[hellothere123]

Ok,

I'm getting a bit of this, but wanted to understand more.

I understand the black wire carries the current to the switch and then the light or the electrical outlet. Ground wire is also easy to understand as it will carry current to ground in case the black wire touches the box for example.

I had 2 questions:

1) If a black (hot) wire touches the electrical box and then travels on the grounding wire to "ground", how does this trip the breaker in the box? I don't understand how that surge would go to the box and trip the breaker that the light is on...or does it cut all power?

2) Neutral wire (white) : At a electrical outlet, the black wire provides the power...is the neutral wire also carrying this same power, basically is it as hot as the black wire? Or is it just there to return unused power back to ground once an appliance is plugged into the outlet.


As you can see, I kinda got the jist of things...but not really!

Thanks for the help!

 

[InPhase277]

Ok,

I'm getting a bit of this, but wanted to understand more.

I understand the black wire carries the current to the switch and then the light or the electrical outlet. Ground wire is also easy to understand as it will carry current to ground in case the black wire touches the box for example.

I had 2 questions:

1) If a black (hot) wire touches the electrical box and then travels on the grounding wire to "ground", how does this trip the breaker in the box? I don't understand how that surge would go to the box and trip the breaker that the light is on...or does it cut all power?

2) Neutral wire (white) : At a electrical outlet, the black wire provides the power...is the neutral wire also carrying this same power, basically is it as hot as the black wire? Or is it just there to return unused power back to ground once an appliance is plugged into the outlet.


As you can see, I kinda got the jist of things...but not really!

Thanks for the help!

The hot and neutral are exactly equivalent, except at the service equipment the neutral is grounded. They both carry the same current, and electrically they each take turns alternately supplying current to the load. What I mean is, AC changes direction 120 times every second, so during one half the cycle, the black is positive, and the white is negative. 1/120 of a second later, the direction changes, and the black is negative and the white is positive. So, the two wires are identical.

We ground the white wire so that stray currents and faults will have a place to go in order to complete the circuit and trip a breaker. At the service equipment, the neutral and the ground wires are joined together. This way, the metal frames of our appliances will be connected to the ground, and therefore the neutral. If a short occurs, like in your box example, current does NOT go to ground. It travels along the ground wire until it reaches the point where the ground wire joins the neutral. This completes the circuit and trips the circuit breaker.

It is a huge misunderstanding that the ground wire goes to the Earth. We do connect it to the earth, but as far as short circuits are concerned the key is being connected to the neutral. So, the neutral is as "hot" as the hot wire, but everything around us that we can touch that is grounded is at the same voltage as the neutral at that instant in time. When voltages are equal, no current can flow, so we don't notice that the neutral is carrying a current. It appears to be dead, but that is only relative. So, relative to grounded objects around us, the neutral is at 0 volts. But relative to the hot wire, it is at 120 volts.

For the purposes of illustration and clarity, diagrams usually show the hot wire bringing current in and the neutral taking it away. But that is simplistic view that is really the case.

Stubbie has some good diagrams showing this and the paths that fault current travels. Maybe he will chime in with some. I hope that didn't confuse you even more.

 

[Yoyizit]

But the other end of the neutral might be at 1vac or so with respect to ground, depending on the current through it and the wire size.
In this case, 120vac goes in and 1vac is lost in the hot lead and 1vac is lost in the neutral lead, leaving your appliance with 118vac.

 

[220/221]

If a black (hot) wire touches the electrical box and then travels on the grounding wire to "ground", how does this trip the breaker in the box?

It carries more current than the breaker can handle. If more than 15 amps tries to flow thru a 15 amp bteaker a mechanical switch inside snaps open.


An small incandescant light bulb is a dead short that only caries a fraction of an amp.

A toaster is a dead short that draws about 4 amps.


A quick touch of the hot wire to the grounded box may not draw enough to trip the breaker.

If the contact is solid, it will trip the breaker quickly (if the breaker is functioning properly)

At a electrical outlet, the black wire provides the power...is the neutral wire also carrying this same power,

It is easier to think of the neutral as a path back for the power.

You won't feel any power by sticking your fingers on the neutral because the neutral phase is grounded. You will however feel the power quite profoundly if you get in between an open, loaded neutral.

 

[hellothere123]

The hot and neutral are exactly equivalent, except at the service equipment the neutral is grounded. They both carry the same current, and electrically they each take turns alternately supplying current to the load. What I mean is, AC changes direction 120 times every second, so during one half the cycle, the black is positive, and the white is negative. 1/120 of a second later, the direction changes, and the black is negative and the white is positive. So, the two wires are identical.

We ground the white wire so that stray currents and faults will have a place to go in order to complete the circuit and trip a breaker. At the service equipment, the neutral and the ground wires are joined together. This way, the metal frames of our appliances will be connected to the ground, and therefore the neutral. If a short occurs, like in your box example, current does NOT go to ground. It travels along the ground wire until it reaches the point where the ground wire joins the neutral. This completes the circuit and trips the circuit breaker.

It is a huge misunderstanding that the ground wire goes to the Earth. We do connect it to the earth, but as far as short circuits are concerned the key is being connected to the neutral. So, the neutral is as "hot" as the hot wire, but everything around us that we can touch that is grounded is at the same voltage as the neutral at that instant in time. When voltages are equal, no current can flow, so we don't notice that the neutral is carrying a current. It appears to be dead, but that is only relative. So, relative to grounded objects around us, the neutral is at 0 volts. But relative to the hot wire, it is at 120 volts.

For the purposes of illustration and clarity, diagrams usually show the hot wire bringing current in and the neutral taking it away. But that is simplistic view that is really the case.

Stubbie has some good diagrams showing this and the paths that fault current travels. Maybe he will chime in with some. I hope that didn't confuse you even more.

Usually there has to be confusion before you can have understanding...I hope you stay with me here:

I think the thing that is confusing in diagrams is the neutral wire (white wire) is connected to the neutral bar (which is connected to "ground). So I was assuming that all of the electrical power that flows through the white neutral wire is waste and simply goes to ground. I can see that the power that flows on the neutral wire goes back to the service panel and THEN returns unused current back to the power source, not to ground.

Even on a diagram of an electrical outlet, it is labelled black "hot" or "live" wire and the white is just labelled as neutral. So you just assume that only the black is live and the white is dead like a ground wire.

The reason I am asking, is that I started to renovate this house, and I found that on the neutral (white) wires that this guy wired electrical outlets so that the clump of wires were millimetres away from touching the metal electrical box..some people I talked to said that I was not to worry as that is just the neutral wire anyway.


I was going to ask you about how power can flow back through a switch when it has one white wire connecting instead of 2 black wires (black in and black out)...but I think I get it, because the only way for electricity to flow is when there is a completed circuit.

 

[hellothere123]

It carries more current than the breaker can handle. If more than 15 amps tries to flow thru a 15 amp bteaker a mechanical switch inside snaps open.


An small incandescant light bulb is a dead short that only caries a fraction of an amp.

A toaster is a dead short that draws about 4 amps.


A quick touch of the hot wire to the grounded box may not draw enough to trip the breaker.

If the contact is solid, it will trip the breaker quickly (if the breaker is functioning properly)



It is easier to think of the neutral as a path back for the power.

You won't feel any power by sticking your fingers on the neutral because the neutral phase is grounded. You will however feel the power quite profoundly if you get in between an open, loaded neutral.

Thanks for the explanation of how a black wire touching a box will increase the flow and then the breaker will blow on the black wire because of the increase...got it...thanks!

This statement : "You won't feel any power by sticking your fingers on the neutral because the neutral phase is grounded. You will however feel the power quite profoundly if you get in between an open, loaded neutral."

confuses me. How can I touch the neutral wire as I thought it is as live as the black wire...and I assume you can't touch the black wire?

 

[InPhase277]

Thanks for the explanation of how a black wire touching a box will increase the flow and then the breaker will blow on the black wire because of the increase...got it...thanks!

This statement : "You won't feel any power by sticking your fingers on the neutral because the neutral phase is grounded. You will however feel the power quite profoundly if you get in between an open, loaded neutral."

confuses me. How can I touch the neutral wire as I thought it is as live as the black wire...and I assume you can't touch the black wire?

OK, you have to understand voltage and current. Voltage is the pressure at which electricity is pushed through a circuit. Current (amps) is the quantity of electricity. In the timeless old water analogy, volts is pressure, and current is gallons per minute. Resistance of a wire is like friction in a pipe.

Current only flows when there is a difference of voltage between two ends of a circuit. In a household circuit like we are discussing, this difference (pressure) is 120 volts. The neutral wire and ground wire are connected together at one end, therefore the difference in voltage between them is 0. What that means is that because there is no difference in voltage, no current can flow. So if you touch a grounded object, your body is at the voltage of that object. When you reach out to touch a neutral at the same time, the voltage between your body and the neutral is 0, so no current will flow, and you will feel no shock.

I think the thing that is confusing in diagrams is the neutral wire (white wire) is connected to the neutral bar (which is connected to "ground). So I was assuming that all of the electrical power that flows through the white neutral wire is waste and simply goes to ground. I can see that the power that flows on the neutral wire goes back to the service panel and THEN returns unused current back to the power source, not to ground.

Nope. Ther is no such thing as unused current. If a current exists at all, it is being used for something. The current in the neutral makes its way back to the transformer outside. Inside the transformer, it flows through the coil and back to the hot and back to the load again. It is a closed loop. One half of the AC cycle the current flows from the transformer on the hot and to the load. The other half of the AC cycle the current flows from the transformer on the neutral to the load. The white wire plays an equal role to the black wire, because they both swap positions 120 times every second.

We just think of the white wire as dead because everything is bonded to it and thus at the same voltage. We don't get shocked from the neutral because there is no difference in pressure. That leads us to consider it dead, but from the perspective of the hot wire, it is in every way an equal. If we reversed the connection at the transformer and grounded the hot wire, and left the neutral ungrounded, we would not be able to tell the difference. All our grounds would now be bonded to the old hot wire, and the same 0 voltage would exist to ground. This time, touching the wire wire would shock you while the black wire would appear dead.

 

[goose134]

How can I touch the neutral wire as I thought it is as live as the black wire...and I assume you can't touch the black wire?

You can touch the neutral because it has (if done correctly) no potential to ground. You assume correctly. There are times when professionals need to do so, but I'm not going to enumerate the ways to touch a hot wire without a shock here. The hot wire has the voltage potential to ground that the neutral does not. This is because the neutral is BONDED to the ground in the panel. This simply means that electrically speaking they are holding hands.

There will be more questions. I don't want to confuse this by adding too much.

 

[InPhase277]

The neutral in your box should have a wire nut on it, because while it will not short out if it touches ground, it is still a good idea to try to limit all the possible paths for current to travel on to just the exact circuit we planned. If we allow grounds and neutrals to come together downstream of the service equipment, we introduce a path other than what we intended, and unintended current paths can be dangerous.

And a white wire hooked to a switch is just an extension of the hot wire from the light. It just happens to be white and should be colored to identify it as such, like with a piece of tape. So, the hot wire comes into a light, and is diverted down to the switch on the white wire of another cable. It returns from the switch on the black wire and powers the light. In this case, that white wire is not a neutral, but simply an extension of the source hot.

 

[hellothere123]

OK, you have to understand voltage and current. Voltage is the pressure at which electricity is pushed through a circuit. Current (amps) is the quantity of electricity. In the timeless old water analogy, volts is pressure, and current is gallons per minute. Resistance of a wire is like friction in a pipe.

Current only flows when there is a difference of voltage between two ends of a circuit. In a household circuit like we are discussing, this difference (pressure) is 120 volts. The neutral wire and ground wire are connected together at one end, therefore the difference in voltage between them is 0. What that means is that because there is no difference in voltage, no current can flow. So if you touch a grounded object, your body is at the voltage of that object. When you reach out to touch a neutral at the same time, the voltage between your body and the neutral is 0, so no current will flow, and you will feel no shock.



Nope. Ther is no such thing as unused current. If a current exists at all, it is being used for something. The current in the neutral makes its way back to the transformer outside. Inside the transformer, it flows through the coil and back to the hot and back to the load again. It is a closed loop. One half of the AC cycle the current flows from the transformer on the hot and to the load. The other half of the AC cycle the current flows from the transformer on the neutral to the load. The white wire plays an equal role to the black wire, because they both swap positions 120 times every second.

We just think of the white wire as dead because everything is bonded to it and thus at the same voltage. We don't get shocked from the neutral because there is no difference in pressure. That leads us to consider it dead, but from the perspective of the hot wire, it is in every way an equal. If we reversed the connection at the transformer and grounded the hot wire, and left the neutral ungrounded, we would not be able to tell the difference. All our grounds would now be bonded to the old hot wire, and the same 0 voltage would exist to ground. This time, touching the wire wire would shock you while the black wire would appear dead.

Very interesting read...thanks for the explanation.

Ok, real world stuff (I know I would always turn the power off at the panel, but this is examples just for arguement sake):

1) If I touch the hot black wire (the wire is still connected to the electrical outlet or switch) with my finger - do I get a shock?

2) I assume if the black hot wire is off of the electrical outlet and I grab it with my fingers, then it would try to complete the circuit to ground, because of the difference in voltage, and I would be shocked.

3) If I take the white wire off of the electrical outlet and hold it with my fingers, since it is grounded or 0 volt difference that you mentioned, then I would not get a shock?

4) Lets say you plug in an extension cord, didn't realize it had a break in it and you make contact with the live wire...does your hand have to be wet to be shocked?

5) In the above examples when you would get shocked, the breaker would blow, so based upon how much current you would recieve before the breaker trips, is it most likely that you would be shocked but not killed?

Thanks again

 

[hellothere123]

You can touch the neutral because it has (if done correctly) no potential to ground. You assume correctly. There are times when professionals need to do so, but I'm not going to enumerate the ways to touch a hot wire without a shock here. The hot wire has the voltage potential to ground that the neutral does not. This is because the neutral is BONDED to the ground in the panel. This simply means that electrically speaking they are holding hands.

There will be more questions. I don't want to confuse this by adding too much.

I hear you and thanks for the reply... I assume that what electricians do is (as my dad has always said) work with one hand behind your back...and always to assume the wire is hot even if you know the power is turned off.

 

[InPhase277]

Very interesting read...thanks for the explanation.

Ok, real world stuff (I know I would always turn the power off at the panel, but this is examples just for arguement sake):

1) If I touch the hot black wire (the wire is still connected to the electrical outlet or switch) with my finger - do I get a shock?

Only if you are grounded somehow or touching the neutral at the same time. If you are insulated from the earth and not touching any grounded objects, then your body's voltage becomes that of the hot wire, and no current flows so you feel no shock.

2) I assume if the black hot wire is off of the electrical outlet and I grab it with my fingers, then it would try to complete the circuit to ground, because of the difference in voltage, and I would be shocked.

Same as above, no shock unless there is a complete circuit through you to ground or neutral.

3) If I take the white wire off of the electrical outlet and hold it with my fingers, since it is grounded or 0 volt difference that you mentioned, then I would not get a shock?

Correct, as long as everything is wired correctly. Do not assume that it is.

4) Lets say you plug in an extension cord, didn't realize it had a break in it and you make contact with the live wire...does your hand have to be wet to be shocked?

No, it doesn't have to be wet (that doesn't have much to do with it). But there has to be a complete circuit. If all you are touching is one wire of any circuit, and have no other paths to complete the circuit through you, you will not be shocked.

5) In the above examples when you would get shocked, the breaker would blow, so based upon how much current you would recieve before the breaker trips, is it most likely that you would be shocked but not killed?

Thanks again

No, the average house circuit is 15 or 20 amps. The current that flows through your body to cause a painful or lethal shock is only fractions of an amp. The resistance of your body is much too high to allow enough current to flow to trip a breaker. In other words, if you are unlucky enough to get caught in the wrong circuit, that circuit will happily supply enough current to kill you, and continue to supply that current for as long as the path exists.

 

[InPhase277]

I hear you and thanks for the reply... I assume that what electricians do is (as my dad has always said) work with one hand behind your back...and always to assume the wire is hot even if you know the power is turned off.

Some foolish electricians may have done that, but the fact is, it is just plain stupid. The theory is right, but the application is dumb. While I have been guilty of working on live circuits, it is far wiser to turn off the power. Besides, you can get much more work done with two hands.:thumbup:

 

[220/221]

If you think of electricity in simple forms, it may be easier to understand than the technical explanations..

Electricity flows kind of like water thru pipe.


Picture a tube filled with tennis balls. When you push a tennis ball in one end, one comes out the other end. Change the tennis balls to electrons and you have flowing electricity.


With electricity there are electrons in the wire that are pushing to get out to "ground". There are billions of electrons behind them pushing to get out. They cannot just pop out of the open end like the tennis balls, they can only leave if they have a path to "ground".

Your 120V power is in the black wire, waiting to get out. It runs to a switch where it is stopped by the open connection. Close the switch and it flows up to the light bulb, thru the filiment and back to the source on the neutral.

On the black, hot, ungrounded side of the circuit, the electrons will take any path to ground. If you touch the black and you are grounded, the electrons will flow thru you. If you are not grounded, they wont. In this case, the electricity cant go into you, it can only pass thru you. No ground, no path.

The neutral side of this circuit is grounded. The electrons are flowing thru the white wire back to the source but that side of the system is tied together with the "ground" and all things metal. The electrons will take the easiest path back which will be thru the wire.

If you interupt this neutral path by opening a wirenut, you can become the new path to ground by getting in line with the neutral or being grounded.

 

[Bigplanz]

This might make it easier to understand. At the transformer, the primary (high voltage) enters and the secondary (reduced windings to 'transform' the high voltage into lower voltage is split. The secondary measure 240V from one end to the other, but it's split in half with a ground tap that is called the (neutral) line. Both halves are 120V, but the sine wave is split. The top half is 120, the bottom half is 120. Add together and they are 240. That's why a strictly 240V circuit doesn't need a neutral. The circuit is 'complete' at 240V.

This diagram shows what I'm talking about.

 

[hellothere123]

Thanks for all the help...I understand a TON more

 

[RST]

This discussion is really helping to clear some things up for me. After reading it another 5 times I think I'll understand how things work for a 120V outlet and why the backwards-wired outlets at my house work!

But, I started reading this thread because I don't understand how a 240V circuit does not need a neutral. I spent an hour trying to figure out why a 12-2 wire to my well was not on a single-pole breaker, and then I was really confused when I saw it was on a two-pole breaker. (Same for the electric heater in my bathroom.)

I do understand why my dryer and range are wired with neutrals, it is for the 120V circuitry inside them.

But I'm still having trouble understanding why a 240V-only circuit does not need a neutral. Is it because the current flows through the one wire, then back through the other wire as a complete sine wave, alternating at 60Hz? Doesn't this normally happen with a 120V circuit? Returning to the 240V circuit, why can we use standard wires? Wouldn't the extra voltage (240V) cause some sort of problem? Along those lines, why does current (amps) decrease? I know the mathematical relationship of Ohm's law, I just don't understand the physical basis why voltage and current are inversely related . (This is where the voltage = pressure, amps = size of pipe analogy falls apart for me.)

Thank for your continued help! R.S.T.

 

[Stubbie]

I'm a little late getting here but if you guys want we can have a discussion and I'll post up some of my diagrams and other graphics to help understand this neutral thing, what causes a breaker to trip and what compromises a 120 volt circuit and a 240 volt circuit. So if the OP and RST are still here you guys let me know and we should have an interesting discussion. I will start a new thread and we can go thru each issue starting with the first two questions asked by hellothere123 in post #1.

 

[InPhase277]

Yes Stubbie, I figured you would chime in and post one of your diagrams, as I'm too lazy to look one up or draw one myself:laughing:.

And for RST, the 240 V circuit works exactly like the 120 V circuit, the same physics apply. But the reason why we put it on a double pole is because, unlike a 120 V circuit, one of the conductors is not grounded. If we used a single pole breaker, there would still be a hot wire present at the load when it was turned off, and that would be dangerous since the difference in voltage exists to ground, unlike the 120 V circuit where the neutral is at ground voltage.

In one half of the AC cycle, current is flowing into the circuit on one conductor and out on the other. This reverses on the other half of the cycle. This is identical to the 120 V circuit, the only real difference is the voltage is twice as high. Why does the current go down as the voltage goes up? Well, power is the product of voltage and current, V x A. So, if we double the voltage, for the same power we only need half the current.

 

[220/221]

Is it because the current flows through the one wire, then back through the other wire as a complete sine wave, alternating at 60Hz?

That's pretty much it.

I think of 120 as out on the hot and back on the neutral 60 times a second.

I think of 240 as out on one hot and back on the other, then it swaps, 60 times a second. Power out on phase A, back on B then power out on B, back on A.

Returning to the 240V circuit, why can we use standard wires?

240V simply = two 120's.

Besides, the wire we commonly use is rated for 600 volts.

Along those lines, why does current (amps) decrease?

You are spliiting the load between two phases so the amps per leg is half that of the same item on a 120V circuit.

Lets say that a dual voltage motor 120/240 will draw 6 amps on the single hot leg when wired for 120V.

It will draw 3 amps each on the two hot legs @ 240V

The total power consumption is the same, it is just split up differently.