Single Phase or Three Phase 2

[McEleney]

Could someone please explain to me why when a device is tied into a three phase panel, and is fed with a two pole breaker, why is that device refered to as a single phase 208? From my understanding, it is drawing from two of the three phases 120 degrees from one another. Thanks for any assistance.

 

[alehman]

If you have just two wires, the voltage between the two wires is a single value, regardless if the source is one-phase or three-phase. There is no way to determine, by measuring the single voltage, that it originated from 3-phase source. It appears and in fact is one and the same as a voltage from a single-phase source.

If you add a neutral to the circuit, then you could argue that it should be called 2-phase. But 2-phase is usually intended for sources that are 90 degrees out of phase rather than 120 - and is fairly unusual in power systems.

 

[stevenal]

faq238-777

 

[alehman]

Should'a looked there first...

 

[JBD]

A phase is not a single conductor or reference point. Phases require two points either usually Line and Line. A 1-phase system has 1 set of L-L conductors, a 3-phase system has 3 possible sets of L-L conductors, and the very rare 2-phase system has 2 sets of L-L.

You probably don't refer to a household range as a 2-phase device even if it is connected to (2) hot conductors. So when describing a single phase transformer, instead of saying "it is drawing from two of the three phases" say it is drawing from 2 of the 3 line conductors and see if it is easier to explain.