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Thread: Xfmr Reverse Connected

  1. #1
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    Xfmr Reverse Connected

    When a typical 150 kVA, 480V, 3PH delta to 208Y/120V, 3PH dry type xfmr is reverse connected with a 208V, 3PH, 3W primary, is there any negative impact on the xfmr because it is connected with a 3PH, 3W feed in lieu of a 3PH, 4W feed. Are there any engineering or NEC principles violated. Any thoughts.


    Michael

  2. #2
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    Nope, it's common to do, but not always the best idea from a design standpoint. No special hazards created.

  3. #3
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    This was just discussed and I don't have time to find the other post.
    A step down transformer is 480 delta by 120/208 Wye.
    Reversed its 208 wye by 480 delta. The secondary by 250.30 and 250.20 must have a grounded conductor. The delta would become a corner ground.
    I maintain its a violation unless the transformer is listed for the reverse wiring. Sq D makes a step up transformer. Its delta by wye....
    Moderator-Washington State
    Ancora Imparo

  4. #4
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    Quote Originally Posted by tom baker
    This was just discussed and I don't have time to find the other post.
    A step down transformer is 480 delta by 120/208 Wye.
    Reversed its 208 wye by 480 delta. The secondary by 250.30 and 250.20 must have a grounded conductor. The delta would become a corner ground.
    I maintain its a violation unless the transformer is listed for the reverse wiring. Sq D makes a step up transformer. Its delta by wye....
    I was asked this question before and quite frankly I was at a loss for words on a reply. Mr. Baker just made it very clear to me what the answer is. Thanks. I am now enlightened.
    When in doubt, empty the magazine. Semper Fi

  5. #5
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    I agree Tom. Having established that, is a delta to delta xfmr in a typical commercial building application legal. I wouldn't think so, & IEEE standard 241, & other IEEE standards, also recommends not using the application unless special engineering considerations are used. Yet this application is frequently used for equipment hookup, ie a 480V, 3PH delta to 400V, 3PH or 380V, 3PH delta. Any Thoughts anyone.

  6. #6
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    Tom,
    The secondary by 250.30 and 250.20 must have a grounded conductor.
    The code does not require that the secondary of this transformer be grounded. The system that supplies the transformer is required to be grounded, but there is no requirement that the grounded conductor be connected at the transformer.
    Don
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

  7. #7
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    Quote Originally Posted by tom baker
    ...I maintain its a violation unless the transformer is listed for the reverse wiring. ...
    tom -

    When you get some time, perhaps you could lead us to this listing reference prohibiting reverse feeding.

    Jim Dungar has a pretty good response about the mfg requirements http://forums.mikeholt.com/showthread.php?p=86265

    carl
    Using the code for a design guide is a sign of incompetance

  8. #8
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    There is a problem in my opinon in having the secondary ungrounded at 480 volts. It can lead to overvoltages in a fault up to 2200 volts.
    And even if grounded, its 480 to ground, not 277, leading to a higher voltage stress on conductors.
    As far as the listing, it makes sense the transformer would have to shown with voltages by the mfg to be able to use it as a reverse connection. The UL white book does not mention this at all under the catagory "transformers"
    Moderator-Washington State
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  9. #9
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    Quote Originally Posted by mjobee
    ...IEEE standard 241, & other IEEE standards, also recommends not using the application unless special engineering considerations are used.
    The engineering considerations typically include:
    Addressing the increased inrush current and probable difficulty in selecting the OCPD.
    The HV will be lower than expected.
    The poor performance of the taps not being on the primary side.
    Dealing with all of the factors associated with grounding delta systems.
    The design of the transformer windings such as T-connections and compensation windings.
    Making sure the X0 terminal is not connected.

    edited to add a consideration
    Last edited by jim dungar; 05-02-07 at 08:20 PM.
    Just because you can, doesn't mean you should.

  10. #10
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    Quote Originally Posted by tom baker
    I maintain its a violation unless the transformer is listed for the reverse wiring.
    Tom,
    Have you ever found a written statement prohibiting the reverse connection of general purpose transformers or is it just your opinion?

    I have seen manufactures saying specifically not to reverse connect some transformers (i.e. the fan cooled GE units). But every other statement has been "while not prohibited, it is not advised" primarily due to the problems with inrush currents and voltage adjustments.
    Just because you can, doesn't mean you should.

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