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Most light fixtures intended to mount to a wall or ceiling box don't actually intrude into the box cavity itself, leaving the entire box space for wiring.

Is there some code (in the US NEC, or in California) that requires fixtures to stay clear of the cavity of the box? Or is that mainly a practical concern -- it's harder to build practical insulation and it's a somewhat ill defined space to rely on? (I imagine this is in some code -- but I don't have the books, so someone who does, can probably answer!)

The reason I wonder is that it would be totally possible to build a LED fixure that's close to flush with the wall/ceiling if it's OK for the power supply to poke into the cavity half an inch, but I don't find manufacturers selling such fixtures. Fixtures I've found advertised as "flush" are thicker than I'd expect. (And then there's recessed lighting, which is something else.)

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    This is actually quite common for "Junction-box-mount" (or J-Box) LED fixtures. I have two examples in hand. There's a minimum of 4 cubic inches allowed for "device fill" in calculating "box fill" (or more, if not a 14Ga wire 15A circuit.) Happens the manufacturer pictures hide that on the example I just checked, though. – Ecnerwal Jun 13 at 22:33
  • Thanks for the answer! Feel free to promote it to a real answer and I'll accept it. – Jon Watte Jun 13 at 22:39
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    It will probably be in the UL White Book, the standard for appliance certification in America. Of course, lots of crud can be bought mail order (Amazon, especially) that was not manufactured to that standard. Such lights are illegal to install and are probably not safe. – Harper - Reinstate Monica Jun 13 at 23:37
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    As ecnerwal mentioned there is a device fill required. The reason most do not totally enclose the power supply is because of the heat loading. The same is true with dimmer controls many are only rated to 500w max for the same reason. – Ed Beal Jun 14 at 14:44

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