OK. Suppose you're thinking about wiring on your boat. You have three
"house lights" along the port side -- one over the nav desk, one over
the settee, and one in the V-berth. A reasonable thing to do
is run a pair of wires (+12 and GND) and take "taps" off them
at each point, for each lamp. Something like this

+12-----|--------|--------|
| | |
GND-------|--------|--------|
| | | | | |
X X X

where each "X" is a lamp. (View with constant width font!)

That's what's done in lots of boats, and lots of houses too. It
works OK. You have to size the "bus" wires to handle the aggregate
current of the three lamps, in case they're all on at once. You have
to worry a but about how you're going to attach the "taps" to the
bus wires. And you have to live with the fact that if the fuse/breaker
blose, you lose all three lamps at once.

Alternatively, you could run three sets of 18/2 wire (assuming low
wattage bulbs, short runs, etc.), but that seems a little silly.

Now...suppose you have a navigation light as well.

It's powered by a different circuit, with its own breaker. I'll call
this "12A".

Choice 1: run a 16/2 pair from 12A/GND-strip out to the nav light.

Choice 2: run a 16/1 wire from 12A to the nav light. Attach the other
side of the nav light as another "tap" on the ground bus shown above,
carefully resizing it to handle the larger load if necessary.

Assuming that you're doing all this in the *planning* stages rather
than after the fact, choice 2 doesn't seem so bad. Ignoring the
resistive effects of the wire (which are relatively small in this
application), it's electrically equivalent: the GND half of the 16/2
pair is attached to the ground -strip at the directribution point,
as is the GND bus above, so they're at the same potential, to a first
approximation.

As I'm thinking about wires and cabling, etc., it makes we wonder: "Do
I want to have dozens of circuits coming back from some cable, with
one wire going to the breaker panel, and the other wire from each
going to one of dozens of screws on a ground-strip? Or you I rather
just have one "master ground" that serves as the ground for ALL
the circuits that come out of that cable?"

Any thoughts here?

-John Hughes