Alternating Current and the Neutral Wire

[derSturm]

Hi, guys. Newbie here. And I have to start out with a question that has very little to do with electronics; it's all about wiring my workshop. I've arrived at a crossroads and I'm a bit stumped...

OK, so Alternating Current (AC) in the United States is wired to a standard 120V application (outlet, lamp socket, etc.) with a hot wire (black), a netural wire (white) and a ground wire (bare). Easy enough. So why is it that a 240V application (like a clothes dryer, air conditioner, etc.) can be wired with 2 hot wires and the bare ground, with NO neutral wire whatever?

I understand the 2 x 120V phases that distinguish 240V from 120V. I'm not interested in sine waves or Ohm's perspectives or the law according to Voltaire and blah blah. I'm just wondering:

If the neutral wire isn't necessary for 240V applications then is it actually necessary for 120? And if so, then why isn't it needed for 240?

What exactly is the purpose of the netural wire? Is is just essentially another ground wire?

Any help will be greatly appreciated.

[budm]

Re: Alternating Current and the Neutral Wire

Home power distribution has two hots (L1, L2, the center tap of the split phase is connected to Earth Ground (safety gorund), the Neutral wire is also bonded to this junction (Earth ground) in the circuit panel box). Your 240V dryer uses L1 and L2 of it to get 240V power to run the 240V load. The Neutral is common to both circuit of L1 and L2, you can get 120V between L1 and Neutral, or L2 and Neutral.

[LDSisHere]

Re: Alternating Current and the Neutral Wire

Out of curiosity, what would the result be on wiring 120V if you were to use the bare earth ground in place of the correct neutral wire?

[budm]

Re: Alternating Current and the Neutral Wire

Since the bare wire for the safety wire is there for diverting the fale current to ground only, if you use that ground wire for the load current return, if the chassis is connected to the ground wire, then there will be voltage drop between that chassis and the ref ground point at the breaker panel due to the wire will have resistance for voltage to be developed on that gorund wire, when you touch the chassis, you will get electrical shock. The chassis should not be part of the load current path.



http://www.ul.com/global/eng/pages/c...omewiring/diy/

What will happen if the power switch is on and the load return wire is attached to the chassis ground which is connected to the ground wire and run back to the panel, if that ground wire has open circuit, imagine what will happen to you when you touch that chassis!

[LDSisHere]

Re: Alternating Current and the Neutral Wire

Thanks for all that information, you have helped fill in a lot of gaps in my knowledge on AC power. I had several classes on AC theory in the way past, but trying to relate that information to what you find in the real world in not always easy. Where I work we have to measure the current on the neutral lines in our power panels monthly. I have always wondered why the panels with a lot of 110V circuits had high neutral readings while the ones with only 208V circuits have little to none. With your explanation I now know why this occurs. As for using the bare wire for a neutral, that is something I would never consider doing, even for an experiment, which is why I asked. And if I ever run across a device that gives off a shock, slight or great, I will now have an additional area to check for a possible cause.

[budm]

Re: Alternating Current and the Neutral Wire

208 3-phase Y system, the center will be where you hook the Neutral wire and the safety earth ground wire to, They use 208 3-phase in office buildings. you will get 120 if you use L1 and center, same for L2 & center, L3 & center.
If you use L1 and L2 (or L2 & L3, or L3 & L1) , you will get 208V because L1, L2, L3 are 120 degree out off phase from each other so you get the Vector sum of the voltage. You want to measure the Neutral so you know that you have balance the loads so the total current of all the Neutral will not exceed the wire current rating, you can overload and burn the Neutral wire due to high harmonics generated by the bad reactive loads like motor, switching power supplies, Electronics ballast for the fluorescent lights, etc.

[severach]

Re: Alternating Current and the Neutral Wire

Simple math explains 120v and 240v.

Each of the two hots vary from +120v to -120v to neutral. The neutral and ground are 0. If we subtract to find the voltage between a hot and neutral we see that the voltage varies between +120-0=+120 and -120-0=-120. This is the voltage expected at a 120v AC outlet.

Note that the voltage does not switch between +120v and -120v akin to a square wave. It's a sine wave where the voltage smoothly rises from 0, touches 120v, then smoothly goes back down across 0 to touch -120v, then smoothly back to 0 to repeat. Twice each cycle there's no voltage on the line. If you could touch the wires 120 times per second at the exact right times you wouldn't get a shock, but who can do that? The voltage also doesn't vary between -120 and +120 but -160 and +160. 160v is the peak voltage and 120v is the RMS value. A 160v peak voltage sine wave provides the same heating value as a 120v DC voltage. It's done this way so a 120v light bulb works at the correct brightness without burning out whether driven by a 120v DC voltage or a 160v peak 120v AC RMS voltage. To someone making purchasing decisions 120v AC RMS is the same quantity as 120v DC. Only the engineers need to know that the voltage is actually 160v.

If we subtract for the voltage across two hots we see that the voltage varies between +120-(-120)=+240 and -120-(+120)=-240. That's the expected voltage but how does it come from two 120v sources? Look closely at the signs. Notice that the two hots are always of opposite sign. When one pole is +120 the other is -120. When one phase switches over to -120, the other switches over to +120. Subtracting numbers of opposite signs leads to addition.

Say we place our subtraction meter on two random breakers. If the probes land on opposite poles we get the subtraction above and our meter shows 240v AC. Let's see what happens if we place the probes on two breakers of the same pole. The voltage varies between -120-(-120)=0 and +120-(+120)=0. The voltage on each probe is going up and down but always at the same voltage and sign. The subtracting meter always sees zero, just what we'd expect from placing the probes on the same buss bar.

To further demonstrate the difference between the poles let's redo the first example on the the other pole and in our imaginary world we're going to measure the other pole at the exact same time as we measured the pole in the first 120v example. We saw the first pole to neutral vary from +120 to -120. By subtraction we see that this other pole varies from -120-0=-120 to +120-0=+120. See the difference?

+120-0=+120 to -120-0=-120 (one side)
-120-0=-120 to +120-0=+120 (other side)

Same voltage but the sign is opposite. When one pole to neutral is providing +120v the other is providing -120v and as the one pole changes to -120v the other pole is changing to +120, always in lock step. When we foolishly placed the probes on the same pole the voltages always subtracted to zero. When we correctly placed the probes on opposite poles the voltages add by subtraction.

The real answer is that the poll transformer is in fact a 240v output transformer. It always produces 240v across the poles. If we only wanted to run 240v loads the transformer would not have the 3rd wire or we would leave it disconnected. Here in the USA most of what we run is 120v so a center tap is added to the transformer. This splits the 240 into two equal but opposite sign 120v transformers. It's a cheap, simple, and reliable technique to run any number of 120v and 240v loads out of one transformer. Because it's AC it doesn't matter that one device in the house sees -120v when another device is seeing +120v. The work is the same and the meter always shows positive 120v AC. Sign only matters when you use the two poles together.

The +120 -120 math comes from our arbitrary choice of zero ground. We could have chosen anywhere along the transformer as ground but grounding either pole would have caused the voltage sequence to be 0V, 120v, 240v. In this setup 120v devices operated on the 120v-240v side would not have a near zero voltage side so 2 wire devices could not be safe. By grounding the center tap which is available to both sides two wire devices become reasonably safe by having a near zero voltage wire for parts that can be touched with little difficulty.