Thanks for the replies.
1) You have a couple of wiring options and some items to take into consideration.
A) 1st off, if you ever decide to sell the property and the next owner decides to turn the barn back into a real agricultural barn for animals, hay storage, etc. then either Gray electrical PVC Schedule 40 conduit or possibly rigid metal conduit would be required due to animal vapors, corrosion, dust, hay, wash-down areas, etc. If this is a possibility, you would need to confirm the acceptable wiring methods with your local electrical inspector. Keep in mind that local electrical codes can always take precedence over the National Electrical Code. The local electrical inspector always has the final say-so.
Also keep in mind that this is a barn and a future home buyer will most likely retain a state licensed home inspector and the inspector would write up the barn as not being up to code compliancy if the next home owner decides to turn the barn back to a working agricultural barn. The NEC has specific requirements on agricultural buildings, thus the reason I mention these items that need to be considered:)
B) In general, if the loft area will strictly be used for office space, then I recommend installing Metal Clad cable (type MC). The MC cable will need to be protected from any possible damage such as stapled to the wall studs and the wall stud area covered by drywall or paneling, etc. The MC cable can also be secured directly to the ceiling beams and protected where required to prevent any future damage to the cable. Installing MC cable near the floor area would require some form of protecting the cable.......ie........ boxing the cable with 2" x 4" wood or some other form of protection.
Reply back and let me know if this helps and I can answer any other questions you may have..........Thanks...........Kevin:)