Conduits, circuits, and outlets.

[David G Baker]

Terry,
In 220 circuits the two 110 legs complete the circuit. The only reason you would need a neutral leg is if there was something on your machine that required 110 volts, for example if your machine has a lamp or another device that needs a 110 volt supply. Someone else can explain the technical side of 220 better than I can.

[John Coloccia]

Not trying to hijack this very useful thread but I have a simpler question that's always nagged me: none of my 3 or 5 HP 1-Phase machines gets a neutral wire. They all double up two 110 conductors at the outlet to make up the 220. Why no neutral to return current? Is it the motor design? (Never mind the ground.)

first: I'm not an electrician...but I did just rewire my whole shop. I'm just telling you what I ended up doing, and it seems to work well (and passes inspection)

The two "hot" legs are the returns, so to speak. There are 2 phases going into your house, 180 out of phase with eash other. To make it as simple as possible, when one is "pulling" the other is "pushing" and vice versa. That's how you get 220V when each phase is only 110V to earth....they're always 220 to each other.

re: everything else

There are simple calculations you can do to determine box fill based on the size of the conductor...it has to do with how many wires and connectors you can reasonably fit in the box and still be able to tuck it all back in. Be careful...I had to install a two gang outlet box along a run JUST for box fill.

Conduit fill has to do with how much wire you can reasonably pull through the conduit. The size of the conduit has nothing to do with the derating....in conduit, all the wires will be laying on top of each other at the bottom of the conduit regardless how big it is, so the conduit size is irrelevant for heat generation.

I just rewired my shop with conduit, and would strongly suggest wiring a 110 outlet everywhere you run a 220. You'll use it for lights and other things. You may also want to consider running everything as 10AWG. If you run 10AWG, you'll be good for 30 amps on some runs, and 20 amps on runs where you really pack the conduit (depends on the derating). I can take practically any 110V outlet in my shop and convert it to a 220V-20A (3HP) by changing the plug and the breaker. Some of them I can convert to 220V-30A (5HP). Makes for a flexible layout.

I also ran stranded wire. Solid wire is a royal PITA to pull, especially the longer runs.

Anyhow, good luck. You'll be expert at pigtailing and pulling by the time you're done!

[Terry Brogan]

I'm dense, and I'm still not getting it. I do understand the two sine waves being out of phase with each other by 180, so, in your terminology, one is pushing one pulling, so to speak, but in 110 circuits you could get half of that with one black conductor. A push or a pull or whatever. My question is about the white. A 220 delivers twice the current, but nothing goes back, whereas, if I understand Electricity and The Universe right, on the 110 side a wire is needed to go back. Why is that.

[John Coloccia]

I'm dense, and I'm still not getting it. I do understand the two sine waves being out of phase with each other by 180, so, in your terminology, one is pushing one pulling, so to speak, but in 110 circuits you could get half of that with one black conductor. A push or a pull or whatever. My question is about the white. A 220 delivers twice the current, but nothing goes back, whereas, if I understand Electricity and The Universe right, on the 110 side a wire is needed to go back. Why is that.

There is current going back on the 220V....back and forth, actually, at 60Hz.

110V with a neutral is like having a water pump on one end of a hose, and a pool of water on the other end (the neutral is the pool). The pump alternately sucks water out of the pool and pumps water back into the pool 60 times a second (hence the name "Alternating Current").

A 220V system is exactly the same but there's a pump on BOTH ends. You can see that having a pump on both ends gives you twice the power for the same amount of water.

This is why a 220V system will typically draw less current than the equivalent 110V system. Ultimately, they use approximately the same amount of power, but the analogy is that the 110V uses more water at lower pressure, and the 220V system uses less water at higher pressure. The practical benefit of this is that you can run a 5HP motor with 220V@30AMPs and use reasonably sized wire, as opposed to 60amps@110V with ridiculous wire. There's lots of other problems with the 110V system to0, but it will all boil down to wire size, wire length, effeciency, packaging, etc.

In an AC system, there's no net movement of charge through the wire (not exactly true, but any movement is incidental and can be ignored). The electrons don't actually have to "flow" anywhere to be useful. They jiggle back and forth, and you can get work out of that, just like you can get work out of moving a hand saw back and forth, even though the net effect of all that motion is that the saw doesn't actually go anywhere....it just moves back and forth, more or less around the same point.

edit: What most folks don't realize is that there are 2 pumps coming into your house, and it's setup so that they're always doing the opposite thing (sucking or pumping). For 110V you randomly choose one side or the other, and hookup a neutral. Panels are designed so that when you plug in a ganged 220V breaker (it doesn't matter where you plug it in), it always selects 2 phases that are exactly opposite. That's why the 220V breaker is exactly twice the size of the 110 breaker. It's literally doing exactly what 2 seperate breakers would do. In fact, you could use 2 seperate breakers and it would still work. It wouldn't pass an inspection anywhere on the planet, and it's not at all safe (NOT SAFE!) but in terms of getting power from point A to point B, it works just dandy.

[David Parker]

I'm dense, and I'm still not getting it. I do understand the two sine waves being out of phase with each other by 180, so, in your terminology, one is pushing one pulling, so to speak, but in 110 circuits you could get half of that with one black conductor. A push or a pull or whatever. My question is about the white. A 220 delivers twice the current, but nothing goes back, whereas, if I understand Electricity and The Universe right, on the 110 side a wire is needed to go back. Why is that.

Hi Terry,

Actually, John beat me to this answer and did a better job at it anyway. I'll post mine anyway so you can read the same thing in different words.

In 120 volt AC circuits, the current is first pushed from the black wire, through the load, and into the white ("neutral") wire. 1/120 sec. later, the current is pulled from the white wire, through the load, and into the black wire. The force of this push or pull averages out to 120 volts (also known as the effective voltage or "rms" voltage. Actually the peak voltage is about 170 volts).

In 240 volt AC circuits, the current is first pushed from black wire #1, through the load, and into black wire #2. Black wire #1 is pushing with 120 volts of effective force and at the same time black wire #2 is pulling with 120 volts of force. This results in a combined 240 volts of effective force moving the current through the load. 1/120 sec later things are reversed, black wire #1 is pulling the current and at the same time, black wire #2 is pushing the current.

[Terry Brogan]

So, now, two more questions.

1. W=AV. Since the utility company charges us for Watts, and since doubling the voltage allows half the amperage (whicfh makes thoe 220 circuits so efficient), and since, in your helpful examples, the electrons are merely jumping back and forth, as opposed to being pushed through in one direction only like a water hose, can I then assume that the net minus of all that jumping is equivalent to the wattage consumed?

2. What's unsafe about taking the 220 off two separate breakers, i.e. two separate circuits? Let's assume they're both on the same side of the main bus.

[Tom Veatch]

...
2. What's unsafe about taking the 220 off two separate breakers, i.e. two separate circuits? Let's assume they're both on the same side of the main bus.

First, if both breakers are on the same side of the bus (same leg of the incoming service) you won't get 240v at the outlet, you'll get 0 volts. At a given instant of time, one leg of the service is 120v above ground potential while the other leg is 120v below ground potential. That gives you a potential difference (voltage) of 240v between the legs. The potential difference between 120v above ground and 120v above ground, which is what you'd have if your breakers were both on the same leg, is zero (0) volts. The two breakers have to be on opposite legs.

CLARIFICATION: You'd have zero voltage between the "hot" wires of the circuit if both breakers were on the same leg. That does NOT mean there's no voltage on the wires. You would still have 120v between each hot wire and ground or neutral. So you could still get zapped by the hot wires, but you couldn't run a 240v load off the circuit.

Why is it dangerous to use two separate single pole breakers? Because if one leg developed a short to ground, the breaker on that leg would trip and break the circuit. However, with two separate breakers, the breaker on the other leg would not be tripped and that particular wire at the load would still be hot.

A 240v, 2 pole breaker is actually two separate breakers in the same body. The two breakers are interconnected so that if one breaker sees an overcurrent for whatever reason and trips, it forces the other breaker to trip also. That cuts power in both legs so that both legs are "cold" at the outlet/load/etc.

[David Parker]

So, now, two more questions.

1. W=AV. Since the utility company charges us for Watts, and since doubling the voltage allows half the amperage (whicfh makes thoe 220 circuits so efficient), and since, in your helpful examples, the electrons are merely jumping back and forth, as opposed to being pushed through in one direction only like a water hose, can I then assume that the net minus of all that jumping is equivalent to the wattage consumed?

That is correct. As long as there is a load (motor, light, etc.) connecting the black ('hot') wire and white ('neutral') wire, or the two black wires in the case of 240 volts, there is current flowing back and forth in the wires. It takes power measured in Watts to produce that current movement. That is what your electric company charges you for.