see how the top AC waveform alternates.
now imagine the 'white' or 'nuetral' of your 3 wire as the line in the middle, it will see the 'return' current of both 'phases', but they will cancel each other out.
i.e- if your 3 wire were to be served via 2 breakers on the red & black, and said breakers were of phase A & B , each pulled 10 amps, no current on the nuetral.
If A pulled 10 amps, and B pulled 5 amps, there would be only 5 amps 'return' on the nuetral.
should the breakers have been both A phase, the nuetral would 'return 20 amps, and 15 amps in the above examples.
i hope this helps, others may imbelish....
[This message has been edited by Electrical Inspector (edited October 16, 2002).]
The three wire version of this circuit was invented by the edison electric company and was used with Direct Current. This is why the three wire version of the circuit is called an "Edison Circuit" to this day. The original edison circuits were supplied by series wound generators and balancer sets. These three wire "Edison Circuits" were later converted to AC by center tapping single phase step down transformers. The four wire version was developed by Tesla and is only used with three phase Alternating Current.
DC Edison circuits are still used in modern practice to supply emergency lighting panels in some buildings. Under normal power the three wire feeders to the emergency lighting panels are supplied from a single phase AC supply. When the AC supply is lost and the transfer equipment drops out onto the DC supply a 220 to 240 volt battery array that is center tapped to limit the voltage to ground to half the end to end voltage carries the load.
Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.