Generators can be inside buildings, so I'd say there is no minimum distance. And since we don't know what country you are in, it's hard to be very specific.
There may be some issue regarding the location of the fuel storage, assuming this is a diesel generator. This may also depend on the occupancy and the fire rating of the walls.
Other considerations (without providing a code reference, as I'm not aware of them pertaining to your location) would include:
Noise.
Discharge of exhaust and proximity to any air intake points for the building.
Fuel Storage and fire rating as per dpc's comment.
Access for servicing and maintenance.
Access to operate the unit (I'm assuming from the question its a standby generator).
This site has an NFPA forum that is pretty active. If you post a more specific question with all relevant details, I suspect you will get a good answer.
The relevant codes could include and or all of NFPA 30, NFPA 37, Georgia Building Code, Georgia Fire Code, NEC, and whatever the local building authorities say. Sorry but you really need to check them all.
Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.
@dpc - 130KW gen set, commercial building, two story. Used for emergency and stanby power (for nonemergency power). Looks like 5' is the absolute min, but I'll recommend 10'.
