Assuming they are 1/2" deep steel it would depend on how they are attached to the joists. Are they on brackets that span between joists? Are they attached directly to joists? How heavy are the fixtures?

I think you mean a square box with no attachment to framing, only held to drywall with ears, clips or tabs. They don't belong in the ceiling, they can take no weight. Ceiling boxes need to be attached to blocking or framing.
"Old work box" can also mean a metal box that is modular and can be ganged together, and was often used to retrofit electricity to houses 75-100 years ago, they can be fastened to framing and be really sturdy.
Casey

FYI, they do make old work ceiling boxes.

http://www.menards.com/main/maintenance-repair-operations/lighting-electrical/wiring-connecting/electrical-boxes/pvc/carlon-old-work-box-with-3-zip-mount-retainers/p-1420208-c-6431.htm

The manufacturer wont tell you a weight limit for liability reasons. Their main purpose is as a junction box. It is not the same idea as an anchor on a wall, the biggest problem is the drywall fastners holding the weight of the drywall up against gravity to the ceiling joists. Too much weight on the ceiling drywall can pop the nails or screws. Of course the plastic tabs can only hold so much, but ive installed plenty of old work boxes for wall outlets and they are pretty much in there, I cant forsee ever pulling them out.

http://www.carlonsales.com/techinfo/faqs/FAQ-ZipBoxes.pdf

"The B618R is a round old work outlet box and is not listed for fixture support in the ceiling. Only approved ceiling boxes are allowed to support fixtures in the ceiling. These boxes will be permitted to support a minimum weight of 50 pounds. The B618R can be used in the ceiling as a junction box. Do not hang a fixture from it since it relies on the strength of the ceiling itself."

For boxes in the ceiling supporting any weight, I would only install ones that were attached to the joists.

I would never use an old work, plastic box in a ceiling for any purpose. Even if it was just a junction box with a cover plate, someone could always come along later and decide to hang something from it.

analogmusicman:

I'm assuming you haven't covered any of these boxes with drywall?

got my answer: the "old work" ceiling boxes you buy at Lowes are made by "Cantex" and are rated for fixtures up to 6# per the Cantex website.

tnx,

Can you use a 1/2 x 4 round pancake box screwed directly to a joist?

just a comment then I'll go away: it's ridiculous to assert that "old work" ceiling boxes are only good for junction boxes and not support ANY fixture since they basically attach to the ceiling just like "remodel cans" and I have yet to hear even the most "rabid" code junkie claim that THOSE can't support anything! :-)

tnx,

Remodel cans are a fixed, light weight of 1-2 pounds, and not the same thing as an old work box which someone might attach anything to, like a chandelier or a fan.

They can support some weight, but remember if you have a fair amount of insulation weight pushing down on the ceiling and then add a point load of 10-20 lbs of light fixture, the ceiling might break. I agree the box will hold weight but the ceiling might not. Dont overdo it. These boxes might actually be best for things like vanity lights and sconces. The box will hold the weight fine but you dont have to worry about the wall pulling away from the studs.