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As part of a home renovation project I discarded my old ceiling fan and have bought a new one. When installing the new one the electrician tells me that I have a 3" junction box and that I'll need a 4" junction box to install the fan. Now, this is a Manhattan high rise and getting to make any changes to the concrete ceiling would be impossible (to replace the 3" junction with a 4" junction).

Can someone advise me of a workaround or perhaps a fan that will work with a 3" junction box?

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Are there more wires necessitating a larger j box? (did you add lights with the fan?).. Its not clear WHY you need a larger box. Why would clarify WHAT can be done.. Photos? –  HerrBag May 13 '13 at 22:06
    
Can you perhaps provide a picture of the current box with nothing attached to it? You may be able to use some all thread, greenfield, stainless anchors and what's called a pancake box - but I'd really like to see it before I endeavor to write up the recipe for such a kludge :) –  Tim Post May 25 '13 at 16:00
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2 Answers

If the ceiling is concrete, you can mount the fan to that. Fan boxes are meant to support the fan, but a concrete ceiling will support it just fine. As long as there's enough room in the box for the wiring, use the 4" fan bracket to mark two holes outside the box on the concrete that won't interfere with the wiring coming out of the box, and use appropriate tools and fasteners to attach the fan bracket to the concrete around the old box.

If you can't modify the box, chances are good, being embedded in concrete, that it'll still hold the heavier load of the fan designed for the bigger box, and you can modify the fan bracket, or use an adaptor.

If none of these solutions seem workable you can buy a 4" box, and mount it to the 3" box, then mount the fan to it. It's not ideal, as the fan mount would no longer be flush to the ceiling, but it should work just fine, and provide all the room you need for wiring.

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In the case that you need the 4" box because of the fan mounting points then I can think of two solutions:

  • Drill out the fan mounting bracket to include holes with 3" spacing
  • Fabricate a metal 3" to 4" adapter plate. You may be able to purchase one adapter patent
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