You are right. what we have in north america for residential homes is not the three phase system I thought.
see http://answers.yahoo.com/question/index?qid=20090809202958AAKJkjf
The fundamental difference is Three Phase vs single Phase distribution system...
120/240 is a single phase system...... typically used in
residential homes in north America....
The single phase transformer (center-tapped) produces 240VAC.
However it provides two 120VAC feeds :
one 120VAC @ 0 deg., another 120VAC with 180deg phase difference and one neutral (Center-tap)
The 240VAC is used only for major appliances ( Stone, Fridge)
The 240VAC (Line-to-line) is obtained using the two 120VAC feeds instead of the one 120VAC line and neutral...
The 120/208YVAC system describes the output of a
3 phase transformer, used only in commercial applications.
The 3-phase output consists of three 12OVAC lines with voltages signals being 120 degree apart from each other.
i.e. these are 120VAC between Line-to-Neutral.
This type of connection is referred as a WYE configuration.
208VAC voltage sources are obtained by connecting
the load accross to lines feeds.
208VAC = 120VAC Sqrt(3)/2
These voltage are referred as Line-to-line Voltages.
The type of connection is referred as a DELTA configuration
Source(s):
http://en.wikipedia.org/wiki/Center_tap
http://en.wikipedia.org/wiki/Transformer
