Phase refers to difference between two or more alternating current waveforms. There are 360 degrees in a complete cycle (as a generator completes one rotation). 3-phase systems produce three sets of sine waves that are 120 degrees apart or "out of phase" from each other (3 x 120 = 360). That is phase a-b, b-c, and a-c are 120 degrees apart from each other.

However, when you use only two of the legs of a 3-phase system, it is single phase since there is no other phase that it is "out of phase" with. The term two-phase is a misnomer, although many electricians use it. There is only ONE sine wave, hence it is single-phase.

>Also, where IS the neutral (Ok, return) line of 3 phase?

Well, that depends on the type of 3-phase system. A Delta configuration (230 or 460 Volts line-line) doesn't have a neutral. It has a SAFETY GROUND, but that's different. One line returns to the next line. When DELTA comes into a building, it then goes to a transformer where the winding across two of the lines is center-tapped, producing 115 Volts from each line to that center-tap. The center-tap becomes the neutral (return) for 115 V lighting circuits. Also, this neutral is connected to a good earth-ground near the transformer.

            B
            *
          *   *
       A * *:* * C 
            :
            :
            :
         Neutral (Center-tap. Connected to earth-ground. Return for 115 V)

[ ed. note - As you can see from the above, A,B, and C are equidistant, there for at the same potential to each other. A and C are equidistant to Neutral, at 115V each. B, however, is farther away from neutral ( whether on the path through a, or through C ), therefor a 'high leg'. ]

A WYE configuration is another matter. There is 208 Volts from line-to-line. There is 120 Volts from line-to-neutral (208 V / 1.73 = 120 V). This is often called 120/208 V, 3-phase, 5-wire. The obvious advantage is that you get three-120 V lines for lighting circuits as opposed to the two that you get in the DELTA system above.

         
        B *     * C
           *   *
            * *
             *------- Neutral (Connected to earth-ground. Return for 120 V)
             *
             *
             A

As far as I know, these are the two standards used in the United States today. It is important to know exactly what you have. Whenever my company sells a refrigeration system, we ask if the customer requires 208 or 230. We usually get a reply like, "Oh, 220V, whatever". Well, it makes a big difference since most compressors (over 1/2 HP) are designed to work at 230 V nominal, no lower than 208 V. If the customer has a 208 V line that is on the low side, it can be as low as 187 V. That can cause starting problems, causing the compressor to draw lock-rotor and never start! Also, some systems require the neutral of a 208 V WYE to develop 120 V for the control circuits. If the customer only has 230 V DELTA available, the machine must have an internal control transformer to develop 115 V (proper designed 208 V units should have an internal transformer too, but not everything is properly designed).

There are only two true nominal values in the U.S. -- 115V (from a delta) or 120V (from a wye). Anything else (110V, 220V) is a misnomer, based on archaic nominal values from the 1920's and '30's.

BTW, The advantage of 208V for industrial sites is that you get three-120V lines for lighting/appliance circuits, whereas with 230V you only get two-115V lines.

The relationship of 208/120V is that the neutral is not a center tap. It is the node where three phases come together. It is 208V divided by the tangent of 60 degrees or 208V/1.73=120V.

Hope I haven't rambled too long. Feel free to post this elsewhere if you think it would be helpful to others.

Peter Kesselman. Opinions are definitely my own.