National Electrical Code Articles and Information

1. It must completely enclose the device on sides and back, provide substantial support for the box, and not have its mounting screws serve double duty as device supports [314.19].
    

2. A quarter inch [319.20].
    

3. An eighth inch [319.21].
    

4. Zero. Use the correct depth box. If you need a surface extension, you can use an extension ring listed for the purpose [312.22]. You can't use another box as an extension ring.
    

5. It can contain splices and splicing device, but no other devices. It can support equipment such as a luminaire or ceiling fan. It can't be bigger than a hundred square inches (the NEC doesn't specify a max depth or volume). It must have threaded entries or hubs identified for that purpose. It must be supported by at least two raceways, each of which must be supported within 3 feet of the box (or 18 inches of the raceways are on the same side of the box). You will find these requirements, and five exceptions, in 314.23(E).