I have a existing 6 circuit 100 amp sub panel.It has the grounds running with the neutral wires. I need more circuits. I bought a 20 space main 100 amp breaker box. It doesn’t have a ground bus bar. It has a 100 amp breaker with two poles for the hot wires and a neutral bus bar, no ground bar. My understanding is that you cannot connect the ground to the neutral bus bar in a sub panel. Do I just add a ground bar? If yes does it have to be grounded to the earth with a grounding bar? The sub panel is 32 feet from the main. Or could I remove the connecting bar on the neutral bus bar and use one side for the ground bus bar and one side for neutral bus bar.
If you want to use that panel, you will need to buy accessory ground bar(s) for it. The label will state the specific model numbers of ground bar that will perfectly fit the sites already provided for mounting ground bars. Expect to pay under $10.
They omitted the ground bars to make the panel cheaper, since they expect most users will be using it as the main panel.
DO NOT break up the neutral bars. They probably gave you just enough, and you'll need them. Yes, they are expecting grounds on the neutral bars, but they expect you to put 3 grounds on 1 neutral lug. So only 33% more neutral spaces than needed for neutrals. A 30-space panel typically has 40 neutral spaces, etc.
You need a ground wire back to the main panel. Fullstop, final answer. (unless of course you have EMT, IMC or RMC metal conduit, where the metal conduit shell is rated to be the ground wire).
If you are in an outbuilding, then you need a ground rod ALSO. It does a different job than the wire.
The ground and neutral bus/bar must be electrically separated/isolated from each other in a sub panel.
If you have two buses that are/have only a wire connecting them, then removing that wire should do it.
The neutral bus/bar needs to be insulated from the metal of the panel.
The sub panel if in the same building just needs the ground bus/bar connected by a ground wire/metal conduit to the ground of the main panel. The neutral bus needs connecting to the neutral of the main panel also.