All references are based on the 2017 edition of the NEC, unless otherwise noted.
When it comes to deciding whether a fan-rated box should be installed at a luminaire location where a paddle fan could be installed, there seems to be confusion in the Code rules. There are no rules in the Code that require you to install a listed fan-rated outlet box — that is, of course, unless you are not actually going to install a fan. Yes, it’s true. You might need to install a fan-rated box in a location where no fan is being installed.
The second paragraph of Sec. 314.27(C) states, “where spare, separately switched, ungrounded conductors are provided to a ceiling-mounted outlet box, in a location acceptable for a ceiling-suspended (paddle) fan in one-family, two-family, or multifamily dwellings, the outlet box or outlet box system shall be listed for sole support of a ceiling-suspended (paddle) fan.” Now if a 2-wire cable is run to this location, a fan-rated box would not be required. Likewise, if a 3-wire cable is run to this box, where both ungrounded “hot” conductors were on the same switch or if one “hot” wire was simply not switched at all, a fan-rated box would not be required. But a fan-rated box would be required at a luminaire location if a 3-wire cable was run, and two separate switches are installed to control the “hot” conductors — one conductor being connected to the luminaire and the other one capped off as a spare for future use on the fan, which might never be installed. I find this all a bit ironic.
Section 422.18 also comes into play on this topic. It states that paddle fans shall be supported independently of the outlet box. It also gives us the option of supporting the fan directly to the box if a fan-rated box is used. But paradoxically, it never actually requires us to install a fan-rated box.
Yes, I understand the intent of Sec. 314.27(C) is to provide for the possibility of a paddle fan being installed in place of a luminaire. However, because paddle fans can be supported independently of the outlet box, and fans can be installed at luminaire locations with no “spare” conductors, I suppose the irony of installing fan-rated boxes where fans are not installed, and installing non-fan-rated boxes where fans are installed, will simply continue.