Originally Posted by Jim Port
The box only needs to be fan rated if it provides the sole support for the fan. It sounds like the bracket screws into a framing member and not the box.
Some plastic fan boxes also have a metal plate above the box to provide support.
I suspect you have a box that straddles a joist. A pic would confirm this.
The fan bracket screws into the box, using a pair of what are probably #10 screws, which are very long, but I doubt they go any further than the bottom of the bosses in the box.
Since the fan was mounted this way before, and the other one is mounted same way, I don't think it's going to be a problem, but I think I will go up in the attic and see how the box is secured. Should be easy, since the floorboard was removed or cut to install the box.
EDIT: Just checked the RACO site, and learned of something called "The Insider", a plastic insert for the box when type NM cable is used. This may be what I have. I will have to check from the attic.