Would appreciate your opinion on this--
A client has approached me to wire a 60' x 40' barn. It now has a 240V, 30A dedicated branch circuit supplying a well pump. Fed with 10/2 WG romex via 3/4" GRC underground from main panel at house (about 90').
Theere are now no other electricals in barn. Client would like to add a few fluorescent fixtures and a few general-use outlets. No plans for shop in the building, however, there is a partial second floor storage area that may be converted later to a mother-in-law apartment.
To meet immediate need, I propose to: pull out romex, pull in three AWG 8 THWN conductors (protected at main panel at 40A) to a new subpanel, using GRC as the EGC. Drive a ground rod. Bond panel to the well casing via galv supply pipe. Feed 240V to the pump from the subpanel. Wire in two 20A branch circuits--one for lights, one for plugs. Wiring in the barn will be exposed, so I will use MC cable. GFI receptacles.
Now, what if the client wants to instead run a bigger feeder (say 60-70A) to barn now to provide power to the future apartment? I'd run new GRC (or sched 80 w/4-wire feeder) to a barn subpanel. Should I leave well pump on the existing branch ckt from the main panel, or abandon the branch circuit and feed the pump from the new subpanel?
I don't see the requirement that a building have only one service as applicable here. This is not a service. I also do not see any obvious hazard from having a feeder and a branch circuit from the same panel come into the barn. Keep in mind that I would run both a neutral and an equipment grounding conductor to the subpanel (because the branch circuit conductors are parallel metallic paths between the two buildings. I didn't notice if the water pipe was galv. iron or PVC. If it's galv iron, another reason to run a 4-conductor feed).
Any thoughts about my general approach or the specific question?
Thanks in advance...