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?
You're going to have to get rid of the circuit to the well if you go with the larger feeder. Article 225 deals with outside branch circuits & feeders & is frequently overlooked. You're right, you aren't running a service, but you're only allowed one feeder or one branch circuit (generally speaking). See 225-30(b)
You'll also need 2 ground rods unless you are able to test the rod resistance & it ends up less than 25 ohms.
Few things are harder to put up with than the annoyance of a good example.
Re: wiring a barn#4664 10/08/0107:25 PM10/08/0107:25 PM
Thanks very much. Tom, I see that 225-30 is the pertinent requirement. My situation does not fit any of the exceptions. And your point about ground rods is well taken.
Sparky, no animals in the barn. It's more a storage area for boat, tractor, etc. I also thought about animals & ground currents.
Your thought on Sec 220 & load calcs is right on; I wouldn't install a new raceway & feeder until I got an idea of how the mother-in-law unit would be set up (HVAC load, kitchenette?). Although the MIL unit is "just a thought" right now, I want the client to be aware that the upgrade we're talking about will not give them the power for the apartment, IF they ever do it.