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.