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Thread: Single Phase 480V - Single Phase 120V Xfmr & Distribution Question

  1. #1

    Single Phase 480V - Single Phase 120V Xfmr & Distribution Question

    Hello,

    This question is most probably asked and answered several times on this forum but in the search I could not find exact question/answer, so I am posting this. Thanks for understanding....

    I have a 1-ph xfmr with primary 480V (fed from two phases of a 480V MCC) transformed to 1ph 120V on the secondary. There is no neutral terminal. There is only one ground terminal.

    I have a 1-ph distribution panel on the secondary side with main breaker and 42 fdr ckts. There is also a neutral bus and ground bus terminals on this panel. But we call it 120V, 1ph, 2 wire panel. We intend to use this xfmr-panel distribution circuit for building receptacles, lighting etc.

    Question: What should be the connection between xfmr and distr. pnl? Should it be a simple L1-L1, L2-L2 and ground to ground and leave the neutral unused? I will also run ground wire from MCC to xfmr for continuity. The 42 fdr ckts then will feed L1-L1, L2-L2, grnd-grnd for each receptacle and lighting circuit and other aux. power circuits. Is this correct?

    My concern is about the return path in the circuit when the secondary side of xfmr has no neutral (like in 120/240V case) Should one of the legs on xfmr secondary, let's say L2 to be grounded? then that will groun one of the phases on panel as well.

    Please some one educate me. Thanks for your help!!!

  2. #2
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    Too much of what you are describing is not making sense.

    Unless there is something unusual about your transformer there is no L1 and L2 but rather a H1, H2, H3, and H4 on the primary, and X1, X2, X3, and X4 on the secondary. There should also be a diagram of the connections and corresponding voltages.

    Also what kind of panel do you have that is fed with only 120V? Is it a single buss panel?
    If Billy Idol or John Denver is on your Ipod go and re-evaluate your life.

  3. #3
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    250.20 will require this to be a grounded system. The grounding rules will be found in 250.30.
    Don, Illinois
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  4. #4

    Name Plate of xfmr attached

    Hi All,

    Thanks for your replies. See attached the name plate info of this distr xfmr. H1,H2 is primary. X1-X2 is secondary

    Yes, the panel is 1ph, 2 wire, grounded neutral.

    According to 250.20, I noticed that most of the demonstrations have neutral that is grounded except one, exhibit 250.4 which shows one of the legs on secondary to be grounded. So, grounded secondary should be connected to L2 on the panel?

    Thanks
    Attached Images Attached Images

  5. #5
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    One step at a time..........

    Quote Originally Posted by kvramesh View Post
    I have a 1-ph xfmr with primary 480V (fed from two phases of a 480V MCC) transformed to 1ph 120V on the secondary. There is no neutral terminal. There is only one ground terminal.
    Please verify the transformer nameplate rating. Typical would be 480V - 120/240V. By verifying the nameplate information may answer many if not all of your questions. If this is not a typical transformer, and you really do have a 480V-120V, then.... lets take that as it comes.

    Quote Originally Posted by kvramesh View Post
    I have a 1-ph distribution panel on the secondary side with main breaker and 42 fdr ckts. There is also a neutral bus and ground bus terminals on this panel. But we call it 120V, 1ph, 2 wire panel. We intend to use this xfmr-panel distribution circuit for building receptacles, lighting etc.
    Again, unless you have a special design panel, it is single phase, but will require two 120V lines hooked up to it, which will give you 240V when measured line-line. This would match the "typical" transformer as mentioned above.
    "Just because you're paranoid, doesn't mean they're not out to get you"

  6. #6
    Hi King,

    I attached the xfmr details. Doc.pdf
    Unfortunately it is 480V-120V transformer. There is no 240V neutral point.

    The panel description is:

    1 - SECTION
    P1H42QH175CTS, System Voltage: 120 1Ø 2W Grounded Neutral AC, IR @
    10,000 AIC, Top Feed, Surface Mount, Bus Material: Copper, Plating: Tin,
    NEMA 3R/12 OUTDOOR.
    1 - INTERIOR
    1 - NEMA 3R Enclosure
    1 - Std Al/Cu Gnd Connector
    1 - Master NP Secured -Adhesive
    1 - Std Plastic Sleeve
    1 - 175A /2P-QJH2 MB
    1 - (1)#6-300Kcmil Cu / #4-300 Al
    24 - 30A /1P-BQD
    18 - 25A /1P-BQD
    1 - Enclosure, Catalog Number CUSTOM

  7. #7
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    OK, so it is non-standard.

    You essentially have 250A at 120V. Your panel looks ok although if it has a 175A MB then you are not getting the full capacity out of the transformer. Short circuit is good, you will only have about 6.5kA.

    Also, the MB says 2 Pole, I am assuming that they have factory jumpered the bus in the panel? On the transformer, its looks like a big control power transformer. Just ground L2, and that becomes your neutral.
    Last edited by kingpb; 12-13-11 at 08:36 AM.
    "Just because you're paranoid, doesn't mean they're not out to get you"

  8. #8
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    Quote Originally Posted by kvramesh View Post
    Hello,
    My concern is about the return path in the circuit when the secondary side of xfmr has no neutral (like in 120/240V case) Should one of the legs on xfmr secondary, let's say L2 to be grounded? then that will groun one of the phases on panel as well.
    We routinely use 415V primary to 110V secondary single phase transformers for the control circuits in the electrical panels we make.
    As in your case, the primary voltage is derived from line to line of two phases of a three phase supply.

    Like this arrangement:



    As long as it's double wound (primary and secondary electrically isolated from each other), you could designate either end of the secondary as neutral. The secondary on the ones we use are marked as shown above and the 0V is used.

  9. #9
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    A two wire source does not have a "neutral" Ground one of the leads and that becomes the "grounded conductor".

    Do not expect a transformer to have a "grounded" and "grounding" terminal. The transformer simply has a winding with two ends. Any additional terminals are just extensions of those two ends.

    From the point that the grounding is done keep current carrying "grounded conductors" separated from "equipment grounding conductors".

    This is basic rule for all grounded systems whether two wire 120 volt, 120/240 two or three wire, or a corner grounded delta system of any voltage.

  10. #10
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    You chose not to reveal your location, but you are posting in a forum based on the US National Electric Code. So assuming you are here or an OEM sending equipment here, that is not the correct type of transformer for what you appear to want to do; connect to a panelboard. US panel boards are designed for 120/240V single phase connections with two hot lines and a grounded neutral. you are not going to get that from that transformer.
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