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Plastic ceiling boxes in fire rated ceilings have to be spaced certain distances apart depending on the manufactured product. What about metal ceiling boxes? I have never seen any restrictions on their installations. Looked all over, called UL, no one semms to know - but I'm sure its out there someplace.

I believe you need to refer to the UL listing book that deals with fire rated assemblies (I'm sorry I cannot recall the proper title now and I'm not in my office). That is where the info is on the permitted penetrations for each rated assembly. You may be able to get some of the info on their website but I think for a specific assembly (construction and fire resistance time)you will have to go to the actual listing.

I have looked through the UL fire resistance directories and they contain ceiling penetrations for recessed and fluoresent lighting but no surface mounted fixtures that would require fixture boxes. The general descriptions in the front of Vol 1 cover metal wall boxes but not ceiling metal boxes.