How can you have the same 208 voltage at 2 pole single phase
And 208 voltage 3 phase, when there is an additional (3rd) wire.
Please help.:confused:

In this area 120 208 voltage is all multiphase. Usually find it in large apartment houses or downtown area. Where there are 3 transformers. This allows a lot of apts to be run off the transformers and load to be balanced. The voltage comes from how electric is generated and how the phase would be drawn off the transformers.
For example:
The apartments are 120 208 using 3 wires 2 hot and one neutral.. you could also pull 120 208 4 wire off this bank with 3 hot and 1 neutral wire. The apt ould uses 2 of the 3 transformers and the for 3 phase all of the transformers are used.

how can you have the same 208 voltage at 2 pole single phase
and 208 voltage 3 phase, when there is an additional (3rd) wire.
please help.:confused:

Simplest answer is that the conductors are out of phase not all 3 conductors are hitting at the same time at the load.

The simplest answer is really that the voltage is measured across only two conductors at a time.

Single phase is two wires, A & B. Measured: A-B
Three phase is three conductors, A, B & C. Measured: A-B, A-C, B-C

Simplest answer is that the conductors are out of phase not all 3 conductors are hitting at the same time at the load.

Hurray for that answer.
As I go through this site many answers are loaded with jargon. Your simple answer helped me to understand those long explanations.
Thanks...

Glad I could help. I work on a lot of 3-phase compressors and when I first started I didn't fully understand it either.