A/C voltage understanding

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  #1  
Old 01-25-11, 09:21 AM
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A/C voltage understanding

im a begginner and confused, dont worry im not an electrician haha
i get d/c voltage, its the basic of the two. but what i dont get about a/c voltage is this:
i thought that (to simplify things) a hot wire comes out of the power plant and into your house, through what ever receptacle etc, through your lamp, through the neutral wire and back to the power plant. also that the current alternates back and forth. so with this understanding if you place a test lamp lead in the hot receptacle slot and the ground it will light, but also that if you place it in the neutral and ground it would light also. i know now that the neutral to ground wont light it.

my questions are do the neutral lines go back to the plant or does the current actually alternate into the earth for a ground?
if the current alternates why wouldnt both lines be "hot" and why wouldnt putting the test light from neutral to ground make it light?
that like saying only the black wire is hot which makes it out to act like dc voltage
 
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  #2  
Old 01-25-11, 09:33 AM
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I'm not an expert, but there are 2 hots and a neutral, between either hot and the neutral is 120 volts, between both hots is 240 volts. I think the neutral goes back to the power plant.

The current alternates between both hots, not between either hot and the neutral.

Any expert correct me if I'm wrong. I also understand DC much more clearly than these 2 phase AC lines.

Could anyone post how the 2 hots and neutral are connected at the generator/power plant? This would help me understand better.
 
  #3  
Old 01-25-11, 10:06 AM
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Could anyone post how the 2 hots and neutral are connected at the generator/power plant? This would help me understand better.
The neutral isn't and the hots only indirectly. The neutral begins and ends at the transformer that supplies your house. Near your house the power company have a step down transformer with a center tapped secondary. The output is 240v. That is the two hots to your house. The center tap being half the winding provides 120v when reverenced to either of the hots.

In addition the center tap is grounded that is why the technical name for it is "grounded conductor". Under ideal conditions voltage between the neutral and earth is 0v but variations in soil conductivity can alter that. That is why you can't rely on a connection to earth to safely trip a breaker. Therefore you also are required to have a metallic path such as a wire or metal conduit back to the panel so the breaker will reliably trip if there is a fault.

Above applies to single phase residential.
 

Last edited by ray2047; 01-25-11 at 10:51 AM.
  #4  
Old 01-25-11, 10:07 AM
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These are good questions, but probably can't be fully answered in this forum. But let me try to explain AC. Yes, you have two hots and one neutral and one ground. think of each hot and being +voltage for 1/120 of a second, then - voltage for 1/120. The neutral conductor is grounded for safety purposes, and so the voltage on the neutral does not fluctuate very much, but it will a little. The voltage you see on the neutral is caused by the current flow from the load back to the panel and to the transformer. The "ground" conductor is also grounded but it does not normally have current flowing through it, so the voltage on it remains "ground".

So each hot is + then -, then +, then -on and on. But while one hot is + the other is - {Mod edit: up to this point the post is correct but the voltage isn't additive and there are not two 120 v There is a single neutral from a center tap on the transformer which provides a 120 v with either 240v hot leg.} so the voltage across, hot to hot is additive and you get 240 from each of the two 120 legs.

Does that help at all?
 

Last edited by ray2047; 01-25-11 at 10:54 AM. Reason: Correct mileading information
  #5  
Old 01-25-11, 11:37 AM
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im still pretty confused. ill refer to this picture link to show you what i mean
http://www.school-for-champions.com/...dc_circuit.gif
im comparing
dc source-power plant
wire to resister-hot wire to outlet
resister-lamp plugged into outlet
wire returning to dc source-neutral wire
but its constantly switching from + to -
thats what ive been basically thinking of, about a house circuit

@DIYmaster: when you say "The neutral conductor is grounded for safety purposes, and so the voltage on the neutral does not fluctuate very much, but it will a little." im assuming that the neutral wire travels out to the transformer then to ground like ray2047 stated. so do you mean that since the neutral is attached directly to ground at the transformer it cant switch enough to positive to light a test light from the neutral to ground sockets? basically it is still switching but the majority of its switching is negative because of its direct link to ground?
sorry for all the confusion and questions if im asking to many
 
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Old 01-25-11, 11:47 AM
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I won't try to confuse things...but you can't compare DC to AC. AC "polarity" (not really the correct term, I don't think) is constantly changing...DC remains constant.

You might Wiki it....?
 
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Old 01-25-11, 11:57 AM
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i know you cant compare them, thats just what i was going off of since dc was all i was taught. ive wiki'd it and seached a bunch but cant really find the answer anywhere
 
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Old 01-25-11, 12:29 PM
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Any voltage between neutral and ground comes from a difference in voltage between the two. For example due to resistance of the soil at the point of measurement you have a voltage drop of 3 volts from 120v to 117 volts. However the neutral is on a copper wire with no measurable voltage drop so it is 120 v. So if you measure between the two you see a difference of 3 volts. However that is if you rely on earth only. With an uncompromised ground wire back to the main panel there should be no measurable difference.
 
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Old 01-25-11, 12:33 PM
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The difference between AC and DC is not important for the scope of your question about the grounded neutral. For current to flow in any circuit, there needs to be a difference in voltage. Voltage is essentially the "pressure" in an electrical circuit which forces electrons to flow through the resistance. If there is no difference in voltage between two points, there will be no current flowing through the load.

In your example, the difference in voltage between the hot and neutral is 120V which is a significant amount of pressure to drive current through the load. When you connect a resistor between the hot and neutral, the difference in voltage between the two points causes current to flow through the resistor. When you connect a resistor between the neutral and ground, which differ in voltage by one or two volts at most, drives very little current through the resistor; certainly not enough to light a lamp filament.
 
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Old 01-25-11, 12:58 PM
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Your analogy of DC compared to AC is appplicable to a degree. But I would do it this way:

DC source - transformer secondary winding
wire to resistor - hot wire to outlet
resistor - lamp plugged into outlet
wire returning to DC source - neutral wire

Now the analogy continues.....For and AC source such as the transformer secondary winding in our case, the source voltage polarity changes very often. It is plus then minus 120 times a second. So think of it thes way: the electrons don't go around in a circle like for DC, they go down the line a bit then back up, go down the line then back up, over and over.

The other point that is confusing is the concept of "ground". Voltages are ALWAYS really measured as a voltage difference. When you measure voltage it is always relative to something else. When you say I have 120 volts, that (usually) means 120 volts to "ground". But any point can be chosen as the reference point, the "earth" is the biggest and the most common reference point, and it is usually called "ground" or "earth ground". Your AC voltage at your outlet for a hot conductor, alternates in polarity relative to ground, it is positive, then negative. The neutral side is "grounded". So the voltage there is something very close to ground compared to "ground". The only reason there is a voltage at all is because there is a small voltage drop in the neutral conductor leading back to the panel. V=IR. If there is current, there is voltage drop. There is current in the neutral so there is voltage drop.

Back to your DC diagram. The negative side of the DC supply is tied to ground. So the only voltage (relative to ground) on the neutral conductor at the outlet is what is dropped in the conductor from the outlet to the source connection because of the current flow.
 
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Old 01-25-11, 02:12 PM
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I have a slightly related question. For years our neutral connector from the power company to the house was completely broken off and was just recently fixed.

Why did we never notice any problem with the power with a completely disconnected neutral? I think the ground did the job of the neutral but wouldn't there have been some noticeable issue like lower voltage or something?
 
  #12  
Old 01-25-11, 02:18 PM
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Originally Posted by aabsc View Post
I have a slightly related question. For years our neutral connector from the power company to the house was completely broken off and was just recently fixed.

Why did we never notice any problem with the power with a completely disconnected neutral? I think the ground did the job of the neutral but wouldn't there have been some noticeable issue like lower voltage or something?
Very likely, your water pipe carried the "return" current back to the utility. Do you have copper piping?
 
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Old 01-25-11, 02:21 PM
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Yes, I would have thought you would have noticed, but since you didn't it means you have very good ground rods and a very conductive earth. The unbalanced load from the panel was making it's way back to the center tap of the transformer through the ground rods and grounding conductors. There is a ground rod at the house and another one at the transformer with the earth in between acting as the neutral conductor.
 
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Old 01-25-11, 02:22 PM
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Originally Posted by aabsc View Post
Why did we never notice any problem with the power with a completely disconnected neutral?
If you are in a city with a copper water system, a combination of the earth itself and the water pipes can take the neutral current; however most households that experience a broken neutral will have a lot of bad side effects. Voltage can fluctuate, damaged electronics, shocks from the plumbing, etc.
 
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Old 01-25-11, 04:13 PM
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If I'm not mistaken I believe my UPS and multimeters used to read 118 volts, now it reads 120-121 volts.

And yes, we have copper piping
 
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Old 01-25-11, 05:09 PM
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well im glad to say that this thread has now cleared up a good amount of things up for me, thanks guys i really appreciate it!
 
  #17  
Old 01-26-11, 07:24 AM
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Originally Posted by aabsc View Post
If I'm not mistaken I believe my UPS and multimeters used to read 118 volts, now it reads 120-121 volts.

And yes, we have copper piping
So, your copper pipe (tubing really) took virtually all your "neutral" (unbalanced) current from your home to your neighbors pipes and thru their service entrance to the POCO feeder. Not much of this current will travel from ground rod to POCO, that is far higher in Resistance.
 
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Old 01-26-11, 12:52 PM
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But the unbalanced load current must make it back to the center tap of his transformer. So how is that again?
 
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Old 01-26-11, 12:55 PM
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Doesn't the same transformer feed several houses?
 
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Old 01-26-11, 01:09 PM
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One transformer generally feeds several houses, and the grounded secondary neutral is interconnected across many transformers in the neighborhood via grounded messenger wires. Not to mention they are all earth grounded, which if done well can handle substantial current.
 
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Old 01-26-11, 03:31 PM
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Originally Posted by ibpooks View Post
One transformer generally feeds several houses, and the grounded secondary neutral is interconnected across many transformers in the neighborhood via grounded messenger wires. Not to mention they are all earth grounded, which if done well can handle substantial current.
the stranded messenger only terminates on one transformer,the one going to your service and the 2 or three houses that are fed off the same xformer.
 

Last edited by oleguy; 01-26-11 at 04:08 PM.
  #22  
Old 01-26-11, 04:05 PM
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Originally Posted by oleguy View Post
the stranded messenger only terminates on one transformer,the one going to your service and the 2 or three hiuses that are fed off the same xformer.
hmmm, no. The CT of the secondaries of ALL the transformers are also connected to POCO earth. Vertical ground from each pole to ground rods, horiz ground pole to pole also. All bonded together.

I should say also, that there are lots of documented cases of water utility workers making lots of sparks as they work connecting and disconnecting water meters. You can see how the water meter can interrupt the neutral current, going from one building to another. Of course, it also takes a neutral line open, but this can go unnoticed for years.
 
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Old 01-26-11, 06:12 PM
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Originally Posted by telecom guy View Post
hmmm, no. The CT of the secondaries of ALL the transformers are also connected to POCO earth. Vertical ground from each pole to ground rods, horiz ground pole to pole also. All bonded together.

I should say also, that there are lots of documented cases of water utility workers making lots of sparks as they work connecting and disconnecting water meters. You can see how the water meter can interrupt the neutral current, going from one building to another. Of course, it also takes a neutral line open, but this can go unnoticed for years.
read his ans.quote,they are all connected to the stranded messenger.my point is the messenger does not loop to all xformers.this is how his post sounded.quote:his quote,one trans generaly feeds several houses,and the secondary neutral ia interconnected accross many xformers in the neighborhood via the grounded messenger wires.yes the xformer neu is grounded at each xformer.my point is,he made it sound like the messengr is looped to ea xformer in the neighborhood.then he say's they are all earth gronded.just clearing it up.to me he is trying to say the messenger goes all over.and no they don't run a ground pole to pole.some times a messenger goes from a xformer pole to another no more than one pole to tie to a pole ground but not to every pole.if thats what you mean.
 

Last edited by oleguy; 01-26-11 at 06:30 PM.
  #24  
Old 01-27-11, 03:51 AM
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I believe it would pretty much get me upset if I found out there was large amounts of current flowing through my water pipes because of an electrical problem at my neighbor's house. But at least I don't have to worry in my "current" house, as it has plastic pipes.
 
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Old 01-27-11, 05:57 PM
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Originally Posted by telecom guy View Post
I should say also, that there are lots of documented cases of water utility workers making lots of sparks as they work connecting and disconnecting water meters. You can see how the water meter can interrupt the neutral current, going from one building to another. Of course, it also takes a neutral line open, but this can go unnoticed for years.

Hey, don't forget the gas lines!


Gas utility lines can be steel pipe that can act as a grounding system when the regular grounding system fails.

The gas utility I worked for required that bonding cable be connected between the house gas fuel line and the utilitiy's gas service before disconnecting the gas meter.

Once in a while you'd find line voltage or thereabouts between ground and the customer's fuel line. A variety of things might cause the fuel line to be energized, and I suppose a failed grounding system might be one of them.
 
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