Conduit / Best practices for a shared ground between branch circuits in residential?
You can barrow a ground from another circuit as long as they come from the same panel or grounding electrode conductor.
The size of the grounding conductor depends on the branch circuit. See table 250.122.
the maximum required size is 6awg when connecting to the existing GEC. (Even if tying into a #4 ufer ground or concrete encased electrode)
The minimum size is 14 awg if you are grounding a 15 amp circuit and since you won’t find anything smaller that makes 15 amp circuits Easy.
When you have a larger circuit let’s say a 60 amp you have to use a #10 ground
So you have to ground off of a #10 ground or larger (most electricians will pull a (#10 and use that for multiple branch circuits) if I am doing an entire home I will pull a #10 unless actually grounding a sub then I go #6 (I don’t carry #8 normally)
I run the length of the home in the crawl space or attic and drop a j box every 20’ or so. (Yes boxes are needed and the box it self requires a ground to the box if there is a splice in the box.)
If tapping the ground from a higher amperage circuit inside that is ok.
I do not run grounds in conduit unless outside and smaller than #6.
So just run 6 to a outside location.
Thhn/thwn dual rated green # 10 is my normal wire but it can be bare.
If in conduit, No you don’t color the conduit green.
I think I hit all of your points.