Question about using 2 phases off three phase 208

Discussion in 'Power Electronics' started by Greco, Nov 29, 2017.

  1. Greco

    Thread Starter New Member

    Nov 29, 2017
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    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?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    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.
     
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  3. Greco

    Thread Starter New Member

    Nov 29, 2017
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    That's what I thought thank you
     
  4. Greco

    Thread Starter New Member

    Nov 29, 2017
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    So I need a bigger main or smaller breaker for the transformer depending the the full load amps.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Yes.
    Max.
     
  6. Greco

    Thread Starter New Member

    Nov 29, 2017
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    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
     
  7. gerty

    AAC Fanatic!

    Aug 30, 2007
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    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.
     
  8. Greco

    Thread Starter New Member

    Nov 29, 2017
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    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
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    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.
     
  10. Greco

    Thread Starter New Member

    Nov 29, 2017
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    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
     
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