I get the wiring now, but can anyone explain why a manufacturer wants two different systems in a fairly simple appliance. Why can’t they, as we do in Europe, run all the controls etc at the same voltage?
Because 120v clock motors are made in enormous quantities, and 120v switches are cheaper and last longer than 220v ones.
I wonder if it has anything to do with the Edison and Westinghouse fight.
Edison wanted to use direct current for power distribution, and Westinghouse believed that alternating current was more safe.
Edison pointed out that alternating current was used in the electric chair. “Carnivorousplant was Westinghoused to death last night at Folsom Prison”.
We run two 120v circuits on the same neutral. They are carrying different amounts of current with various devices on each side which are unequal, so there is current in the neutral. I get it, thanks.
I have a BS in electronics, an MS on instrumentation and electronics, and I never had a course on AC power, current or radio.
That’s known as a multiwire branch circuit. As long as the two hot wires are out of phase from each other, it’s OK, because the neutral will carry somewhere between zero and the full load for one hot leg. If you mistakenly connect the two hot wires to the same leg, you can end up with 2x the allowed current in the neutral, which is a fire hazard.
Then why worry about the timer using one leg of the 220v?
If we did move to 220v, a guy selling step down transformers could clean up. 
engineer_comp_geek thanks for the explanation, I get it now.
One last question if you don’t mind. In the 240V, hot, hot two wire configuration, how does that work without a neutral or a ground? I don’t know a lot about AC, but your last explanation was at a great level.
Thanks again!
Yes, but at the cost of a higher voltage everywhere. I’ve had electricians tell me that 220/240 volts is the most dangerous for an electrician: if you grab a live 120V circuit, you can let go, a live 480V or higher circuit will throw you off, but 220/240V won’t throw you off but paralyzes you enough to prevent you from letting go, so you just sit there and fry. (I’m not sure how accurate that is, but it’s a common belief for USA electricians.)
Westinghouse thought AC was more efficient for a distribution system (because it could use transformers to easily change voltage – that’s much harder in DC). I think the talk about safety was mostly a PR battle; I don’t know that there are significant differences between them in safety. Shocks from either one can kill you. (I think the human body uses a DC system internally.)
Actually, Edison did more than point out the (AC) electric chair – his lobbyists encouraged states to use it, and then he tried to use that as an argument to push his DC system. And tried to get ‘Westinghoused’ as a synonym for ‘electrocuted’. Though that might have been partly due to personal animosity between them.
It works just like the UK system, really. One of the 2 hot’s could be called the return, if we used UK terminology. (Either one, since it’s AC (alternating) the current alternates between coming & returning 60 times per second (or 50 in the UK). Then there is a 3rd wire for safety, you call it ‘earth’, we call it ‘ground’.
(Why can’t you British speak English correctly?
[QUOTE=beowulff]
Assuming a correctly sized conductor, the voltage drop between Ground and Neutral should be only a fraction of a volt, thus posing no danger.
[/QUOTE]
“Should” is a great weasel word.
If all the connections between the appliance and the service entrance are good, the neutral wire will be at earth/ground potential. But we live in an imperfect world where wire nuts corrode, screws loosen and people do stupid things like putting switches in the neutral instead of the hot wire.
Sure, but that’s not what boytyperanma implied.
In 99% of cases, touching Neutral is safe.
Okay, to help you folks who are having trouble with it, I think you need to consider the transformer. A transformer is basically 2 coils of insulated wire wrapped around an iron core. The core is laminated iron and is shaped like a block O. The input side is called the “primary” and goes on one side of the O. The output side is the “secondary” and is on the other side of the O.
In home split phase wiring, you get your electricity from 3 wires going from a transformer mounted on a pole to a box outside your home with a meter on it. One of those wires, let’s call it “the black one” is connected to one end of the coil on the secondary side of the transformer. Another wire is called “the red one”. You can tell which it is because the electrician wraps some red electrical tape around the black insulation, about 1.5 inches from where the insulation ends and the copper conductor keeps going. This is connected to the OTHER end of the coil on the primary side. The third wire is connected to the center of the coil (via a ‘center tap’) on the secondary. This wire has white electrical tape wrapped around the insulation and is called “the white one”. These 3 wires are connected to one end of the socket where the power company plugs in the meter. The other socket has much shorter wires, similarly marked, which lead through the exterior wall of the house into the main electrical box. And underneath the meter box is a bare copper wire (or sometimes a braid) leading to a copper stake that’s hammered into the ground. This is your Ground.
Now, if you measure the voltage between the black and red wires on a voltmeter, you’ll get 240Vac. If you measure with an oscilloscope, you’ll see a sine wave. If you measure between the black and white wires, you’ll get 120Vac on a voltmeter and a sine wave on an oscilloscope. Same if you measure between the red and white wires…
But…
Most oscilloscopes nowadays are 2 channel. You can simultaneously look at TWO different signals. you can adjust the height of each one and move each one up and down. So, if you set up channel A to look at Black + White and channel B to look at Red + White, and set it so one is above the other, you’ll see 2 sine waves, but they’re 180° out of phase. When the sine wave on top peaks, the one on the bottom is at its negative peak and vice versa.
So, for 120V you need to go from EITHER BUT NOT BOTH the red or black wire to white. For 240V you need to go from black to red. No white. That’s why in the UK, where they go with 220V, they only wire Black/Red + a ground (green or bare). And in the US, where, for something heavy duty like a washer, drier, or stove, they’ll have 4 wires, black, red, white, and ground. They’ll need the white for 120Vac circuits but go black/red for the 240Vac circuits. Heating elements need more electric and some motors do also.
The relationship between electrical power (which is what you’re using), voltage, and current is described in one of Ohm’s Laws, P=I*E, where P = power and is measured in watts, I = current measured in amps, and E = energy measured in volts. 1000w at 240V draws 4.166 amps. 1000W at 120V draws 8.333 amps. Halve the volts and you double the current. The more current you use, the heavier gauge wire you need. It’s thicker, heavier, and more expensive than smaller gauge wire. THAT is why appliances and tools that draw a lot of power will come in 240v so you can use the smaller gauge wire. Either voltage, they’ll still use the same amount of watts.
Not this one.
It all can kill you.
Notions (as above) always make me cringe. One cannot reliably just let go of 120V, and a higher voltage doesn’t exist that magically throws a person off the line. These are myths.
There has been, generally speaking, some very good replies in this thread, and this seems to be the norm on SD concerning electrical questions… However, questions and answers of this nature are easily misinterpreted due to the complexity of the subject, and I am always very hesitant to participate: A simple misunderstanding can result in property damage from fire, personal injury, or death that I’m no willing to be party of.
IMHO it surprises me that electrical (power) questions are not treated in such fashion as the legal questions are handled here on SD.
Because they invented a language they cannot speak.
Do not speak of this again, it is very embarrassing for them.
Let it go.
And I was concerned that I was off topic.
Been there done that. 480 delta system so it was 480 to ground on the leg I touched. Buckled my knees and broke the connection. Hurt but not a long term zap. 120 is like “Oh yeah, I forgot to turn this off. I must be careful.” Worst I got was 240 to ground. Could not let go until I started to blackout out and lost balance. Slipped off the ladder I was on (lowest rung.) Two holes burned into my fingers. Chest hurt like hell. Didn’t “feel right” for about 24 hours afterwards.
Yeah, I’ve heard this before (many times). It is a common belief. It’s mostly wrong, but there is an element of truth to it.
Electricity tends to kill you in one of two ways.
If you start out at low currents, a very small current won’t really do much to you. In the U.S. most safety standards are built around 5 mA being the maximum “safe” current. Above this, the current has the capability of screwing up your heartbeat. This is kinda hit and miss, as the heart is more sensitive to disruption at certain times during its rhythm than others. Generally speaking though, the higher the current, the more likely you are to get the heart to go into fibrillation, where the heart ends up with this chaotic beat pattern and just kinda sits there and shakes instead of pumping blood. Your heart has a funny design that this fibrillation state is stable, meaning that if you get your heart into fibrillation, it generally won’t come out of it on its own. Your heart just sits there and shakes, you pass out, and a few minutes later, you die from the lack of blood flow through your body. Better hope that someone has a portable defibrillator handy.
Like I said, the more you increase the current, the greater the risk of screwing up your heartbeat. But once the current levels get high enough, instead of going into fibrillation, the heart muscles all just tend to clamp. Your heart isn’t pumping blood at this point, so if someone doesn’t remove the source of the electricity fairly quickly you’re going to be in a whole lot of trouble. But surprisingly, once the electricity is removed, the heart generally tends to start beating normally again.
But then, if you keep increasing the current, you run into the second way that electricity tends to kill you. It simply burns you to death. This is how the electric chair works. It cooks you. Literally.
So it does end up working out that as you increase the current (and generally, more voltage leads to more current), the fatality rate rises, then drops, then rises again.
That said, none of those voltages are safe to touch. The human body isn’t a simple resistor and it’s actually fairly difficult to model the human body electrically. A lot of times we use over-simplified models such as a resistor in series with a parallel resistor and capacitor, but the human body is so non-linear with respect to electricity that the values of those components changes dramatically depending on the voltage levels involved. Measure your resistance with a multimeter and it will measure hundreds of thousands or maybe even a few millions of ohms. At higher voltages, enough current flows through your body that its effective resistance drops to 1000 ohms or less.
Lower voltages, like the common 12 and 24 volts, are safe to touch. The electricity can’t overcome the body’s skin resistance. Above 50 volts is where you get into trouble.
If all you are looking at is electricity flow, you don’t need a ground connection.
Consider a simple transformer. It’s just a couple of coils of wire around a hunk of iron, basically. If we ignore what is supplying the electricity and only look at the load side of the transformer, you’ve got a coil of wire in the transformer and we connect a wire to each end of the coil. Now when we connect those wires to a load, current flows. Simple enough. At this point, there’s no difference between the wires. Let’s call the wires L1 and L2 for line 1 and 2 respectively. If you swap around L1 and L2, the load doesn’t really care much. You reverse the phase of the sine wave, but other than that, everything is the same. so who cares.
The funny thing is, this is actually safe to touch. You can touch either L1 or L2 and touch earth ground, and you won’t get shocked because you aren’t completing the circuit. The transformer isolates the entire load side circuit from ground.
So you’re probably wondering, if this is safer, why don’t we always do it this way? Well, sometimes we do. Hospitals run isolated circuits like this in operating rooms and other places that they call “wet” locations, and a lot of ships have isolated systems as well. When you get into residential power though, it’s just not practical. In large power systems, mother nature will tend to randomly insert grounds into your system by blowing tree limbs onto power lines and such. If your system is isolated, it’s kinda hard to detect these faults. In large systems, it is safer to intentionally ground one of the lines. That becomes your safe “neutral”, and the wire that you didn’t ground is now no longer safe to touch, so it becomes the “hot”. If the neutral shorts to ground, it’s already connected to ground, so no biggie. If the hot shorts to ground, now you’ve got a direct short between hot and neutral and it blows the fuse/breaker. Easy to detect.
Then, for all of the reasons I mentioned before, it becomes safer to run a separate safety ground instead of relying on the neutral for safety, so now you have a hot, a neutral, and a safety ground.
In the U.S., power doesn’t typically come from a single coil transformer like this. In the early days, we settled on voltages around a hundred-ish because we couldn’t make lights that didn’t burn out at higher voltages than that. By the time we figured out how to make better light bulbs, there was too much equipment running at 100 volts or so, so we couldn’t just bump up the voltage everywhere. But there were times when we wanted more power than you could get from a 100 volt line. So then we started using split phase transformers, aka center tapped transformers. Basically, all you do is take a regular single coil transformer and attach a wire to the center of the coil (this is your center tap).
So now you have three wires coming off of your transformer, one wire at each end, and the center tap off of the middle. We ground the center tap and call it our neutral (N), and we have L1, L2, and N now. And we’ll go ahead and add a safety ground connected to the neutral, just as before. That’s how modern houses are typically wired today. We’ve also bumped the voltage up a bit over the years. 110/220 used to be common, now it’s typically 120/240. The 240 lets us run higher power devices that would require very thick wires on 120, so now we can have our newer higher powered stuff, and the 120 circuits give us backwards compatibility with the old stuff so we didn’t need to replace every electrical device in the entire United States when we first put this in.
Europe started later with electricity, so by the time their electrical systems started getting popular we had already figured out how to make higher voltage lights that wouldn’t fry themselves to death, so Europe went straight to 220 and didn’t bother with this lower 120 volt type stuff. The actual history of electrical systems is a lot more complicated than this. I am very much over-simplifying here.
Anyway, if you call N our zero volt reference, then as L1 goes positive during its sine wave, L2 goes negative. They both peak, then come back to zero. Then L1 goes negative and L2 goes positive. L1 and L2, when measured from the neutral, are sine waves that are 180 degrees out of phase with each other. So what you end up with is 120 volts between L1 and N, and you also get 120 volts between L2 and N, and you get 240 volts between L1 and L2. Simple enough.
So if you have a 120 volt load, you just connect it between one of the lines (either L1 or L2) and the neutral (N). All of the 120 volt outlets in your home and your 120 volt lights are wired this way. Roughly half of them will be connected to L1, and roughly half will be connected to L2. If you need a 240 volt circuit, you just connect between L1 and L2. So in your typical breaker box, the breakers alternate. The first breaker is on L1, the second on L2, the third on L1, the fourth on L2, etc. If you need 240, you get a 240 breaker that covers two slots so it gets L1 and L2.
In your 120 volt circuits, the current goes out through the hot (L1 or L2) and back through the neutral. This is AC so technically the current goes back and forth in both directions 60 times a second, but for visualizing circuits it is easiest to think of hot as out and neutral as back. If you think of it that way though, it gets confusing when you look at your 240 volt stuff. For your 240 volt circuits, the current goes out L1 and comes back on L2, and again switches direction 60 times a second. In your 240 volt circuits, the neutral isn’t involved at all. Since L1 and L2 are both lines, there’s no obvious out and back, so it gets harder to visualize. Just remember, in a single coil transformer, there’s no difference between the two lines until you intentionally ground one of them. Then that grounded line becomes the neutral. We often envision L1 and L2 as the hots and the current coming back on N, but in reality, the current is switching direction between the hot and neutral 60 times a second. It’s the same between L1 and L2, it’s just that neither of these is grounded.
This is why many 240 volt dryers can run with two hot wires and no neutral. They just run everything (heating elements, motor, control circuits) off of 240. You still want the safety ground though, but it has nothing to do with the operation of the dryer. It’s just there to keep the metal case safe. All of the real action is between L1 and L2.
ONe thing everyone is over looking on a dryer not just the controls are 120 VAC. The motor is also 120 VAC.
The neutral wire will carry current and the ground should only carry leakage current.
And so we conclude Home wiring 101.