You are assuming wire is electrically same at both ends.
Not true. As wire carries more current, then those electrical
differences become more obvious. Others will provide (and
have previously posted) why these neutral and safety ground
wires remain separate. However underlying all those reasons
is one fact: wires are electrically different at their
opposite ends. When measured, those differences were not
detectable for electrical reasons. Connect a major load such
as a steam iron to that circuit. Then take those voltage
measurements again.

That single neutral ground bus bar is the central safety
ground that every building must contain for specific safety
reasons.

Sasha wrote:
I have a fundamental electrical question. I have several years old
Siemens 150 A main panel that has single neutral/ground terminal. All
ground and neutral wires are attached to it. There are also only three
incoming wires into the panel: two 220V hot wires and one ground wire.
So what I see there is no difference between ground and neutral wire so
the questions is why wiring is done with three wires instead of two? Is
it possible (legal) to connect ground and neutral wire together in a
switch box? As far as I understand from books I read most panels have
only single neutral/ground terminal.