DG wrote:
I'm wiring a subpanel in free standing garage.
70A circuit, on 4/3 conductor with a ground.

There is something that I don't understand:
The main panel has a bus that combinds the neutral and ground.
As I read about sub-panel wiring, the neutral and ground should be
seperate.

The seperate neutral and ground at the subpanel run to the combind bus
at the main panel, so electrically speaking, there is continuity
there..

Why wire them seperate at the subpanel?



They are not the same. There is current flowing on the neutral wire,
and because its resistance is not zero there is a voltage drop from one
end to the other. So the neutral bus is not quite at the same potential
as the ground when there is a load (unless it is a completely balanced
240V load)

You said it is a free standing garage, so you /can/ combine the ground
and neutral if the box is rated as "service equipment" and if you do it
right (there's lots of rules.) You're better off in my opinion, since
you have the separate ground already, to add a ground bus kit and remove
the bonding screw that electrically connects the neutral bus to the box.

Bob