || +-------------------+----------------+ || ( 1 | | || ( --- 110VAC | >----+ || ( ----- socket | ) || ( Neutral | ----- 220VAC ) || +-------------------+ ----- socket ) || ( 2 | | >----+ || ( ----- 110VAC | || ( --- socket | || ( Hot Circuit 2 | | || +-------------------+----------------+ 3

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the
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What do you mean by "show only line 1 and line 2"? I have a transformer on a power pole in my back yard, and I see three lines come out of the transform and head towards my house. One of these lines also splits and goes down the pole into the ground, so I assume that's my neutral coming into the house, and the other two are the 110VAC lines. If the neutral leg is grounded at the pole, how can it ever become hot? I must be missing something here. Brian

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Electrical types use the word 'line' to refer to a 'hot' wire (significant voltage relative to ground). You see three WIRES coming out of the transformer. One, as you noticed, is connected to ground, and this is the neutral. (The neutral is one of the three wires headed toward your house. Most likely a bare aluminum wire with the two black hot lines wrapped around it. The neutral is grounded both at the pole and at your circuit breaker box)
--
-Mike

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Thanks Mike. You're right, coming from the transformer in my back yard is a bare wire, and two black insulated hot wires. My question is, under what circumstances does the neutral become hot? I think Phil said in one of his posts that the neutral wire can become hot. Brian
writes:

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writes:

3...
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neutral coming

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must be missing

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and this is the

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They've probably answered this for you already, but I'll give it a shot.
You reason easily how a meter with a D'Arsonval movement coil reacts and points to a place on a scale due to the magnetic reaction from the applied emf (voltage) to ground/neutral., the principal for a digital meter is the same.
How does it measure 220/240VAC from 2 two/three phase conductors without ground reference ?
The same way a 220vac appliance or motor uses the 2 Hot Leads of the circuit and runs perfectly without a path or need of a Neutral or Ground {of course they add needed safety & versatility for 110/125 vac use}
you almost answered yourself:)
With 2 separate phase Voltages a distinctive EMF (electromotive force) is present on the appliance/motor or meter equal to the sum of their parts and moves motors, induce heater coils etc.
That is why you do not need a neutral or ground to operate a 220vac aire conditioner< ~ < because the 2 conductors of the 2/3 Phase System carry enough current and potential (emf) between them that it is more efficient than how it's done in the 110/120/125 VAC 1 & 2 Phase System (generally called single phase) with Ground & or Neutral applied.
Making everything 220/240vac in the US was found to be not that productive, and was Coded for certain appliances and devices only.
RQT
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Thanks Roy. So I understood correctly, that a 240V appliance can run off just the two hot wires. And the ground is there in case one of the hot wires touches the case of the device.
My original question has been answered. My only confusion now is the ground(green wire) and the neutral(white wire) of a in-house circuit, and how these wires all work together. I'm gonna track down some other posts on grounding to see if I can help myself out. Thanks. Brian

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give it a shot.

coil reacts and

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for a digital

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devices only.

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Brian Dugas wrote:

The ground and neutral wire should be at the same potential (though they may not be, exactly). They should be connected together at one point, the service entrance, and should also go to a ground rod there. It is important to maintain the ground at zero potential with respect to actual earth ground as a safety measure. That requires keeping currents out of the ground leg in normal operation. Thus, the neutral wire, and not the ground wire, must be the return path for the so-called 'hot' wire, or there could be voltage drops that keep the ground above earth ground. All normal operating currents come in the black wire, and return by the white wire. The green wire is connected to all conductive surfaces that could come into contact with the power carrying wires in case of a fault. Since it is not carrying any currents, it will be at ground potential. Of course, if there is a short circuit to the chassis, there will be high ground currents for a moment, until the circuit breaker trips or the fuse blows. It is possible to get electrocuted by touching a hot wire while standing in bare feet on a concrete floor, because current can flow back to the transformer through the ground, since it is connected to ground at the service entrance. This is where a GFCI comes in. It breaks the circuit if there is any imbalance between the currents in the hot (black) and neutral (white) wires. One might ask why one side is connected to earth ground at all. If it weren't, both wires from the transformer could be at a dangerous potential with respect to earth ground by capacitive coupling. Similarly, if the chassis of a piece of equipment were not connected to a ground, it could still shock someone in bare feet on a concrete floor by capacitive coupling to the connected power lines.
-- john
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