Your local/state building code applies of course but normally a conduit is not necessary if the wire is enclosed in dry wall or otherwise isolated. For example, I ran my 100 amp subpanel from the main located in the garage to the my shop which is an extension of the garage. We ran the wire inside the wall from the main up over and onto the floor of the garage ceiling and it lays exposed on the ceiling joists until it comes down through the wall in the shop. And to be on the safe side, I used a 100 amp breaker in the main to service the subpanel.
Incidentally, got my subpaner from HD and they did have a 100 amp, 20 circuit at a decent price.