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Thread: Sub panel grounding and bonding

  1. #1

    Post Sub panel grounding and bonding

    Can you stand one more question about installing a sub panel?

    I have installed a 100A sub panel in the stud bay adjacent to the 200A main panel in my garage; they are both Square D Homeline panels. I put a 100A breaker in the main panel and ran #2 wire from the breaker to the 100A breaker in the sub panel. I also ran one #2 from the ground/neutral feed in the main panel to the neutral bus in the sub panel.

    I now understand that I need to add a separate ground wire from the main panel to the sub panel and that the neutrals and grounds should not be bonded in the sub panel. I'm also assuming that I'll need to buy an additonal bus bar and add it to the sub panel and use it to connect all of the ground wires. (Yes??)

    I think I'm okay so far, but need advice on where to attach the separate ground wire in the main panel. Do the ground and neutral wires going to the sub panel both connect to the neutral/ground feed -- from the meter -- in the main panel?

    I saw in other posts on this subject -- here and in other forums -- that if the sub panel is of a certain distance from the main panel that you have to install a separate ground rod or use a water pipe, etc., near the sub panel and run the ground to that. Since my sub panel is within inches of the main panel I think I'm okay running the ground from the sub back to the main panel. (Yes??)

    Regarding the ground wire from the main to the sub panel, I have enough of the #2 wire to use for the additional ground, but wonder if I can get two #2s under the binding screws at either end. Thoughts on this??

    If someone can help me understand where to terminate the grounds and neutrals in both panels, and comment on the issue of using #2 for the ground and whether it will fit under the binding screws at both ends, I would be most appreciative.

    Finally, the whole reason I'm doing this is I want to install a 240V heater in my bathroom, plus I intend to finish out some attic space above the garage as living space, so I'll need more room for the additional circuits and the main panel is full.

    I plan to use a GFCI breaker in the sub panel to feed the heater. That all looks pretty straight forward, but if there are "gotchas" that I should look out for I'm open to input.

    Also, any other comments on the whole job are welcome.


  2. #2
    Join Date
    Nov 2005


    Yes, you will need a separate ground wire from the main panel to the subpanel. This will connect to the neutral/ground bus on the main panel and to the (added) ground bus of the subpanel. This added ground bus is screwed to the enclosure. Do not use the green bonding screw that comes with the panel.

    You do not need a separate ground rod, that only happens when the subpanel is in a separate, detached building.

    I don't understand why you need to get two #2 wires under the same screws, That's not allowed anyway. You may use #2 for the ground, #4 will also be acceptable.

    There is no need to run the bathroom heater off a GFCI breaker. GFCI in the bathroom is only required for the 120V receptacle.

  3. #3
    Join Date
    Aug 2004


    I'll try to simply your application and househelper can chime in if clarification is needed.

    Assuming your using thhn/thwn individual copper conductors (must be in conduit).

    #2 copper thhn/thwn is good for 115 amps if the load centers are rated 75C for terminations. All modern load centers are so rated. But if they are old they may not be and this will change the ampacity to 95 amps.

    You need 4 wires in the feeder the minimum would be 3-3-3-8... in your case it looks like you will have 2-2-2-2 if using a 100 amp breaker to protect the feeder. The neutral can be smaller but lets not go into that. You need a #8 copper ground minimum for 100 amp ocpd.

    Neutral and ground are not electrically connected in the sub-panel in your situation. In a Homeline panel there is a green screw as househelper mentioned that is either factory installed or you install it your self. You do not use it so remove it if it is factory installed. This screw is the bonding means to bond the metal of the panel to neutral bar. It location is shown on the panel door spec. sheet. With the green screw removed you should not have any continuity between the metal of the panel and the neutral bar. If the panel did not come with auxillary ground bars you will need to purchase one maybe 2 for convienience of wiring. You will see some predrilled holes on the back of the enclosure. Usually Homeline has three sets of holes left side, right side and bottom with the lugs or main breaker located at the top of the panel. Your ground bars are fastened utilizing these holes. At the main panel you connect the two hots to the double pole breaker the neutral to the neutral bar lug and the ground to the ground bar. This may be the same bar as the neutral or it may be a seperate bar... regardless in the main panel bonding is required for the neutral and ground so it doesnt matter. At the sub-panel connect the two hots to the main lugs or breaker whichever you have then connect the neutral to the neutral lug. Connect the ground wire to the ground bar you installed. Do not connect it to the neutral bar. All branch circuit grounds from the breakers will connect to the ground bar and all neutrals (white wires) will connect to the neutral bar. Do not intermix.

    You do not install a ground rod.

    Ground bar kits are readily available for square d panels at the big box stores.

    Note: A #2 ground will not fit the ground bar without buying an oversize lug. I suggest due to the short distance between panels just spring for some green #8 copper cut to length at the supply house and forgo the #2 stuff for the ground wire.


  4. #4

    Thumbs up Thank you, thank you!

    Thanks for the detailed responses. This gives me the info I need. I did go to Lowes today and picked up a ground bus bar and a large connector for it that -- I hope -- will accommodate the #2 wire that I already have. If so, I'll use it rather than get more wire. I do plan to wrap the ends of it with green tape to indicate that it's the ground wire.

    Again, thanks for the help!

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