In our area the rule of thumb is 2 ft. Sometimes we have a prob and have been given the ok for 3 or 4. I once got permission to have 5' inside but the AHJ made it clear that they were only granting this permission because the panels (it was actually 20 townhouses in a row, all identical) were in rooms that were separated from the main house by 4 masonry walls, concrete floor and fire rated ceiling (furnace was in the room also).
