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Thread: Transformer XO

  1. #1
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    Transformer XO

    I know this topic has been discussed at length and I finally maybe I think understand the bonding and grounding of transformers in general. Some of the different exceptions and scenarios can still be confusing. My problem or question I’m hooking up a large saw from China I mean really from China not just made there. The manual, controls, everything Chinese. It’s Designed to run at 200V 3phase I’m installing a 3phase 45KVA 480V/277V to 208/120 transformer. Saw rep said quote“208 very good “ Did all the calculations 70Amp breaker on the primary side 125amp fusable disconnect on the secondary side. There is 3 lines and a equipment ground on the primary side EG on a lug at the service entrance. I pulled 5 total lines from the secondary to my disconnect. X1 X2 X3 X0 neutral and a #2 ground wire which I fed thru XO to aid as my bonding jumper and terminated at the ground lug I installed on the transformer metal frame. Here’s my question inside the saw which arrived last night after all other work is done there are only four terminals L1 L2 L3 and PE which is used as green ground then jumps to terminal block for all control grounds for the machine. There is no neutral do I need to use the XO terminal at all do I carry it through to the unit and terminate it at the machine ? I’m just not sure which I should be using. Right now I’m using both through the disconnect with no where to terminate my last 5th wire in the saw. I also have both on the combination block in the disconnect with the bonding screw intstalled. ( I know that needs to come out to keep a fault traveling back to the source) Is XO even needed ? If not I just continue the EGC straight from the source to the machine bonding transformer and disconnect along the way. Clear fault path machine to source. Is a bonding jumper still required at XO in this case. My brain hurts
    Last edited by Smash; 07-25-18 at 11:57 AM.

  2. #2
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    I have a hard time with reading large blocks of text. Please break it up next time.

    XO has to still be bonded to the case and connected to the GES even if do not need a neutral at the equipment.

    A neutral does not have to be ran to equipment if not needed.

    I think you were asking about that?..
    "Electricity is really just organized lightning." George Carlin


    Derek

  3. #3
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    I currently have XO to the disconnect from disconnect to the saw. Smaller #2 ground from transformer frame lug to disconnect combination lug then both to the saw. I used the #1 AWG from disconnect to the PE terminal on saw. All work was completed with a whip off disconnect waiting for saw. I’m just questioning the use of the XO back to the saw. Sounds like XO should have stopped with the bonding jumper only and only carry ground thru disconnect to saw. Wouldn’t removing bond screw in disconnect be the same thing ? It carries straight thru just using multiple wires to accomplish the same thing.

  4. #4
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    Quote Originally Posted by Smash View Post
    I currently have XO to the disconnect from disconnect to the saw. Smaller #2 ground from transformer frame lug to disconnect combination lug then both to the saw. I used the #1 AWG from disconnect to the PE terminal on saw. All work was completed with a whip off disconnect waiting for saw. I’m just questioning the use of the XO back to the saw. Sounds like XO should have stopped with the bonding jumper only and only carry ground thru disconnect to saw. Wouldn’t removing bond screw in disconnect be the same thing ? It carries straight thru just using multiple wires to accomplish the same thing.
    I am sorry, I do not follow.

    XO is a transformer terminal. XO at the disco?
    "Electricity is really just organized lightning." George Carlin


    Derek

  5. #5
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    Quote Originally Posted by Smash View Post
    I currently have XO to the disconnect from disconnect to the saw. Smaller #2 ground from transformer frame lug to disconnect combination lug then both to the saw. I used the #1 AWG from disconnect to the PE terminal on saw. All work was completed with a whip off disconnect waiting for saw. I’m just questioning the use of the XO back to the saw. Sounds like XO should have stopped with the bonding jumper only and only carry ground thru disconnect to saw. Wouldn’t removing bond screw in disconnect be the same thing ? It carries straight thru just using multiple wires to accomplish the same thing.
    Smash,
    Like Jumper I'm having a hard time following your installation. You need to clarify these few terms:

    SBJ -system bonding jumper, from X0 to frame
    SSBJ- supply side bonding jumper, from the X0 to the disconnect
    EGC- equipment grounding conductor, from the disconnect to the saw
    GEC- grounding electrode conductor, X0 to the ground electrode

    Since your SBJ is in the transformer then you do not need to bring a neutral conductor to the disconnect switch since it's not being used. The SSBJ will be used to bond the disconnect switch and will be used for all equipment grounding purposes on the load side of the disconnect switch.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  6. #6
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    Quote Originally Posted by infinity View Post
    Smash,
    Like Jumper I'm having a hard time following your installation. You need to clarify these few terms:

    SBJ -system bonding jumper, from X0 to frame
    SSBJ- supply side bonding jumper, from the X0 to the disconnect
    EGC- equipment grounding conductor, from the disconnect to the saw
    GEC- grounding electrode conductor, X0 to the ground electrode

    Since your SBJ is in the transformer then you do not need to bring a neutral conductor to the disconnect switch since it's not being used. The SSBJ will be used to bond the disconnect switch and will be used for all equipment grounding purposes on the load side of the disconnect switch.
    I apologize the confusion comes in because the transformer the disconnect and a load side whip I installed weeks ago in prep for this saw being delivered. That’s where my contract ended. Everything was set up with the belief a neutral was needed. Saw rep was there to bolt down, level, and tie in the whip I supplied. He used the full sized #1 AWG neutral XO in this case to the PE ground at the saw. Is this a problem since I did install the bonding jumper inside the transformer ? This job is 3hr drive for me and my contract is done. If it’s not correct I will return and fix it but if it’s ok it will save me a day.

  7. #7
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    So, PE at the saw is connected to the XO in the tranny.

    XO to case.
    XO to GES

    Yes?
    "Electricity is really just organized lightning." George Carlin


    Derek

  8. #8
    Join Date
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    I think this is what you are asking
    480 to xfmr 3 ph and a gnd, no x0
    208 to switch 3 ph and gnd, no x0
    208 to machine 3 ph, a gnd, no x0

    in xfmr
    bond 480 gnd lug to 208 gnd lug and to enclosure (lug may already be grounded)
    bond sec x0 to gnd lugs (prim x0 is not used)

    at disconnect
    3 ph in
    3 ph out
    bond ground to case and continue to machine

    machine
    land 3 phases
    land ground to pe lug

    this assumes the machine does not need 120
    if so add neut from sec gnd lugs thru switch to machine
    only connect at machine, not switch

  9. #9
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    Allow me:

    Quote Originally Posted by Smash View Post
    I know this topic has been discussed at length and I finally maybe I think understand the bonding and grounding of transformers in general. Some of the different exceptions and scenarios can still be confusing. My problem or question I’m hooking up a large saw from China I mean really from China not just made there. The manual, controls, everything Chinese.

    It’s Designed to run at 200V 3phase I’m installing a 3phase 45KVA 480V/277V to 208/120 transformer. Saw rep said quote“208 very good “ Did all the calculations 70Amp breaker on the primary side 125amp fusable disconnect on the secondary side. There is 3 lines and a equipment ground on the primary side EG on a lug at the service entrance.

    I pulled 5 total lines from the secondary to my disconnect. X1 X2 X3 X0 neutral and a #2 ground wire which I fed thru XO to aid as my bonding jumper and terminated at the ground lug I installed on the transformer metal frame. Here’s my question inside the saw which arrived last night after all other work is done there are only four terminals L1 L2 L3 and PE which is used as green ground then jumps to terminal block for all control grounds for the machine.

    There is no neutral do I need to use the XO terminal at all do I carry it through to the unit and terminate it at the machine ? I’m just not sure which I should be using. Right now I’m using both through the disconnect with no where to terminate my last 5th wire in the saw. I also have both on the combination block in the disconnect with the bonding screw intstalled.

    ( I know that needs to come out to keep a fault traveling back to the source) Is XO even needed ? If not I just continue the EGC straight from the source to the machine bonding transformer and disconnect along the way. Clear fault path machine to source. Is a bonding jumper still required at XO in this case. My brain hurts
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  10. #10
    Join Date
    Oct 2016
    Location
    US
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    I think I understand. Pull out neutral to disconnect and leave bond screw there. Done.

    Sent from my SM-G930V using Tapatalk

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