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Thread: 120/208, 1Ph, 3 Wire

  1. #1
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    120/208, 1Ph, 3 Wire

    I have a scenario where there is a existing transformer in the field (no data on the existing transformer) and an adjacent Eaton Panel. The nameplate on the panel reads 120/208, 1Ph, 3W. Correct me if I'm wrong but the only way to get this voltage is to pull one phase off of a 3 phase transformer? If that is the case isn't technically split phase? I'm adding some small loads out their and was hoping to add a 40 amp 2 pole breaker to the Eaton board.

    The other solution is to contact the utility and get info on the existing transformer, but that usually takes weeks...
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    The supply voltage originates at a 208/120 three phase source - only way possible other then with some electronics to derive 208/120 volts. This equipment just happens to only be supplied with two "phase conductors" plus a neutral conductor. You have 208 volts line to line available and 120 volts from each line to neutral available. Because of the 120 degree phase angle neutral does not carry unbalanced current on line to neutral loads though, it carries approximately same current as the two ungrounded conductors if they are both the same current level.

    What people typically call "split phase" is a single phase source with a center tap, in that case the neutral does only carry imbalanced current of the other two lines, that is because the phase angle in such setup is 180 degrees.

  3. #3
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    It's 208V line to line and you can supply line to line loads whether you call it single phase or split phase.
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    Quote Originally Posted by ActionDave View Post
    It's 208V line to line and you can supply line to line loads whether you call it single phase or split phase.
    If you call it split phase, this thread is gonna be 300+ posts arguing.
    "Electricity is really just organized lightning." George Carlin


    Derek

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    Quote Originally Posted by ActionDave View Post
    It's 208V line to line and you can supply line to line loads whether you call it single phase or split phase.
    Let's not get the engineers involved on definition of "phase" , but a 120/240 coil is more of a "split phase" IMO then two lines of a 208/120 plus the neutral.

    You can derive "three phase" with just transformer coils from the 208/120 if you wanted to, you can't do that with the single phase coil with a midpoint.

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    Quote Originally Posted by Sean.Day72 View Post
    I have a scenario where there is a existing transformer in the field (no data on the existing transformer) and an adjacent Eaton Panel. The nameplate on the panel reads 120/208, 1Ph, 3W.
    A couple of questions if I may?
    Do you know for sure if the transformer in the field is 120/208V? That would normally be 3-phase. Can you physically look at the name plate?
    Does the Eaton panel need both voltages or is it an either or?
    Si hoc legere scis nimium eruditionis habes.

  7. #7
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    The existing transformer has no name plate on it, just a serial number for the utility. What I think happened is that this transformer has been sitting out in the yard for some time (it looks old) and they recently built a access control gate close by. They installed a rack with a service disconnect, meter, and the eaton panel and tapped into the old transformer. Since the gate only has some cameras and a small motor, 3 phase power was not required so they only pulled 1 phase off of it. I'm adding some small loads close by so the plan is to put a 2 pole 40 amp breaker into this existing eaton panel to feed a small panel to power up some street lighting. I just want to confirm I'm not loosing it

    And I misspoke this is not split phase, it appears to be line-line

  8. #8
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    Quote Originally Posted by Sean.Day72 View Post
    The existing transformer has no name plate on it, just a serial number for the utility. What I think happened is that this transformer has been sitting out in the yard for some time (it looks old) and they recently built a access control gate close by. They installed a rack with a service disconnect, meter, and the eaton panel and tapped into the old transformer. Since the gate only has some cameras and a small motor, 3 phase power was not required so they only pulled 1 phase off of it.
    Sorry to be a pain, but is that one phase line to neutral or line to line?
    Si hoc legere scis nimium eruditionis habes.

  9. #9
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    Quote Originally Posted by Besoeker View Post
    Sorry to be a pain, but is that one phase line to neutral or line to line?

    I'm not sure until the utility gets back to me. My guess is the 208V is line-line with Y connection on the secondary of the existing transformer. The only other way I know how to get 208 is if the existing transformer is a delta with high leg configuration, which I never seen used with this particular client.

  10. #10
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    Quote Originally Posted by Sean.Day72 View Post
    I'm not sure until the utility gets back to me. My guess is the 208V is line-line with Y connection on the secondary of the existing transformer. The only other way I know how to get 208 is if the existing transformer is a delta with high leg configuration, which I never seen used with this particular client.
    Yes, line to line star (wye) would be 208V if that's what the transformer is designed to produce. I'm just a little curious to know how you know it is that and not say, 480/277V if there is no nameplate rating?

    Do you also need the 120V as well as the 208V?
    Si hoc legere scis nimium eruditionis habes.

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