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Previously, I asked another question regarding this ceiling fixture. I wanted some advice on how to go about attaching a fixture with more than one bulb and whether or not the insulation in the old one was possibly asbestos.

Those questions were answered satisfactorily, but Harper commented: "This looks like a common 4” (trade size) pancake box but might be the dreaded 3-1/2". In reality, mine seems to be closer to 3-1/4". Not even 3-1/2"...

Why is the 3-1/2" size pancake box "dreaded"? And if mine is even smaller should I dread it more? Will it affect my ability to find a new fixture?

For reference, the screw holes that the old fixture screwed into are 2-3/4" apart which seems a little more standard.

In my photos, the box looks like it's 3", but it's just the angle. It's really about 3-1/4".

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Many things don't fit 3-1/4" boxes

There are quite a few fixtures and other pieces of hardware that simply do not fit a 3-1/4" round or octagon box, as their mounting holes are too far apart. This can be sometimes fixed for fixtures with the aid of a "universal crossbar" that adapts the mounting holes to the right places, but in some more exotic situations (like where excess box fill or the nature of what's being hung off the box forces you to put a mudring or extension box on for volume or mounting space), the requisite parts simply aren't made.

  • Thank you! I think I understand now. Thankfully, the screws seem to be at a distance that is more common than the 3-1/4" box. So I think a universal crossbar should work. – Tagger Nov 9 '17 at 1:21
  • Do you know if there will be any issue using it with a drop ceiling? I suspect not, but... – Tagger Nov 9 '17 at 1:21
  • @Tagger -- is the drop ceiling combustible? – ThreePhaseEel Nov 9 '17 at 2:52
  • How can I tell? Could that be why there was insulation in the old fixture? I believe the tiles are fairly old - perhaps the 1950's-1980's. – Tagger Nov 10 '17 at 1:17

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