Hi everyone,

I have this question with respect to hot versus neutral wires with AC electricity in household wiring. This is very basic, but if someone could it explain it to me, I would be grateful.

Let's say that you are installing a switch in your house (for example, a light switch). I understand that you are supposed to attach the hot wire -- the one that carries the current -- to the bronze terminal screws. Typically, in the United States, that wire is black.

And the reason that you do this is so that, when the switch is open (off), no current is going to the device (the light bulb in this case).

But here is what I don't understand.

In AC electricity, the direction of current flow keeps changing -- half of the time the current flows in the wire one way, and the half of the time the current flows the other way, right?

So how can we say that one wire is hot (and supplies the current) and that the other wire is neutral (and returns the current)? Why aren't both wires hot half of the time, and both wires neutral half of the time.

Thank you very much.

Stanly

 

Here are some definitions in wiki that might help.

http://en.wikipedia.org/wiki/Ground_and_neutral

 

Here is the best analogy I can come up with.

Imagine your toilet bowl is plugged up and you are going to work it loose with a toilet bowl plunger.

The plunger is your "hot" wire.
The toilet drain is your "neutral".

Without making contact with the toilet bowl, move the plunger up and down in an oscillatory motion. No contact is made with the toilet bowl as yet. The water is undisturbed and stays motionless.

As you "connect" the plunger with the toilet bowl you set the water moving up and down with the oscillatory action of the plunger. You have created the alternating current of water.

The plunger is the oscillating voltage, it carries no current.
When you apply the plunger to the toilet, the current of water sloshes up and down created by the plunger pressure "voltage". That is the alternating current.

"Pressure" creates "current".

Voltage creates current.

Hope that helps.

 

and, all the while, the "neutral" sewer pipe has no pressure (voltage) build up.

 

Hi everyone,

Thank you very much for your replies, but I am still confused.

Look at the very first drawing on this page:

http://www.allaboutcircuits.com/vol_1/chpt_3/8.html

So, with alternating current, isn't there electrical flow half of the time from the source to the load? In other words, isn't there electrical flow half of the time along the "neutral" conductor?

And if there is electrical flow, then why isn't the "neutral" conductor "hot" half of the time?

Thank you.

Stanly

 

It's called neutral because its voltage is maintained at ground potential. In other words there is no significant voltage between it and earth ground. It still can have AC current flowing back and forth but the only voltage you would measure to ground is the small voltage due to the current going through the low wire resistance.

Neutral does "return the current" but return doesn't mean one direction of current, it just means the current is returning from the hot lead, independent of direction. The "return current" is actually alternating in both directions, the same as the hot current.

The hot wire's voltage polarity goes positive and negative with respect to the ground and so the current goes back and forth in both the hot and the neutral wires.

Make sense?

 

Hi,

No, sorry, but it doesn't make sense yet.

Let's say for a moment that the circuit was not grounded. So let's take the ground out of the equation.

Would both wires be equally "hot" half of the time? In fact, would both wires always be "hot" all of the time? Dangerous current would be flowing in one direction half of the time, and dangerous current would be flowing in the other direction half of the time?

Thank you.

Stanly

 

"Hot" does not mean, "negative voltage" only"
"Hot" does not mean, "negative current only".
In house wiring, "dangerous" current flows in both directions and it flows through the neutral wire. The neutral wire is connected to, "ground" so very little voltage can be measured on that wire, no matter which way the current is flowing.

 

Hi everyone,

OK. So now I am getting somewhere.

What makes the wire "neutral" is the fact that it is grounded, right?

If you take away the ground, then both wires -- the so-called hot conductor and the so-called neutral conductor -- have dangerous current flowing through them, right? And half of the time the current flows one way, and half of the time the current flows the other way, right?

Stanly

 

From a conceptual point of view, what you need to do is disassociate your self from the 'Live' and Neutral' and imagine two conductors of a 1phase AC circuit each are identical in nature.
Which essentially makes for two 'Hot' conductors.
All we do is take one to ground so that now one is referenced to earth ground and we term it 'Neutral', nothing has changed as far as the conductors are concerned.
In a N.A. 240v centre tapped transformer supply, we opt to take the centre tap to ground and term it neutral, whereas it is purely arbitrary which one we take to ground and call neutral, all circuits would still function as normal.
I will add that is a NEC requirement to take the C.T. to ground.
Max.

 

Hi everyone,

First, I want to thank all of you nice people for your help. You are making things much clearer for me.

But just to nail it down, I am going to essentially restate my very last post and ask these three questions. I hope that the answer to all three questions is yes. If it is, then I can move on to more fully understand ground.

So, let's say that an AC circuit were not grounded. Let's say that all we had is an AC source connected via a wire (let's call it Wire #1) to a load. And then from the load we had Wire #2 back to the AC source.

Here are my three questions:

Question 1: Do both wires have equally dangerous current flowing through them?

Question 2: Half of the time does the current flow in one direction, and half of the time does the current flow in the other direction?

Question 3: So you could attach an on/off switch to either Wire #1 or Wire #2, and it would make no difference. Is that correct?

Stanly

 

Right. If you were designing your own circuits, you could call anything neutral, ground, or common. In that case, it's just a "convention" to make schematics easier to read. In the case of house power, the Law got involved because it is a matter of public safety. The transformer on the pole would work just the same if we didn't run a wire to the planet, it's just safer that way.

Edit: yes, yes, and, it would work, but it's safer to switch the "hot" wire in house wiring.

 

Just to add, The term 'alternating current' means that there is a reversal of current directional flow in each conductor, in N.A., 60 times/sec.
In a control circuit situation where the 120v supply does not have one side grounded, then both sides require switching to disconnect and also a fuse is required in each line.
As opposed to a circuit that is live and neutral, one side is switched and fused.
Max.

 

Hello, Stanly and welcome to AAC.

You are asking some penetrating questions that are giving our experts pause for thought. Well done.

As to your questions.

Let us suppose you had a portable (single phase) generator on rubber tyres, driving some motorised equipment at a remote site.

In this situation you would have two feed wires that, as Max has already said, would be indistinguishable.

So yes they would both carry powerful and dangerous currents. Touching either would shock you. Either could start a fire.

But the motor would work just fine.

And one switch in either would stop the motor.
But a single switch would not remove the danger.
For this situation you would need a double pole switch. A double pole switch is really two switches mechanically joined together so that you could disconnect both wires on switching off. Disconnecting only one wire would leave the other live or hot.

Finally as to current rushing one way and then the other, this is a convenient fiction to model the way alternating current works. It is a very simple model that has some uses but should not be pushed too far as, like all models, it only tells a small part of the whole story.

 

Hi,

In my very simple model of an AC source connected to a load by Wire #1 and Wire #2, why would you need a switch at both Wire #1 and Wire #2?

I don't understand that. If you disconnect the circuit at either Wire #1 or Wire #2, then why wouldn't that stop the current?

Stanly

 

Technically, yes it would interrupt current flow, but from a safety aspect it is considered the most desirable to disconnect both conductors, by disconnecting one conductor, there could conceivably be a return path via many means, including inductive or capacitive.
In a live/neutral situation, there is only one potentially 'live' conductor.
Max.

 

Hi,

OK. I think that I am all set.

My next job is to understand grounding, and for this it seems that I need to pour over "Safe circuit design" in All About Circuits.

Max (or anybody else), please don't get into this in any great detail because it would quickly go over my head, but in a DC circuit, would it be safer to have a switch at only Wire #1 or Wire #2?

Thanks again to everyone. This has been greatly instructive.

Stanly

 

For instance, if your gasoline powered generator with rubber tires leaned against a wet tree, a "ground" path would be available through the soil. Even worse, if a barefoot person was adjusting the generator and made a ground path through the soil, both people would be in danger.

Edit: This two wire danger is only resolved with an intentional path to Earth. That will make one of the wires, "safe". A car has most of its metal connected to the negative pole of the electrical system. Touching any bare metal in a car is usually considered to be safe. Besides that, 12 to 16 volts usually doesn't hurt people. If you want to work with 50V or 100V of DC, you need to switch both wires or earth one to set a reference for the safe points.

 

My next job is to understand grounding, and for this it seems that I need to pour over "Safe circuit design" in All About Circuits.

Max (or anybody else), please don't get into this in any great detail because it would quickly go over my head, but in a DC circuit, would it be safer to have a switch at only Wire #1 or Wire #2?

With DC, if it is considered Low Voltage it is not absolutely necessary to switch both.
If you want some 'light' reading on grounding and something called equi-potential bonding.
http://www.automation.siemens.com/doconweb/pdf/840C_1101_E/emv_r.pdf?p=1
Max.

 

In my very simple model of an AC source connected to a load by Wire #1 and Wire #2, why would you need a switch at both Wire #1 and Wire #2?

I don't understand that. If you disconnect the circuit at either Wire #1 or Wire #2, then why wouldn't that stop the current?

And one switch in either would stop the motor.
But a single switch would not remove the danger.

I did say that a single switch would stop the motor.

But both wires are live as far as the switch.

So let us say the switch is at the generator. Then switching one wire would make that wire dead from the gnerator to the motor.

But the other wire would still be live. It would just not be passing current.

I also said it would not remove the danger. The danger is there because you have a live wire connected to one terminal of the motor.

If someone touched this they would be shocked.

If the the other (dead) terminal was now connected to earth accidentally or deliberately the motor would start and might cause injury or damage.