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.

The ground wire and neutral wire in a residential electrical
system should be connected together at one -- and only one --
point in the system: where the service enters the house. Thus,
if you have a 220V service, your service entry panel will have
two hot wires and a neutral coming from the utility company.
The panel will be grounded to earth. At that point, the
neutral is connected to ground.

Connecting the neutral and ground together anywhere else but
the entry service could result in a potential difference across the
ground wires throughout the system in case of a short -- a
dangerous situation.