Let us say that we have a fan rated 4" octagon box in the ceiling, but the box doesn't supply enough space for all the wires up there (quite possible if the fan wiring is complicated, for instance with multiple switch loops in a multi-way setup, or with a relay involved). With a normal box, you'd simply slap a matching extension (4" octagon in our case) on the box and call it a day.
However, since there's a big, heavy ceiling fan that needs to hang from this box, how do you attach the ceiling fan securely to the extended box, or does the extension need to be fan rated as well for this to be (NEC 2014) Code-compliant, considering that the fan bracket needs to be solidly mounted to the box for a typical bar-type fan-rated box? (Vs. the fan bracket screws penetrating a joist in the case of a pancake or saddle box, which makes extension easier strangely enough as you simply need a long enough pair of screws)
Best Answer
422.18 paddle fans shall be supported independently of an outlet box Or by a listed box or outlet box system identified for the use and installed per 314.27.C.
There may be listed box extensions out there for paddle fans but I have never seen one. To be code compliant the better route to take with a paddle fan that has more conductors than the box fill allows would be to add a second box. If the ceiling (attic) above the fan is accessible this would be fine. If the ceiling (2 story fan on main floor) above the fan is not accessible a second box could be cut in and a cover plate attached after being wired. After seeing the damage done to the electrical system and home when a DIY home owner incorrectly installed a nice fan I would strongly recommend making sure the fan support box is listed for the weight of the fan or more.
An additional code reference for those hanging chandeliers weighing more than 6 Lbs. If a luminaire weighs less than 6lbs it can be supported on other boxes or plaster rings with no fewer than 2 #6 screws 2014 NEC 314.27.1 exception 1. (even a simple light fixture weighing more than 6lbs requires a listed box).