You do need to be careful about what you post around here, don't you?
Yes Don, across two legs there is no difference as you point out with the stipulation of the same voltage. That is where the wheels come off in this particular instance when comparing the time elements of an AC system; the resulting voltage can be actually different as it is in common trade practice. In the case of using 2 of 3 phases, that is precisely what happens.
You can take a three phase system, use a single leg to supply single phase loads with absolutely no time shift on that leg and have a higher effective voltage if a single phase 120/240 transformer is installed. That is typical in residential distribution where this thread appeared to originate. Single phase loads across two legs of a 3 phase Y (forgive me for not being proper with the word wye; that takes way too many letters to type) do have the same sine wave but not the same performance because the effective voltage is different- 208 vs. 240 typically. That lower effective voltage is directly tied to the phase angles being asymmetrical- limping along in my previous analogy with two peaks on and one missing every cycle.
My point is- the typical single phase with a neutral is not two phase nor is it two phase when derived from a single leg because the time element-phasing- is not there. In this instance only the magnitude of the voltage is different, not the phasing.
Frankly, I am not clear how you can even get a true (symmetrical) two phase from a 3 phase system, I never really gave two phase much thought before this came up here. Considering this timing thing perhaps two of three phases IS two phase but, I humbly present myself to be educated at all times by everyone here and the questions that my posts might provoke.
I too, do not know all I want to know.
(See how this works Smart$?)
