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Thread: Bonding XO when not using a neutral conductor

  1. #1
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    Bonding XO when not using a neutral conductor

    Hello

    Had a debate about the correct way to wire a transformer, hoping for some help. We have a 480V 3ph 4W panel feeding a 480V to 240V delta wye transformer. From the transformer, we are feeding a 3-phase enclosed circuit breaker, which is feeding a CNC machine. The panel is fed with a neutral, but there is no neutral being used anywhere in this particular feed. We have ground wires coming from the ground bus in the panel to the transformer, going out from the transformer to the ground lugs at the ECB, and from the ECB to the machine. There is also a ground wire ran from the transformer to building steel. All grounds are connected to ground bushings at each end of every conduit, and are connected to a factory installed ground bus inside of the transformer itself.

    The question is whether or not we also need to connect the transformer ground bus to the XO.

    Thank you very much for any help you can offer
    Mike.

  2. #2
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    Yes, the transformer's secondary neutral must be bonded to the EGC system, so a fault to ground generates enough current to open OCP devices.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  3. #3
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    Yes, it's required as Larry stated because what you currently have is an ungrounded secondary and a WYE secondary is required to have the X0 connected with a system bonding jumper (SBJ) to the case/EGC terminal. Some helpful terminology, which is important to know when sizing the various "grounds":

    EGC: Equipment Grounding Conductor- the conductor between the panel and the transformer primary and the enclosed circuit breaker and the machine
    SBJ: system bonding jumper- connects the X0 to the case/EGC terminal on the transformer secondary
    SSBJ: Supply Side Bonding Jumper- conductor run with the secondary conductors to the enclosed circuit breaker
    GEC: Grounding Electrode Conductor- conductor run from X0 to the grounding electrode, in this case building steel
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  4. #4
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    Deleted.

  5. #5
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    I think we need some clarification as I note that the OP says that the secondary is 240 volt. If this is really 240 volt 3 phase I think it is likely he has a center tapped 240/120 delta secondary.

  6. #6
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    Quote Originally Posted by Mike_J_H View Post
    Hello

    Had a debate about the correct way to wire a transformer, hoping for some help. We have a 480V 3ph 4W panel feeding a 480V to 240V delta wye transformer. From the transformer, we are feeding a 3-phase enclosed circuit breaker, which is feeding a CNC machine. The panel is fed with a neutral, but there is no neutral being used anywhere in this particular feed. We have ground wires coming from the ground bus in the panel to the transformer, going out from the transformer to the ground lugs at the ECB, and from the ECB to the machine. There is also a ground wire ran from the transformer to building steel. All grounds are connected to ground bushings at each end of every conduit, and are connected to a factory installed ground bus inside of the transformer itself.

    The question is whether or not we also need to connect the transformer ground bus to the XO.

    Thank you very much for any help you can offer
    Mike.
    Mistake or do you actually have a 240 volt wye secondary instead of 208?

    IMO you don't even need a "neutral" to the enclosed breaker.

    If this is intended to be a grounded system though then the neutral is the conductor to be grounded. It can be bonded at the transformer or at the first disconnecting means.

    Between the transformer and first disconnecting means it is called the supply side bonding jumper - important to know that for determining what size of conductor it needs to be.
    I live for today, I'm just a day behind.

  7. #7
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    yes, as noted, required
    in addition to fault clearing if a ph faults to gnd your grounding system/frame goes to ph-n voltage

    if the X0 is floating, ie, 0 seq reactance, you will flow no 0 seq current, ie, gf current
    upon a ph-frame fault the frame goes to phase voltage...and stay that way
    not a shock hazard since 0 seq Z is infinite

    your post is kind of hard to follow, the X0 is isolated/floating? no intentional ground bond?
    or is it internally bonded to the xfmr gnd lug?
    a sketch is worth a 1000 words
    how do you get a neut from a 240 delta?
    The difference between genius and stupidity is that genius has its limits.

  8. #8
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    Quote Originally Posted by Ingenieur View Post
    yes, as noted, required
    in addition to fault clearing if a ph faults to gnd your grounding system/frame goes to ph-n voltage

    if the X0 is floating, ie, 0 seq reactance, you will flow no 0 seq current, ie, gf current
    upon a ph-frame fault the frame goes to phase voltage...and stay that way
    not a shock hazard since 0 seq Z is infinite

    your post is hard to follow, the X0 is isolated/floating? no intentional ground bond?
    or is it internally bonded to the xfmr gnd lug?
    a sketch is worth a 1000 words
    how do you get a neut from a 240 delta?
    He said " 480V to 240V delta wye transformer"

    My guess is either 208 and mistakenly written 240 or he has somewhat rare system - 240 volt wye would have 138 volts each line to neutral.

    Sort of doesn't matter to certain extent - he isn't utilizing any line to neutral loads.
    I live for today, I'm just a day behind.

  9. #9
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    Quote Originally Posted by kwired View Post
    He said " 480V to 240V delta wye transformer"

    My guess is either 208 and mistakenly written 240 or he has somewhat rare system - 240 volt wye would have 138 volts each line to neutral.

    Sort of doesn't matter to certain extent - he isn't utilizing any line to neutral loads.
    I have installed a few 240Y/138 transformers used as drive isolation transformers. 250.20(B)(1) requires this transformer secondary to be connected as a grounded system.
    Don, Illinois
    Ego is the anesthesia that deadens the pain of stupidity. Dr. Rick Rigsby
    (All code citations are 2017 unless otherwise noted)

  10. #10
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    Hopefully he's not using a 240Delta/480VWye in reverse.

    He needs to show a photo of the tag so we can see what he's working with.

    He may not need a neutral for any loads, but it's likely that the drives in his CNC want to have a wye output for the MOV protection.

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