You are right about the 1Ã˜ Transformers being different between a 208Y/120 3Ã˜ 4W bank, and a 480Y/277 3Ã˜ 4W bank.
On the 208Y/120 3Ã˜ 4W bank, each Transformer is (usually) the commonly used 6900/13800V Ã— 120/240V 1Ã˜ Split Coil types. The Secondary Windings (of each Transformer) are connected in Parallel, so the output Voltage is 120VAC. All 3 Pots have the Secondary Windings setup in Parallel, so each one will have 120VAC output.
On the 480Y/277 3Ã˜ 4W bank, each Transformer may be:
A: 6900/13800V Ã— 277V 1Ã˜ 2 Wire type Pot, with Split Coil Primary Windings and a Single (Non-Split) Coil Secondary Winding,
B: 6900/13800V Ã— 480/277V 1Ã˜ 2 Wire type Pot, with Split Coils on both sides (Primary and Secondary).
If option "B" is used, the Secondary of each Pot is connected for an output Voltage of 277VAC.
The physical appearance of the two different types of Pots are:
A: On the 120/240V types, there are three (3) small Bushings on the upper "face" of the Enclosure, which are the Output Terminals.
"X0" - the "Center Tap Point" is between Terminals "X1" and "X2". "X0" is only used on the 120/240V 1Ã˜ 3W systems.
B: The 277V (or 480/277V) types have only two Bushings on the "face", which are about 50% larger than the ones on the 120/240V Pots.
Connection procedures for each of the different classes are as follows:
To form the Star point of the Wye, the "X1 Output Terminal" of each Pot (Transformer) is connected together. The Common Grounded Conductor is derrived from this Star point.
The "X2 Output Terminal" from each Pot become the individual Ungrounded Conductors.
On Delta Connected banks, the Secondary Windings on each Pot are connected in Series Adding, as to yield the higher of the two possible Voltages for the Output.
For a 120/240V 3Ã˜ 4W Delta, one Pot uses the "X0" Center Tap Bushing, and that Circuit Conductor becomes - of course - the System's Grounded Conductor.
Let me know if you have further Q's. I'll check the Transformer drawings I have on-line for something which shows details to cover what's described here.
Man, it has taken me nearly 30 Minutes to write this message! Every 2 Minutes, the Phone rings, someone comes to the door, yadda-yadda-yadda!!!