I have a 30lb light that I'd like to install into an existing light fixture box on a concrete ceiling. My understanding is that most light fixture boxes should be rated to hold 50lbs.
However, 30lbs is reasonably heavy, so I'd like to do a bit more due diligence. I took a look at the fixture box -- it is a pancake style one rated for 50lbs. It is attached to a concrete ceiling with two of the anchors shown in the tech sheet here: https://anchors.dewalt.com/anchors/_documents/uploads/DWANF_ZamacNailin-TP-EN-rB_DDS1.pdf?1697572133
The tech sheet says 'not recommended for overhead use', but does make a comment about using a 20x safety factor for sustained tensile loads. I don't know how deep the anchors are installed, but if we take a guess at 3000psi concrete and 1 inch anchor installation, it looks like each of the two anchors are rated for 700lbs or so... in total, about 40x more than the 30lbs I'm hanging. Of course, my estimates could be wrong, so let’s say that things net out at a 20x safety factor. So, there’s a really good safety margin, but the ‘not recommended for overhead use’ is concerning.
In practice, are these sorts of anchors often used for light fixtures? Have others had experience with these working/failing? It’s not clear to me whether the overhead warning is the company covering themselves, or whether these anchors are really not reliable in this scenario.
If these anchors are not enough, what sorts of anchors would a good contractor use?