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This is basically just to satisfy my curiosity. Please correct me if I'm wrong about any of this:

I believe power supply to a home's main panel usually consists of:

  1. A neutral wire
  2. A hot wire 120 volts AC above the neutral
  3. A hot wire 120 volts AC below the neutral

It shouldn't matter which hot is used to supply 120 volt devices, but is the same one used for all 120 volt devices - so that the other one is only used to provide 240 volts?

Or do both get used, and one goes to each side of the panel?

EDIT: Adding diagram - 3-Wire Single Phase

http://en.wikipedia.org/wiki/Split-phase_electric_power

3-Wire Single Phase

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I just updated This answer with some information you might find helpful. –  Tester101 Mar 22 '12 at 17:38
    
Looks like comment was removed. Correction, everything is not the same phase. Even if it's called single phases, when you measure the actual 2 - 120V phases with an oscilloscope (like someone will do that to their home) the are at completely different degrees. See here for more info. Picture is worth a thousand words? –  lqlarry Mar 22 '12 at 19:00
    
We cleaned up a lot of the extraneous comments and wrong information. Phase was not really important to the discussion. –  dbracey Mar 22 '12 at 21:25
1  
@lqlarry It's not a true two phase system, It's a center tapped single phase. It just looks funny when measuring line to neutral. –  Tester101 Mar 22 '12 at 22:33
    
Yea, I didnt' word that well, but I used to remember all the harmonics, taps and other things but that got killed off in the 70's and 80's thanks to Adolf Coors. –  lqlarry Mar 22 '12 at 23:30

3 Answers 3

up vote 21 down vote accepted

If you look at a service panel without any breakers in it, it will look something like this.

enter image description here

Notice the thick metal plates running vertically, those are your hot bus bars. One is L1 (A), and one is L2 (B). They are each at 120V potential to ground/neutral, and 240V potential to each other (So don't frigin' touch them!).

When you add breakers, they will make contact with the hot bus bars using the tabs that protrude between the bars.

enter image description here

You'll notice on the cover of the panel that each slot is numbered, with odd on one side and even on the other. So for a 20 breaker panel you'll have.

1    A    2
3    B    4
5    A    6
7    B    8
9    A    10
11   B    12
13   A    14
15   B    16
17   A    18
19   B    20

So 1 & 2 will be on leg A, 3 & 4 on leg B, and so on down the panel.

What types of things are fed by each breaker will determine which leg is used more. For example. If breaker 1 feeds the lights in your living room, and breaker 3 feeds your bedroom lights. If you turn on the living room lights and turn off the bedroom lights, leg A will be used more than leg B.

For 240V circuits, you'll use a double pole breaker that spans two slots on the same side (eg. 2 & 4). This means it will have a hot from each leg, and will use each leg equally.

So the short answer is... It depends.

More Information

While we're on the topic, lets tackle some misnomers about phase in this type of system. While this is sometimes incorrectly called Two-Phase, it is in fact a 3-wire, single-phase, mid-point neutral system (Sometimes called Split-Phase).

As you may or may not know, we are dealing with Alternating Current here. So viewed with an Oscilloscope, we'll see a Sine Wave like this.

Sine Wave

However, since we can measure this system at multiple locations, the waves will look slightly different depending on where we measure. This is where the misconception that this is a two-phase system comes from. Lets slow things down, and look at what is happening in the system as the current alternates.

enter image description here

At this point current is flowing in a positive direction on L1, and a negative direction on L2. Because of this if we measure between different points, this is what we'll see.

L1 -> N = +120V
L2 -> N = -120V
L1 -> L2 = +240V

Now as the current swings back the other way, we'll start to see the sine wave appear.

enter image description here

If we take the same readings again, we'll find things have changed.

L1 -> N = -120V
L2 -> N = +120V
L1 -> L2 = -240V

As you can see, it really is a single phase. Things just get a little confused when measuring from Line to Neutral. A true Two-Phase system, would use 4 wires and the phases would be shifted 90°.

In this type of system the two lines are not referred to as "phases", instead they are called "Legs".

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4  
Pictures are always worth 1k words. +1 –  BMitch Mar 22 '12 at 22:18
    
Very helpful, except that the depictions of "sinus" waves aren't. –  cdonner Jul 8 at 21:36

As others have noted adjacent breakers are connected opposite phases. This is intended to roughly balance the load. In normal household use loads will shift and different phases will the most heavily loaded at different times. It also enables two breaker setups using both phases.

  • 240v circuits (as already noted). This usually uses two wires + ground, but may uses a third wire for a neutral lead. Typical uses are stoves, dryers, heaters, and water heaters. Loads should always be balanced unless a neutral lead is required
  • Paired 120v circuits where tripping either breaker will disconnect both phases. This uses three wires + ground and includes a neutral connection. This is commonly used for kitchen wiring so that plugging two devices into the two sockets of an outlet will not overload the circuit. Before the plug is installed a tab joining the hot side of the two plugs is removed and the two hot leads are connected to the two hot screws. The neutral lead will carry the difference in load between the two circuits. (If the load are perfectly balanced, the neutral lead will carry no load.) Loads are usually unbalanced, but running a kettle and a toaster on the same outlet is safe.

Three phase panels work like two phase panel but alternate A, B, and C phases. Adjacent circuits are phase shifted so provide a nominal 208 volts rather than 240 volts. Again the intent is roughly balance the load across phases.

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"Paired 120V circuits" = Multiwire Branch Circuit. –  Tester101 Mar 23 '12 at 17:57
    
Multiwire Branch Circuit = almost a subpanel! –  Jay Bazuzi Mar 29 '12 at 2:50

Each leg of the hot connect to every other breaker down each side of the panel in most current designs (older panels, maybe >30 years, may have a bar down each side). Therefore, two adjacent 120V breakers will be on opposite hots. This means that a 240V breaker takes up two positions on the same side, instead of having to span from one side of the breaker panel to the other. The two legs will be roughly balanced, with the neutral handling any imbalance.

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