Electrical question about neutral wire and power

[NetWareHead]

Reading a bit about AC current (have some rewiring projects at hand) and the way the hot and neutral wires are explained to me is the hot wire carries electricity under pressure (Volts) and amperes is a measure of how much current moving through the wire.

After passing through an electrical load (like a lightbulb) for instance, the electrical current is under zero pressure having given up its volts. Therefore the neutral return wire carries back the charge under zero voltage but there are still amps flowing.

If units of electrical power are measured in watts and watts = volts x amps, then a current flow of zero volts of going to equal zero units (watts) of electrical power, no matter how many amps? Correct? Therefore the neutral is a safe wire to touch in comparison to the hot wire which is fully energized.

Make any sense? I know there is a hole in my theory and likely to kill/injure me so I wont test it recklessly. What am I missing?

 

[pcgeek11]

Reading a bit about AC current (have some rewiring projects at hand) and the way the hot and neutral wires are explained to me is the hot wire carries electricity under pressure (Volts) and amperes is a measure of how much current moving through the wire.

After passing through an electrical load (like a lightbulb) for instance, the electrical current is under zero pressure having given up its volts. Therefore the neutral return wire carries back the charge under zero voltage but there are still amps flowing.

If units of electrical power are measured in watts and watts = volts x amps, then a current flow of zero volts of going to equal zero units (watts) of electrical power, no matter how many amps? Correct? Therefore the neutral is a safe wire to touch in comparison to the hot wire which is fully energized.

Make any sense? I know there is a hole in my theory and likely to kill/injure me so I wont test it recklessly. What am I missing?

A neat analogy to help understand these terms is a system of plumbing pipes.
The voltage is equivalent to the water pressure.
The current is equivalent to the flow rate.
The resistance is like the pipe size.

Note: All neutral wires should be tied to the ground bus inside of the service panel. The problem is you need to ensure that the neutral is indeed neutral before you touch it. Never assume anything with electricity. Use a known good meter to test before touching anything.

So Yes, you could touch a properly grounded neutral wire without any danger as it should be grounded in the breaker panel.

 

[DrPizza]

...Assuming the resistance of wire is zero ohms, that's not an assumption I make though. Also, there is current in the neutral wire, but the potential difference between it and ground is very close to zero.

Some further information for you. Let's say you're running the toaster in your kitchen; the current going through that toaster is around 15 amps. (We're going to assume an ungrounded appliance without a ground fault circuit interupter (GFI).) And, let's say that somehow that neutral wire in the toaster had its plastic coating worn off or melted off, and it came into contact with the metal chassis of the toaster. (There is a layer of non-conductive mica between the heating elements and the chassis of the toaster, so there's no short circuit.) At this point, there are 15 amps of current in the "hot" wire (black), and 15 amps in the neutral wire (white). You decide to touch the toaster with one hand while happening to be leaning against the rim of the stainless steel sink with your other hand. You have just given "the electricity" a parallel path to complete the circuit to ground. It can go either through the white wire, or through the sink. What will happen is that most will continue in the white wire - it's a darn good conductor. BUT, a small bit of the current will go through you. Let's say, 0.1 A. Darn, that's enough to kill you. Good lesson: 15 amp and 20 amp breakers are to protect your house from burning down. They do not protect you from electrocution. GFI breakers protect you from electrocution, that's why they're mandated in locations where it's possible you become the conductor in case of a fault whereby the chassis of your appliance becomes electrically connected to the neutral. The ground wire (usually bare copper) protects you via short circuiting and tripping the breaker if the chassis should become connected to hot.

 

[Mr Evil]

...So Yes, you could touch a properly grounded neutral wire without any danger as it should be grounded in the breaker panel.

Earth and neutral will be connected at a point some distance from you. If that distance is large (it could be all the way back at the substation), or the potential gradient in the earth is large (because you live near a power station perhaps), then there could still be a large enough potential difference between the neutral wire and your feet to be lethal.

 

[C1]

The "white" ground or neutral wire in a home electrical system has a couple functions. One is that it is supposed to be a continuity wire (ie, continuously connected to service outlets throughout the home for grounding and protection against lightning) - this is code. The other is that the neutral wire is the center tap of the transformer outside the home that supplies the 240 volts to the home that is available between the red & black wires (120v between white & Red or white & black). Typically an electric range would use a 240 volt lash up.

Forget about what you think about what happens with a white wire being zero volts at the end of a toaster. If the black or red supply to one side of a toaster exits out of the toaster back to neutral, it doesnt matter what color the wire coming out of the toaster is. If you grab the prong going out of the toaster to ground with one hand and touch the grounded power outlet with the other hand then you will place yourself into the circuit with the toaster. The voltage that will be experienced will be 120 v minus the voltage drop of the toaster (which wont be very much as it is a low resistance device). You should feel the shock. Your high body resistance should keep you from getting badly or quickly cooked.

 

[pcgeek11]

Earth and neutral will be connected at a point some distance from you. If that distance is large (it could be all the way back at the substation), or the potential gradient in the earth is large (because you live near a power station perhaps), then there could still be a large enough potential difference between the neutral wire and your feet to be lethal.

As I said a Properly Grounded Neutral. We are discussing a residential use so it would be grounded in the breaker panel.

As a rule I do not work on or touch energized electrical devices except for documented adjustment of electronics. Note: I am an electrician by trade and have been for >20 years.

I also said:

Note: All neutral wires should be tied to the ground bus inside of the service panel. The problem is you need to ensure that the neutral is indeed neutral before you touch it. Never assume anything with electricity. Use a known good meter to test before touching anything.

I guess I should have been more direct and said never work on energized circuits.

 

[PowerEngineer]

The "hole in your theory" is your assumption that all the voltage drop occurs across the load, and therefore there is no voltage drop across the hot and neutral wires. Rather than "all", it is the vast majority. It might help to consider a traditional toaster using high resistance wires to produce heat. The complete path from your circuit breaker box has the resistance of the hot wire, the resistance of the toaster, and the resistance of the neutral wire all in series. The voltage drops uniformly across the combined resistance. Most of the voltage drop occurs across the high resistance of the toaster, but there are still some small voltage drops across the hot and neutral wires. The sizing (i.e. cross section) of the hot and neutral wires are established to make their resistances, and therefore keep their voltage drops, small. (Yes, I know I've ignored the complexities introduced by AC. But it boils down to roughly the same picture.)

 

[Rubycon]

The important thing to remember is while the neutral and ground buses are at the same potential (in a properly operating system) the ground wire is NOT designed to carry the load current!

And yes, if you're in a live panel and disconnect the neutral line coming from the transformer you will see sparks and get shocked since you're essentially breaking the load on all 120VAC devices connected.

 

[Sheep221]

The important thing to remember is while the neutral and ground buses are at the same potential (in a properly operating system) the ground wire is NOT designed to carry the load current!

That only applies to newer TN-S system.
Earlier networks(sometimes called power grids) were built using TN-C system, where neutral and ground were same wire which was only split in wall socket to ground and neutral terminals and again split on the other side at the base of generator or transformer which means actual grounding infrastructure does carry load current.

 

[freeskier93]

As I said a Properly Grounded Neutral. We are discussing a residential use so it would be grounded in the breaker panel.

As a rule I do not work on or touch energized electrical devices except for documented adjustment of electronics. Note: I am an electrician by trade and have been for >20 years.

I also said:

Note: All neutral wires should be tied to the ground bus inside of the service panel. The problem is you need to ensure that the neutral is indeed neutral before you touch it. Never assume anything with electricity. Use a known good meter to test before touching anything.

I guess I should have been more direct and said never work on energized circuits.

Just a point of clarification... Neutral and ground can only be bonded at the main panel, never should they be bonded at a sub panel. It also isn't (or at least didn't use to be) a code requirement to bond ground and neutral at the main panel.

 

[Paperdoc]

As many have said, yes, the Neutral wire should be at zero volts (with respect to true earth ground) AT THE BREAKER PANEL where it is bonded to the same earth ground as are the bare copper Ground leads. BUT there are at least two things that can cause this to be not exactly true at the OUTLET on your wall, and that might be enough to cause serious injury or death in a few cases, so NEVER rely on the Neutral to be truly zero volts.

The two "things" I'm thinking of:
1. There may be a poor junction in wires anywhere between the wall outlet and the breaker panel, causing higher-than normal resistance in the Neutral line. This means the Voltage at the outlet is above zero. This kind of flaw will also result in heat generation at the faulty junction, but that heat may be so small it goes undetected, whereas the voltage at the outlet of such a Neutral line might be high enough to cause injury.
2. Even without any such flawed junctions, the Neutral wire has a non-zero resistance, and thus when it is carrying a current it will always be slightly above zero volts at the outlet. How much above depends on many factors, so again be cautious and never assume it is near enough to zero to be harmless. Even if there is no load plugged in at this outlet box to feed a return current back on the Neutral line, VERY likely there are other outlets on the same circuit so the Neutral line IS at a slightly elevated voltage from uses at other outlets.

Both these reasons are why the true Ground bonding leads (the bare copper wires in the cable and exposed inside the outlet box) MUST NOT ever be used to carry a current for the circuit. These wires must be Grounded at the breaker panel, and MUST carry no current so that they truly are at zero volts (with respect to Ground) at all points. They are connected to the exterior of the device in use and their primary function is to ensure that, in abnormal situations where a voltage source contacts the exposed chassis of the device, any current flowing as a result will flow unimpeded to the Ground at the panel. This has two effects for user safety: first, it ensures that even in this abnormal circumstance the voltage on the grounded exterior of the device is so low that it is not likely to result in injury; and secondly, the usual result of such a fault is the the current that flows is so large that it trips off the breaker (or fuse) supplying power via the Hot lead, thus completely stopping all current flow and eliminating the dangerous abnormal condition.

A lesser function of the Ground lead with its reliable zero voltage state and good conductivity to true earth Ground is that it can be used in electronic devices as a reliable Ground connection to the chassis and certain circuit Ground points like shield connections. This can route very small induced noise signals on lines to Ground so that they are effectively "removed" from the device and do not interfere with its quiet proper operation. Again, for this to work, there must be NO other current already flowing in the Ground line.

 

[pcgeek11]

I think we are all saying the same thing in different ways.

THE most important being: Make sure the wire is not live before touching it always.

 

[Red Squirrel]

While the neutral is technically safe to touch, keep in mind that if you disconnect it, you are still breaking the circuit. If you were to disconnect it and measure with a volt meter the two ends there would be a voltage reading. If you touch those, zing!

But yeah the way a typical residential system works is that you have 7,200 volts at the pole (typically) going into a transformer, the secondary winding of the transformer has a centre tapped winding, where both ends (hots) will give 240v and centre to any of the ends will give 120. That centre is your neutral. It is also connected to ground. In your panel each breaker alternates between the two hots, which is why a douple pole is used for 240v appliances. In the case of 240v circuits neither wire is safe to touch as it will have 120v potential to ground. Or if you touch both at same time then you get hit with 240.

Oh and if you want some fun read up on floating neutral. That is when the main neutral is broken. It essentially puts both 120v "legs" in series with each other, and really weird things can happen depending on the loads.