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Thread: Bonding screw in the distribution (sub) Panel

  1. #1
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    Bonding screw in the distribution (sub) Panel

    I am a home inspector trying to understand what is correct and why in the distribution (sub) panel. My understanding is that beyond the service disconnect point (main panel) (in what I call the sub panel, I believe is called a distribution panel by NEC) ground (bare copper wire) and neutral (white wire) is separated and not bonded. My question is then is the ground bus bar bonded to the panel it self with the green screw? I have asked building inspector locally and electricians locally and I can not seem to get a clear understanding. So, hopefully someone can lay this out in laymen terms so I can be a better inspector. Thank you in advance.

  2. #2
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    First off, the main distribution panel is the panel with the main disconnect. This is the only place that the neutrals and grounds are connected together. The sub panel as you stated is the correct name for the secondary panel although the nec mentions sub panel only once.

    Once you leave the main panel the ground (equipment grounding conductor) is separated and only carries current when there is a fault. The neutral carries current but not the equipment grounding conductor.
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  3. #3
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    If the sub-panel has a separate ground bar then that is bonded directly to the enclosure. The neutral bar is not bonded past the service disconnect because doing so would cause neutral current to flow on the EGC and and possibly the metallic parts of the system.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  4. #4
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    So, should the ground bus bar in the sub panel be bonded to the metal panel itself with the green screw in case the panel becomes energized?

  5. #5
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    "If the sub-panel has a separate ground bar then that is bonded directly to the enclosure. The neutral bar is not bonded past the service disconnect because doing so would cause neutral current to flow on the EGC and and possibly the metallic parts of the system."


    Thank you. That make sense.

  6. #6
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    Quote Originally Posted by HI9869 View Post
    So, should the ground bus bar in the sub panel be bonded to the metal panel itself with the green screw in case the panel becomes energized?
    Sort of yes because the EGC run with the feeder to the sub-panel would be connected to the EGC bar which is connected to the enclosure.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  7. #7
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    Quote Originally Posted by infinity View Post
    Sort of yes because the EGC run with the feeder to the sub-panel would be connected to the EGC bar which is connected to the enclosure.

    Thank you!

  8. #8
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    Quote Originally Posted by HI9869 View Post
    My question is then is the ground bus bar bonded to the panel it self with the green screw?
    The "subpanel" that has a metal enclosure, that is, the panel box is made of metal, has to be bonded to the Equipment Grounding Conductor (EGC), of which there are many types.

    Very possibly, the more, if not most, confusing type of residential dwelling EGC will be a metal conduit feeder, containing only two hots and a neutral, that brings power from the main panel to the subpanel. The metal conduit is the equivalent of a bare copper wire EGC. The connection of the metal conduit to the subpanel metal enclosure establishes the EGC.

    Mechanically securing a terminal bar (as you say, the ground bus bar) to the subpanel metal enclosure provides terminals to "land" the branch circuit ground wires, whether green, bare or other color (except white or gray). The ground terminal bar is usually available in a bag with mounting screws and labels. The mounting screws do not have to be any particular color, they may be green, but that is actually unusual.

    Now, a different path for the EGC happens when the subpanel gets its power from the main panel through a four-wire feeder Cable, two hots, a neutral and a "ground" wire. The ground wire has to land in a terminal made for wire-type grounds, so, the feeder cable ground "lands" under one of the terminal screws of the "ground bus bar". Mounting the "ground bus bar" to the subpanel box with its mounting screws (of any color) then, in fact, "bonds" the feeder cable EGC to the subpanel metal box.
    Another Al in Minnesota

  9. #9
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    Quote Originally Posted by al hildenbrand View Post
    The "subpanel" that has a metal enclosure, that is, the panel box is made of metal, has to be bonded to the Equipment Grounding Conductor (EGC), of which there are many types.

    Very possibly, the more, if not most, confusing type of residential dwelling EGC will be a metal conduit feeder, containing only two hots and a neutral, that brings power from the main panel to the subpanel. The metal conduit is the equivalent of a bare copper wire EGC. The connection of the metal conduit to the subpanel metal enclosure establishes the EGC.

    Mechanically securing a terminal bar (as you say, the ground bus bar) to the subpanel metal enclosure provides terminals to "land" the branch circuit ground wires, whether green, bare or other color (except white or gray). The ground terminal bar is usually available in a bag with mounting screws and labels. The mounting screws do not have to be any particular color, they may be green, but that is actually unusual.

    Now, a different path for the EGC happens when the subpanel gets its power from the main panel through a four-wire feeder Cable, two hots, a neutral and a "ground" wire. The ground wire has to land in a terminal made for wire-type grounds, so, the feeder cable ground "lands" under one of the terminal screws of the "ground bus bar". Mounting the "ground bus bar" to the subpanel box with its mounting screws (of any color) then, in fact, "bonds" the feeder cable EGC to the subpanel metal box.

    Is one feed more dangerous than the other? My question comes from I have been told that in a three wire feeder system it really does not matter if the neutral and ground are separate but that it does matter in a four wire feeder system. I see over and over that neutral and ground are to be separated in the sub panel and I understand that. It threw me when the person I spoke with said it only mattered in a four wire system.

  10. #10
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    Quote Originally Posted by HI9869 View Post
    Is one feed more dangerous than the other? My question comes from I have been told that in a three wire feeder system it really does not matter if the neutral and ground are separate but that it does matter in a four wire feeder system. I see over and over that neutral and ground are to be separated in the sub panel and I understand that. It threw me when the person I spoke with said it only mattered in a four wire system.

    Unless you're dealing with a separate structure and prior to the 2008 NEC you will almost always need a "4 wire" feeder. IMO there wouldn't have been a code change in 2008 if the CMP felt that a 3-wire feeder were just as safe as a 4-wire feeder.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

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