Provisions pertaining to the installation and use of all boxes and conduit bodies used as outlet, device, pull and junction boxes are in Article 314 of the National Electrical Code. Section 314.16 covers provisions for sizing boxes and conduit bodies containing conductors 18 AWG through 6 AWG.
Jim Dollard has an extensive background in codes and standards. If you have a query about the National Electrical Code (NEC), Jim will help you solve it. Questions can be sent to firstname.lastname@example.org. Answers are based on the 2014 NEC. Point of supply