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Thread: Bonding screw in the distribution (sub) Panel

  1. #11
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    Quote Originally Posted by infinity View Post
    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.
    So from my prospective as a home inspector I am usually inspecting homes that are 10-20 years old and I see and point out that grounds and neutrals should be separated in the sub panel. How important (as a safety issue) should this be corrected. Had a building inspector tell me it is not required to correct unless they are adding/upgrading the sub panel.

  2. #12
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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.
    Careful, here. I think you may be mixing two very different ideas.

    BOTH of my post's feeders are FOUR conductor. In the first, the metal conduit with three conductors, the metal conduit itself is the fourth conductor.

    Your original post question is about "bonding" the grounded terminal bar (EGC) to the metal subpanel enclosure. Your question defines a four conductor feeder situation.

    Actual three conductor feeders exist, but in special situations.
    Another Al in Minnesota

  3. #13
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    Quote Originally Posted by HI9869 View Post
    So from my prospective as a home inspector I am usually inspecting homes that are 10-20 years old and I see and point out that grounds and neutrals should be separated in the sub panel. How important (as a safety issue) should this be corrected. Had a building inspector tell me it is not required to correct unless they are adding/upgrading the sub panel.
    IMO if the sub-panel is within the same structure it needs to be corrected because it has never been allowed.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  4. #14
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    Just to clear up what might be confusing to the OP. The "green" screw is usually found in the neutral bar. It is only to be used at the main panel. Since the neutral bar is mounted on a non-conductive mount, the screw is tightened in so that it contacts the metal enclosure. That bonds the neutral to the metal enclosure and to the ground (EGC) bar.

    If the panel has a "green" screw and it's not the main panel, then the screw should NOT be tightened in. Now sometimes the neutral bar and ground bar are both mounted on a non-conductive mount, neither of which "bond" or contact the metal enclosure. These also have been isolated from each. Technically these are both neutral bars but one is being used as a ground bar. If it is to be used as the ground bar then it has to be bonded to the enclosure. If the "green" screw happens to be in the bar to be used as the ground and there is no connection to the neutral conductor, then the screw can be tightened in. This is unusual but does happen.

    To sum up, if the "green" screw is tightened in then it either should be the main panel or, if in a subpanel, it should not have any connection to the neutral. The "green" screw is only to bond the bar to the enclosure.
    If you aim at nothing, you will hit it every time!

  5. #15
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    Quote Originally Posted by al hildenbrand View Post
    Careful, here. I think you may be mixing two very different ideas.

    BOTH of my post's feeders are FOUR conductor. In the first, the metal conduit with three conductors, the metal conduit itself is the fourth conductor.

    Your original post question is about "bonding" the grounded terminal bar (EGC) to the metal subpanel enclosure. Your question defines a four conductor feeder situation.

    Actual three conductor feeders exist, but in special situations.

    I understand..... a 3 phase system with 4 wires. Ok, I'm with you.

  6. #16
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    Quote Originally Posted by Little Bill View Post
    Just to clear up what might be confusing to the OP. The "green" screw is usually found in the neutral bar. It is only to be used at the main panel. Since the neutral bar is mounted on a non-conductive mount, the screw is tightened in so that it contacts the metal enclosure. That bonds the neutral to the metal enclosure and to the ground (EGC) bar.

    If the panel has a "green" screw and it's not the main panel, then the screw should NOT be tightened in. Now sometimes the neutral bar and ground bar are both mounted on a non-conductive mount, neither of which "bond" or contact the metal enclosure. These also have been isolated from each. Technically these are both neutral bars but one is being used as a ground bar. If it is to be used as the ground bar then it has to be bonded to the enclosure. If the "green" screw happens to be in the bar to be used as the ground and there is no connection to the neutral conductor, then the screw can be tightened in. This is unusual but does happen.

    To sum up, if the "green" screw is tightened in then it either should be the main panel or, if in a subpanel, it should not have any connection to the neutral. The "green" screw is only to bond the bar to the enclosure.
    Awesome! This really clears it up for me along with the other responses. Love the Zig Ziglar quote! It is so true........

  7. #17
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    Thank you all that have responded. You have cleared much up for me. Enjoy your weekend!

  8. #18
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    Quote Originally Posted by HI9869 View Post
    I understand..... a 3 phase system with 4 wires. Ok, I'm with you.
    Sorry, no. Single phase 240 / 120 Volt. The feeder from the main electrical panel to the subpanel you are asking about is four wires in a single phase 240 / 120 Volt system. There are two hots, one neutral and one "ground" -- four wires.
    Another Al in Minnesota

  9. #19
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    Quote Originally Posted by al hildenbrand View Post
    Sorry, no. Single phase 240 / 120 Volt. The feeder from the main electrical panel to the subpanel you are asking about is four wires in a single phase 240 / 120 Volt system. There are two hots, one neutral and one "ground" -- four wires.
    So what does a 3 phase have? 3 hots, neutral, and ground? Thank you for clarifying.

  10. #20
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    Quote Originally Posted by al hildenbrand View Post
    Sorry, no. Single phase 240 / 120 Volt. The feeder from the main electrical panel to the subpanel you are asking about is four wires in a single phase 240 / 120 Volt system. There are two hots, one neutral and one "ground" -- four wires.

    I agree with you and we all seem to call this a 4-wire feeder but we should really call it a 4-conductor feeder because the EGConductor (4th "wire") can be a suitable cable armor or a metal raceway, etc listed in 250.118.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

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