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I have added a sub panel next to my main panel. It is fed from a 100A breaker using 3-3-3-5 CU Service wire. I removed the ground screw to isolate the Neutral and I installed the ground bus that came with it on the left side. It is attached with 2 screws directly to the panel. I connected the #5 ground wire to this ground bus.

The question: The ground bus is SHORT with only 8-10 usable positions so I have added an additional ground bus on the right side of the panel. It is made specifically for the panel, Square D and attaches with 1 screw, centered over 2 nubs that lock it in place. Is the screw enough to ground this additional bar or do I need to interconnect it to the first ground bus with wire. If so, what size?

Thanks VERY much in advance for your assistance!

Follow up:

INSTRUCTIONS!! Those would VERY handy indeed. The problem is that they SUCK. Number 4 gives all the info about how to wire it, but I found it less than useful. Here is the entire text of the instructions:

1) Turn off all power to the enclosure

2) Mount grounding bar to the back of enclosure:

  • if one mounting hole and two nubs are provided, mount with outer holes over nubs and mounting screw through center hole.

  • if two mounting holes are provided, mount with two mounting screws.

    NOTE: If no mounting holes are provided, use #26 drill bit to drill two 0.147 in diameter mounting holes using grounding bar as a template.

3) Apply equipment grounding label to enclosure beside grounding bar.

4) Install wiring to grounding bar. See equipment grounding label for binding screw torque.

So are they assuming this is the ONLY grounding bar and therefore the #5 ground feed from the main panel will get attached to it? Or is the lack of instruction to jumper it to an existing main ground bar an indication that the screws are enough and that it grounds through the body of the enclosure to the original ground bar? I sure can't tell from what little info they provided.

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Given a metallic panelboard cabinet, ground (equipment grounding) bars all bond to the panel enclosure, which is itself grounded either through the system bond point (for service equipment) or the feeder equipment grounding conductor (for a subpanel), which may be connected directly to the panelboard cabinet or to a ground bar terminal.

In other words, since UL and the Code let you connect a wire-type feeder ground to the ground bar via a screw terminal and have that ground the panelboard cabinet via the ground bar mounting screws, then certainly a second ground bar can be grounded to the already-grounded panelboard cabinet via its own set of mounting screws.

  • Thanks! Great explanation and exactly what I was trying to find out. – GBJohns Feb 8 '17 at 13:28
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Follow the instructions. You are obliged (NEC 110.3) to follow the extra grounding bar's instructions. That's because those instructions are the basis for UL granting it a listing as safe.

As long as you met the instructions, feel free to exceed. So you can add a ground wire even if it's not required. Generally you don't downsize a service wire (i.e. you wouldn't run 14AWG to the last single 5-15 receptacle of a 20A circuit), so you'd stay with 5AWG Cu ground wire.

Any type is fine, as long as it meets the original current rating. So don't pay for 2' of 3-3-3-5 just to obtain 2' of #5, use #5 or #4 THHN instead if it's cheaper.

  • If OP adds a second grounding wire at the panel, isn't it possible and/or likely that he will create a ground loop? – Karl Katzke Feb 7 '17 at 17:35
  • Interesting question, love to hear the opinion of some of the bigger code wonks. My guess is "no". The term "ground loop" is not mentioned in NEC at all, unlike neutral, ground is not a conductor and it does not carry current (by NEC's notion) except during fault conditions. It appears to be a radio/AV issue. This would occur entirely within a steel panel, which should be opaque to radio waves. I would worry more about the two mandatory ground rods. – Harper Feb 7 '17 at 18:51
  • Ok, so looking at some other forums I came across this that states explicitly that when adding two, manufacturer compatible ground bus bars, you do not need to bond them. The metal of the panel IS sufficient. Midway down the page there is a nice diagram of a sub-panel and the details stated above. The info is at diychatroom.com/f18/… – GBJohns Feb 7 '17 at 21:23

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