Frank,
The 5 Amp current is flowing in a circuit thru the resistance of the ground rod plus the resistance of the wires, connections, fuse, the source impedance and, most importantly, the connection back out of the earth to the transformer.
The circuit resistances (other than the ground connections) will get larger as the transformer is smaller and further away, and as the conductors in the circuit are smaller.
A residential setting with a pole mounted transformer two spans away and with the ground rod tied at the service disconnect will likely have between a quarter and a half an Ohm in the path through the wire and connections.
Then, the current goes into earth at the ground rod, and, in the real world with several services connected to the same transformer, the current returns out of the earth at the pole ground below the transformer, perhaps a butt plate or a ground ball, as well as at any other ground rods that have parallel return paths to the transformer neutral terminal.
I'd hazard a guess that your theoretical ground rod has 12 to 22 Ohms in its connection to earth. The fewer returns out of the ground, the closer the resistance will get to 12 Ohms.
Al
