I am replacing an old fuse box in our (detached) garage with a modern breaker panel. What I've discovered is that the breaker boxes I can get from Home Depot have a neutral bus in them, but no ground bus. Am I just supposed to buy a ground bus and screw it into the bottom of the box? How does it make sense for a sub panel to have a neutral bus but no ground bus?
The detached garage requires a "disconnect switch" and several things qualify.
- All the breakers can be the disconnect if you can turn them off with 6 hand movements. (feel free to use handle-ties here including 3-phase handle-ties if your panel isn't Homeline!)
- A panel with a main breaker can be chosen (they're cheap) and then the main breaker counts as the disconnect, even though it's utterly useless as a breaker.
However, if you buy a panel with a main breaker, they will assume you are using it as a main panel, which does not require a separate ground bar. So they won't give you one. They are easily purchased as accessories, and they are cheap. The panel label will list models designed to fit on the pre-tapped screw holes and nubs on the panel now.
If you buy a main-lug panel, the manufacturer ought to guess it'll be used as a subpanel, but some manufacturers refuse to throw in the accessory ground bars because they're cheap. That's whatcha get for buying cheap.
Yes, in any modern subpanel, you will need ground and neutral separate. Note that many older subpanels installed before a separate ground was required from main panel to subpanel did not have that ground wire and therefore had ground and neutral together in the panel. When that's the case, you don't need a separate ground bar because grounds can go on the neutral bar. That being said, even if not required (i.e., even if grandfathered), a separate ground wire is a good idea, and if you install one then you should definitely have a separate ground bar (and no neutral/ground bond) in the subpanel. If you increase the capacity compared to the previous fuse panel then you will need a new feeder cable from the main panel anyway, which will include a ground wire.
A few additional key things:
Generally speaking, when you have a subpanel in a detached building you need to have a disconnect and ground rods. As I understand it there are some exceptions, such as when the two buildings are physically detached but connected by a covered walkway (gross oversimplification, but I am pretty sure the rule is something like that). So what you call "detached" may not have these requirements. That being said, a disconnect is really easy if you use a "main" panel instead of a "sub" panel, as you have a main breaker. That is convenient for doing any work on the panel, as well as a required safety feature.
If you are adding more electric things to the garage, such as electric vehicle charging, then you should do a load calculation. Actually two load calculations - one on the main panel/utility service (to determine how much power you can send to the garage) and another on the garage (to determine how much you can use for big loads such as EVSE or welding or other things).
Do not be confused by "circuits vs. spaces". A panel might say "8 spaces/16 circuits", but that depends on using 1/2-size breakers. Those 1/2-size breakers are not available with GFCI or AFCI, and almost every circuit in a garage will require GFCI. A big main panel can be well worth the cost compared to a small sub panel. For example, in a quick search on Home Depot, I found a main panel with extras for $90, including 100A main breaker, 20 spaces, 1 20A breaker (use that for a receptacle circuit with a GFCI/receptacle) and 1 double 30A breaker (perfect for EVSE, which doesn't need GFCI/breaker) and a ground bar.
Keep in mind that you can feed a 100A panel from a 60A breaker in your main panel, as long as you do a proper load calculation to show that 60A is sufficient.