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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?

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  • "Not recommended for overhead" might be legalese. A weight falling on your head is usually a bigger lawsuit than falling on your toes. On the other hand you are depending on fiction and someone else workmanship to hold weight above your head.
    – crip659
    Commented Oct 18, 2023 at 12:43
  • On page 3 of the PDF, it specifies the spacing of fasteners. For spacing, if I read it correctly, it recommends "10d" (I believe that's 10 diameters of the fastener) for full anchor capacity. Don't know your anchor diameter, but if it's 1/4", that would require the anchors be 10/4" (2-1/2") apart for them to make their fully rated tensile strength. If they're closer than that but more than "5d" then you only get 1/2 the rated tensile strength.
    – FreeMan
    Commented Oct 18, 2023 at 13:51

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I have seen these used in many new building with concrete ceilings and have personally installed them in transformer vaults for racking cable. I've never had one pull out. It all depends on how well the instructions were followed as far as hole size. When dealing with concrete ceilings, I have always used lag shields and lag screws. You will probably be OK since there are two of the anchors installed but if it's going to keep you awake at night, go for the lag shields.

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