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Sub-Panel Grounding

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  • Sub-Panel Grounding

    I'm adding a 60-amp sub-panel for a woodworking shop in my basement. I have a new 60-amp bkr in the main panel, and ran #6-4 copper cable to the sub-panel (I'll have some 240V and some 120V circuits). I added a ground bar because I know the common and ground have to be separate in the sub-panel (and that there can't be any jumpers that connect the two in the sub-panel). I have a ground lead from the sub-panel's grounding bar to the house copper water line.

    I think I'm in good shape, but I don't fully understand the grounding issue. If the common and ground ARE tied together in the main panel, and both the main panel's ground and the sub-panel's ground are tied to the copper water lines in the home, then in reality the common and ground bars in the sub-panel are connected electrically - just not 'inside the sub-panel'. But what is the difference, and why is it an issue?

    Regards,
    Fred

  • #2
    Re: Sub-Panel Grounding

    Not an electrician, but this is my understanding. The neutral/common wire (I think it is now called a grounded conductor in the NEC) is meant to have current running through it as part of normal operation. The equipment grounding conductor, i.e. ground, is not meant to have current running through it as part of normal operation. This is the reason that the grounding conductor can be smaller than the neutral/hots for your subpanel. If you were to tie them together at the subpanel, then the current would flow through both conductors back to the main panel; but the grounding conductor is not meant to carry current unless it is an exceptional situation.

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    • #3
      Re: Sub-Panel Grounding

      I get it now - if the ground lead in the supply cable to the sub-panel were allowed to carry current during normal operation, the current could/would exceed the capacity of the conductor because it is a thinner gauge wire. By keeping them separate in the sub-panel, load current must go through the white/common lead.

      Thanks!

      Fred

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      • #4
        Re: Sub-Panel Grounding

        cpw. Right on the money. OP. Keep the neutrals and ground separated at the sub. Do not use the green bonding screw that came with the panel.
        You did not need to bond the sub panel to the water line. You already had your equipment grounding conductor (EGC) (Ground) included in your 6/3 cable. Good job and compliant.

        6/3 is 3 #6 insulated conductors and one bare conductor.
        Licensed Electrician

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