we got a 30x40 I did a big job on about 5 years ago, while on the electrical I wired in everything for A/C furnace thermostat, radient heaters, three phase phase converter, electric water heater,cieling fans, lots of 220, I tried to think of every thing to make my life easier as it grows, I still dont have the A/C or furnace in that one or the water heater but because I was covering the walls with metal I wanted all the wire I could think of in there so now adding these things is easy the wires are there where they all need be. we just built a 42x62 building and I'm doing a similar deal to it, the breakers and wireing is in for cieling fans ac furnace, water heater, 100-50-30 amp 220, rv plug for both 30 and 50 amp, garage door openers both buildings the wire is in and hot, makes it easier later. of course theres nothing wrong with running it exposed later, just the way I kinda got that part over with, also put conduit from the house to both buildings thru the slabs for tv computer wires etc and water 4' deep from the house thru the slabs to both buildings. dont forget 30-50 anp rv plugs and 110 plugs outside.