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My main panel has the neutral and ground to the same bus bar and I am planning to connect a subpanel for a detached garage. So I have a few questions.

  1. When I connect the subpanel to the main panel, does it matter which bus bar I connect the ground and neutral?

  2. Do I need to separate all of the other ground and neutral wires on the bus bar when I connect the sub panel?

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  • This is where you must not, confuse, confuse your- grounding and bonding. Remember, you or someone elses safety, as you must by now realize current WILL travel ALL AVAILABLE PATHS TO GROUND, rather (to complete your circuit DESIGN, just not the shortest available, uh, ☝ – Jonathan Todd Oct 26 '17 at 18:43
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bus bar singular?

There are supposed to be two bus bars in every panel. One for neutrals, and one for grounds.

In the main panel ONLY, they are to be bonded together. This is what references your electrical system to earth voltage. It must only be done one place in a system.

Because of the required bond in the main panel, a great many electricians conclude that there is no difference between neutral and ground in the main panel since the N-G bond is right there. And they simply spam all neutrals and grounds onto the same bus. That logic is actually... reasonable... in the main panel.

However... If you want to be a precise and competent worker, then you separate them always, as if every panel were a subpanel, and let the neutral-ground bond do its job. This has a couple of neat benefits for you.

  • if you convert that panel to a subpanel later, it's cake.
  • if you want to troubleshoot a ground fault, you can remove the neutral-ground bond, and replace it with a wire which you loop 10 times and clamp an ammeter around the loop. Now the meter indicates ground leakage at 10x resolution. It'll take you literally 30 seconds of snapping off breakers before you find which breaker it is. That was easy.
  • not that this is typically done in residential in the US, partly because of risk of damage to hardwired loads and controls... but you can separate the neutral-ground bond, then do insulation testing (megging) on the conductors to ground, to affirm no ground faults in the wiring proper. This is especially important at higher voltages like 480/277.
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    This is a great answer, not only explaining what should be done, but giving practical examples of why. Good job @harper, keep it up. – Ryan Griggs Oct 26 '17 at 18:44
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    The logic is reasonable but it is wrong! - Your have to be able to remove the Neutral-Earth link in order to properly test the installation - for example, you cannot do an insulation test between neutral and earth is their conductors are bonded together. – Dale M Oct 26 '17 at 23:43
  • @DaleM sounds reasonable to me, but I assure you, electricians in The USA did not get that memo. In the wild, seems like most panels clump them together, and there is never any insulation testing that I am aware of. At least not in residential, our scope here. – Harper Oct 27 '17 at 13:46
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To actually answer your specific questions:

How do I connect a sub-panel with neutral and ground to the same bus bar in main panel?

In the main panel it doesn't matter on which us bar you land the feeder's neutral and ground, but make sure the lugs you choose are rated for the gauge and type of conductor, and if using aluminum, you should use Noalox and the proper torque.

When I connect the sub-panel to the main panel, does it matter which bus bar I connect the ground and neutral?

No, It doesn't matter in the main panel. Yes, it matters in the sub panel. First, REMOVE the bonding strap that connects the two bus bars in the sub-panel. Then in the sub, it should be visually obvious that one of the bus bars is insulated from the case. If not, use a volt meter to find out. The insulated one is for neutral. The one that conducts to the case is ground.

Do I need to separate all of the other ground and neutral wires on the bus bar when I connect the sub panel?

Yes. In the sub-panel grounds and neutrals must be only on their corresponding bus bars.

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It is correct that the ground and neutral share the same bus bar in your main panel. The ground and neutral buses only need to be separated inside of a subpanel. In other words: Tie the subpanel's ground and neutral wire to either bus inside of the main panel, but keep them on separate buses at the subpanel.

If you have a subpanel feeding another subpanel, you would need to keep the ground and neutral lines/buses separate in both subpanels. They can (and should) only be tied together at the main panel.

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This is not a new topic. I had similar questions a few years ago. See here: Rewiring Garage, building grounds

To answer your questions,

  1. If you have separate neutral and ground busses in your main panel, I would connect it to the neutral as a general practice. The two busses should be connected anyway.

  2. Yes, you will need to separate them for the subpanel. Not the main panel. The subpanel requires either its own ground or a dedicated ground wire back to your main panel. I do not recommend the second option.

The reason for this is to prevent neutral voltage from returning to the main panel in the ground.

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