Alright so our engineer seems to think we can use 2 phases off our 20amp main (control panel) and distribute it to a 25amp double pole breaker for a 208 to 120 step down transformer. On the line side. Wouldn't the main trip before the 25 double pole. I know it's only two phases but it's the same circuit and that breaker will only allow 20 amps. He swears up and down it's fine so it made me question my knowledge of how AC works. I know it's 3 phases separated at 120 degrees but rather if it's just two or three phases it's the same current right?

 

No, using 2 phases only without any other reference (neutral etc) is single phase.
If the source is only 20amps, that is the capacity of the 208 conductors.
Max.

 

That's what I thought thank you

 

So I need a bigger main or smaller breaker for the transformer depending the the full load amps.

 

Yes.
Max.

 

Yes.
Max.

Engineer decided just to make it the same amps even though there's other things putting a load on the main so it'll still trip before the other. Whatever I'm hourly I still get paid the same lol

 

Does the main panel have a neutral (wye) or is it a delta connection. You'll need the neutral for 120 volts, then you won't need the transformer.

 

Does the main panel have a neutral (wye) or is it a delta connection. You'll need the neutral for 120 volts, then you won't need the transformer.

No neutral it's 3 phases and a ground I mean I'm sure the transformer from our building has a neutral center tapped but we're not pulling one in. I need the 208 three phase for servos and vfd's for motor control and need 120vac for 24vdc power supple and cpu of the plc so have to have both

 

The ground must be referenced to some point on the secondary, probably the star(neutral) point of the secondary.
If you do end up with a 120v transformer secondary, you are allowed to earth ground one conductor at the transformer out only and thereby establish a Grounded neutral.
Max.

 

I'm well aware we're going off topic now. I always ground my neutral conductor on transformers and distribution panels. Learn my lesson on floating neutrals the hard way when I was young lol