Is it OK to attach a metal electric junction box and armored cable (MC) to underside of the subfloor in a basement with an exposed ceiling? Or does it have to be attached to the side of the joists in a parallel run?
Some potential problems could be:
- You might not have much wood in the subfloor to screw into, so the box might not hold very much weight (if a ceiling fan or heavy light is possible in the future)
- You might puncture the floor above with the box's screws
- If someday the subfloor were to be replaced that would be a problem for the junction box
- Long screws or nails from new flooring could puncture or be stopped by the junction box.
Why not just cut & attach a 2x4 support brace between the floor joists, and then attach the box to it? Then you can screw securely to the 2x4, either on it's side or bottom. Like one of these:
Or like this view from above (an attic):
You could also buy a special metal ceiling fan support bar, like this one at Home Depot but it's currently $14.28 US, a small piece of 2x4 is likely much cheaper, or even free if it's scrap.