You remove 120 volts from a three phase wye transformer and you have two phases of 120 volt left. Take 240 volts from a three phase transformer and you have 120 volts left.
WHADDYA MEAN?
A three phase wye connected transformer has a FOURTH TAP comprende? It is the neutral tap. Any 1-2 1-3 or 2-3 phase to phase connection is a 240 volt connection.
1-2-3 to the transformer neutral tap is a 120 volt connection (still with me)
Many USA transformers ARE NOT WYE WOUND. THEY ARE DELTA WOUND. They are not shaped like a dot with three equally spaced radiating lines.
Their diagram is like a fat christmas tree triangle with a stump sticking out the middle of the horizontal flat line that is the bottom of the tree. The result is 240 three phases plus 277 volts and 208 volts from two of the phases to the stump. From the two opposite ends of that horizontal bottom line to the stump IS TAH DAH! 120 volts. Across the total length of the bottom line is TAH DAH 240 volts.
Why don't they put a stump in the middle of all the flats of the christmas tree and have an orgasm of stump neutrals?
Every neutral has to see an earth ground bond and whoppee wouldn't that be fun?
Residential service drop transformers are simple H1 / H2 & X1 / X2 transformers.
Wye commercial transformers cost about twice as much as Delta commericial christmas tree with a stub transformers.
And WhooWhee when you suck one phase voltage way down, it tends to elevate the one or two remaining phases. So please read a little. It may "Enlighten" you.
Transformer engineering gets esoteric with high Kw units. Ground loops and eddy currents. Subractive and Additive winding concepts. So if you encounter transformer rules and regulations, it may be confusing.
That is exactly why there are strict electrical code.
