I will be installing new light fixtures in two bathrooms that look like this:
Each fixture will be replacing a light box. Meaning it is a simple chrome plated box about 30" wide with a plastic diffuser and sat on top of a mirror.
The challenge: The old light is connected to the power cable and there is no work box, it was simply screwed to the wall and wired up. The wire comes through the wall via a hole as big as the cable.
I plan on using an old work box to not only contain the cable, but attach the new light fixture to it.
My question is: How much weight can the old work box hold? (As it will be secured to the sheet rock and not a stud).
Best Answer
From what I could find, it doesn't look like these types of boxes are listed for any type of fixture support.
When looking at a box that attaches to a stud/joist using nails (B520P), it states.
With the the old work version(B618R), it states.
A box like this (BH614R), states.
The last page of this catalog shows some round boxes, and the weight they are rated to hold.
Based on this information, I would say the box you want to use will support less than 10 lbs.
The best options in your situation, would be to move the fixture location to allow you to fasten the box to a structural member, or add a structural member to mount the box to.
Update:
I contacted a company that manufactures these, and here is their response.
The link for boxes rated for fixture support, pointed to boxes that attached to structural members.