I thought that the wire size depends on the amperage that the current passes. However in a 3 conductor cable, the size of the neutral wire is the same as the hot wire; which puzzles me. If both hot wires carry 15A current, the neutral wire will carry 30A current.
A simplified representation of a multi-wire branch circuit, would look something like this.
If each part of the circuit had a 120 Watt light bulb installed, it would look like this.
If the switch on
L1 was closed, you'd see 1 ampere on the circuit.
Ohm's law I=P/E
L1 = 1A,
N = 1A,
L2 = 0A
If both switches were closed, you'd still see 1 ampere on the circuit. However, in this case the current flows line to line, and there will be no current on
L1 = 1A,
N = 0A,
L2 = 1A
If we replace one of the 120 Watt bulbs with a 240 Watt bulb, you'll see that the unbalanced load flows on
'L1' = 2A,
N = 1A,
L2 = 1A
So as you can see, the neutral (
N) will never carry more than either line (
L). At least not in a properly wired circuit. This is because of the nature of alternating current, and the properties of a split-phase system.
When it is said that the current cancels each other out, it means that the neutral of a multi-wire branch circuit only carries the imbalance of the current between the two circuits of a properly wired MWBC.
So a load of 5A on one leg, and 15A on the other leg, will result in a load of 10A on the neutral.
For a typical MWBC it is extremely rare that both legs will be 100% balanced, so the neutral will almost certainly carry some current. This is why a straight 240V circuit (which is a line-to-line circuit as opposed to a line-to-neutral) requires no neutral wire. It is a 100% balanced line-to-line circuit.
3 conductor cable is usually used for one of two purposes: An additional/dedicated hot (i.e. feeding a switched light and an always-on receptacle), or a multi-wire branch circuit. In the case of a multi-wire branch circuit, the two hot legs come from different legs of your supply (+120V, -120V), so they actually cancel each other out when properly balanced.
You are correct in that you would not want to return 2 15 amp circuits on the same leg via a single neutral.
The two hot legs are NOT out of phase! They are completely in-phase. No current flows through the neutral when the same load is applied to both legs of the hots because all of the current flows through the hots (only) from one end to the other across the entire length of the (power pole) transformer secondary winding. A difference current flows through the neutral only when the loads on the two legs of the hot are not equal --- not because they are out of phase (because they are not out of phase) but because they are out of balance!
If the two hot wires were out of phase then no current at all would flow into a 240-volt motor when it is wired across the hots.
I'm not an electrician, but I am a certified electronic technician and I do know electricity. I've been wiring circuits since before the Beatles were a band.
As others have said the 3 conductor cable could be used for feeding two circuits on split phases. Transformer on the pole (or in the ground) is used as shown in Tester101's diagrams. However, you must be sure that the breakers in the panel are on two different phases. Generally breakers that are adjacent to one another are on different phases but not always. Some panels have alternating phases at the top of the panel and some at the lower end adjacent breakers are the same phase. Also a dual breaker that fits in a single slot would not be on separate phases.
The exception to this rule is harmonics. Triplet harmonics will SUM in the neutral, in a 3 phase supply. Triplets come from single phase rectifiers (eg most electronic power supplies for computers, TV's, microwaves, etc). In years gone by when there were less electronic consumer products, half size neutrals were used in 3P supplies. Today the minimum is same size neutrals, and even double size in some installations, depending on the nature of the expected load.
There are many varieties of three and four conductor cables that have unequal size conductors for this very reason: