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Thread: Seperately derived system

  1. #1
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    Seperately derived system

    Hello I have a 75kva transformer stand being built with a disconnect switch (100A)(480V) feeding the primary side. I then have a 120/208v panel mounted to the other side of the transformer itself. The conductors going through the RMC will be supplying under 250v. I ran a EGC from inside the transformer to the ground bus in the panel. Also XO is bonded to ground inside the transformer.My question is, do I need a bonding bushing placed on the conduit connecting the transformer to the secondary side panel?

  2. #2
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    No bonding bushing required when the voltage is below 150 volts to ground. Just for clarity the conductor between the transformer is a SSBJ (supply side bonding jumper) not an EGC. That is distinction is important because SSBJ's are sized differently.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  3. #3
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    Quote Originally Posted by infinity View Post
    No bonding bushing required when the voltage is below 150 volts to ground. Just for clarity the conductor between the transformer is a SSBJ (supply side bonding jumper) not an EGC. That is distinction is important because SSBJ's are sized differently.
    If the connection between the secondary side of the xfmr to the panelboard is in RMC all the way, I don’t think the SSBJ was required?

  4. #4
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    Quote Originally Posted by rojay View Post
    If the connection between the secondary side of the xfmr to the panelboard is in RMC all the way, I don’t think the SSBJ was required?
    That is correct, the SSBJ could be a metal raceway in lieu of a wire type SSBJ as long as it qualifies as an EGC. Stick a piece of FMC on the end and then a wire type SSBJ is needed.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  5. #5
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    Clarification

    I have a co worker that is saying that a bonding bushing has to be placed on the RMC raceway inside the XMFR connecting the XFMR to the panel. I said that there is no need since there is no eccentric or concentric knockouts present. He claims they have been bonding the raceway for years. Is there somewhere in the code that says bonding the raceway is not required just so I can show him?

  6. #6
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    Yes I am sorry for referring to it as an EGC. The SSBJ was sized correctly. Sorry for the confusion just need clarification on the bonding bushing on the raceway.

  7. #7
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    Quote Originally Posted by Hoggedy View Post
    I have a co worker that is saying that a bonding bushing has to be placed on the RMC raceway inside the XMFR connecting the XFMR to the panel. I said that there is no need since there is no eccentric or concentric knockouts present. He claims they have been bonding the raceway for years. Is there somewhere in the code that says bonding the raceway is not required just so I can show him?
    The way the NEC is written it would say that it's required if it actually was not the other way around. Ask him for a code reference, you might have to wait a while for him to find it because it doesn't exist.

    Doing something a certain way for years does not make it a code requirement but there's nothing wrong with exceeding what the NEC requires.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  8. #8
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    Quote Originally Posted by infinity View Post
    The way the NEC is written it would say that it's required if it actually was not the other way around. Ask him for a code reference, you might have to wait a while for him to find it because it doesn't exist.

    Doing something a certain way for years does not make it a code requirement but there's nothing wrong with exceeding what the NEC requires.
    I understand doing extra is always okay. He claimed that I had to do it because code says so. If he was right I would have to go back and fix the other 4 transformer stands I already built. That was why the answer was critical. I appreciate it. The answer is, No I do not have to bond the raceways. thank you

  9. #9
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    So, I was told to look at 250.92 services. where it states that normally non current carrying metal parts shall be bonded together.
    So I think the issue here is dictating whether or not this is considered a service or a separately derived system.
    The conductors feeding the disconnect are going to be fed from a cable that will then be connected to a Meltric on the side of a breaker panel. The breaker (branch circuit) will feed the cable, in turn feeding the switch(primary side of transformer) and then the secondary side will feed another panel.
    wouldn't a service only be the wires between the serving utility and the service equipment? making the service only the points where it enters our company? everything downstream of that is feeders and branch circuits? in turn, a branch circuit feeds our switch to transformer, making that a separately derived system. Which would make it not necessary to bond the raceways?
    How do I distinguish what is a service and what is a SDS bonding raceways changes based on which one we are dealing with.

  10. #10
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    Quote Originally Posted by Hoggedy View Post
    How do I distinguish what is a service and what is a SDS bonding raceways changes based on which one we are dealing with.
    Service has a very narrow definition in the NEC. Look at art. 100 definitions.
    If you go and decide to dance with a gorilla the dance ain't over till the gorilla decides it's over.

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