The very dimensions of the (octagon) Carlon J-box were selected for ceilings. Octagon boxes were originally crafted for ye old classic porcelain Edison socket style outlet. (key-less or switched)
Subsequently, other light fixtures were dimensioned to directly affix to said octagon J-boxes – or to enclose an adapter plate for same. (spider ring and/ or lumière strap)
The additional weights involved caused the NEMA players to come out with up-rated designs that could handle ever heavier lumières. These became known as “hanger boxes.”
In the latest incarnation, hanger boxes have been up-rated to “fan-rated.” Not surprisingly, considering the price premiums and the usage, all such boxes became known as ceiling boxes.
That's a thumbnail history of the octagon box, and its (round, ie Romex-BX era) sisters.
[ The octagon provided flat surfaces facing side-ways in an era when piping was common – all of this before BX or Romex. It's that old. ]
If I was piping towards an Edison socket, I might still use an octagon box.(metallic) But, if you visit any big box store you'll notice that octagon boxes are (now) thinly stocked. (metallic or plastic)
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Any ceiling rated box is also sure to be rated for (less demanging) wall usage. You stumped the tech rep.
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I have to say that in my humble experience, pancake J-boxes are the universal norm for stucco exteriors. Hereabouts, stucco is always applied to OSB/ plywood. (exterior grade) So it's no big deal to punch a hole after the sheathing is up and slap on a pancake – which can be placed just about anywhere to suit and at great speed, certainly far faster than setting a box that requires punching a big penetration with a hole-saw.
(The carpenters have finished the rough-in, setting the box and penetration is all on the electrician. The stucco contractor is expected to mesh-in around the pancakes and slather on his green coat, etc.)
Pancakes beat octagons, hands down. Again, these are terminal boxes that only feed a sconce.
The very dimensions of a pancake make it automatically perfect for exterior stucco.
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Lastly, my practice is to always screw such pancakes down. Nails are shunned.