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Thread: Transformer X0 bonding

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    Transformer X0 bonding

    We have a 3 phase 480v step down to 208v transformer. There is no neutral required for the secondary load side. Does the X0 need to be bonded to structural steel if there is no need for a neutral?

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    Quote Originally Posted by Hoodood View Post
    We have a 3 phase 480v step down to 208v transformer. There is no neutral required for the secondary load side. Does the X0 need to be bonded to structural steel if there is no need for a neutral?
    Yes.
    "Electricity is really just organized lightning." George Carlin


    Derek

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    250.32 is your code reference.
    If you don't think too good, don't think too much.

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    Quote Originally Posted by ActionDave View Post
    250.32 is your code reference.
    Separate buildings?

    How about 250.20(B(1) instead?
    "Electricity is really just organized lightning." George Carlin


    Derek

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    Quote Originally Posted by jumper View Post
    Separate buildings?

    How about 250.20(B(1) instead?
    250.30 is what meant. Your reference is pertinent as well.
    If you don't think too good, don't think too much.

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    One might be able to not ground the neutral - but only if it doesn't leave the transformer enclosure. You still have to ground something though, which leaves you with one of the phase conductors, or set up as an ungrounded system and provide ground fault monitoring.

    If the neutral does leave the transformer then it definitely must be the conductor that gets grounded.
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    Quote Originally Posted by kwired View Post
    One might be able to not ground the neutral - but only if it doesn't leave the transformer enclosure. You still have to ground something though, which leaves you with one of the phase conductors, or set up as an ungrounded system and provide ground fault monitoring.

    If the neutral does leave the transformer then it definitely must be the conductor that gets grounded.
    I do not think the code section I cited allows an ungrounded 208Y/120V secondary.
    "Electricity is really just organized lightning." George Carlin


    Derek

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    Quote Originally Posted by jumper View Post
    I do not think the code section I cited allows an ungrounded 208Y/120V secondary.
    Most will read it that way, but I don't know it is worded quite the way it is intended. Do you see a problem other than possibly complying with this section with having a standby generator that is 480/277 to back up a corner grounded delta system and grounding a phase/leaving the neutral float on such system?

    Another example that may be a little more common would be a transformer to supply foreign equipment designed for 240 volt two wire system. In that case a common 120/240 transformer would work but ground one end and leave center tap float. I have no problem with that but strict interpretation might say no. But if one brought the center tap out for any use at all then it should be the conductor that is grounded to comply with this section.

    JMO.

    Add: I have seen 12 lead generator that can be set up for many different voltages, single and three phase. One will have leads that get left floating for different system arrangements - another reason I believe this rule should only apply to the conductors you bring out of the source for possible utilization. POCO's also have floating terminals on transformer banks - they are not utilized as a system conductor.
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    kwired, my little brain cannot figure out all that.

    Can we just keep it simple and bond XO to the steel?

    It a wye secondary, life is much easier that way.

    No muss or fuss.
    "Electricity is really just organized lightning." George Carlin


    Derek

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    Quote Originally Posted by jumper View Post
    kwired, my little brain cannot figure out all that.

    Can we just keep it simple and bond XO to the steel?

    It a wye secondary, life is much easier that way.

    No muss or fuss.
    99% of the time yes, you must bond that neutral. Some special situations one may not be using that neutral for anything, or even desiring to ground some other point of the source for a specific reason. You can't ground two points, that lets smoke out of insulation.
    I live for today, I'm just a day behind.

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