There is no requirement that hot work be done ONLY in areas that are sprinklered. However, OSHA does say at 1910.252(a)(2)(vi) that “Cutting or welding shall not be permitted … In areas not authorized by management” or “In sprinklered buildings while such protection is impaired.” In other words, if a sprinkler system is not functioning, or is down for maintenance, then hot work may not be done until that protection is working again.