I'm wiring a new Square D Homeline, 200 amp main lug/main breaker panel. The meter and a separate main breaker box is located outside the house, so the panel I'm working on is a sub panel, which also contains a main breaker and 40 spaces. The panel has a bus on the right side and a bus on the left side. Will I need to treat both buses as neutrals and purchase and install a separate ground bar? If so, do I bond that ground bar to the panel with the large green screw that came with the panel? If so, where should I install it (or is it just a matter of preference?)? Lastly, since this is a sub panel, do I need to keep ground bar separate from the neutral buses? (I ask that because other posts suggest that I do.)

Bear with me here as I am not an electrician. The good news is that all of my work will be double checked (and ultimately inspected) by an electrician before we power anything up.

Don't use the Neutral Bond screw, Keep Neutral Isolated from ground and bolt ground bar
To back of panel.

Don't use the Neutral Bond screw, Keep Neutral Isolated from ground and bolt ground bar
to back of panel.

So, will I need to buy and install a separate ground bar? What do I do with the large green bonding screw that came with the panel?

Throw that screw in a drawer. If you screw that to the back, you will now have all ground bars and no neutral bar. The Neutral Carries current and should be Isolated, The ground connects to all metal boxes enclosures. You don't want your ground to carry current.

Thanks Strat. I was just reading an old post by Labman and the follow-up exchange between you, tk and Labman concerning the question of why all panels don't come with the accessory ground bar.

So, to be clear, the bars on the right and left side of the panel will be treated as neutrals and the ground bar that I need to buy and install will be screwed directly to the back of the panel. All greens and bares will be attached to the ground bar. I will toss out the green screw (or make it into a necklace for my niece as her favorite color is "gween"). Can you confirm that this is correct?

You've got it. Good Luck.