HPM piping may be installed within the space defined as a wall in an exit corridor provided an automatic sprinkler is installed within the space. It may NOT be installed if the space is less than ____inches in its least dimension?

HPM piping may be installed within the space defined as a wall in an exit corridor provided an automatic sprinkler is installed within the space. It may NOT be installed if the space is less than ____inches in its least dimension?