IWire- nice smile! I don't want to get bogged down in what is a 'different system.' It just doesn't seem like a good thing if the "hot" wire comes from one panel, and the neutral goes to another- even if the two are simply separate load centers from the same service!
I believe the code says all conductors for a circuit must be in the same raceway....I would think that meant the neutral has to go back to the panel the hot comes from. If you don't identify the neutrals, it is all too easy to get them mixed up....and that's where the trouble starts.
So how do you identify a nuetral? My personal favourite way is to run a colored "Sharpie" along the wire, leaving a SINGLE stripe. That way, I know the red stripe goes with the red wire, etc.
