When installing a ceiling box that may be used for a fan, and you install a spare switch leg to that box. Such as 14/3 or 12/3 NM cable, the box shall be listed for support of a fan. This requirement only applies to single and multi-family dwellings. The code panel recognized that home owners and/or renters sometimes change out the light fixture and put a fan in its place. So this requirement will help when this activity is done in a single or multi-family dwelling unit.
