I'm trying to install a new chandelier in the ceiling of a brownstone with an old plaster light medallion. It looks like there is a rectangular junction box in the ceiling above the medallion that is not made to support a lighting fixture. It is screwed directly into a beam. To replace the junction box, I would have to remove the medallion, which may damage it beyond repair. There is no attic/crawlspace access. Is there an alternative way to hang the chandelier?

If you can mount the chandelier with hardware that is independent of the box it should work. Make CERTAIN that you know what you are doing. A 5 to 1 working load weight safety margin is minimum. No threaded fasteners through the box where the wiring is. And I assume no responsibility for how you apply my advise. It is up to you to be aware of the applicable codes and standards that apply here.

EPM

You say that the box is screwed to a beam. What makes you think it is not designed to support a lighting fixture? How heavy is the chandelier?

You say that the box is screwed to a beam. What makes you think it is not designed to support a lighting fixture? How heavy is the chandelier?

The only threads on the box are tiny ones that appear to be for a cover. There is no other way to attach the strap hanger for the lighting fixture.

Get a strap similar to the ones shown. If the box does not have unused holes, drill two holes through box to joist. Use wood or sheet screws to attach strap to joist. Screws should be 1" longer than the box is deep. Drill pilot hold into wood. Its old and dry and hard, you don't want to wring off a screw as you are screwing it in. Use threaded lamp rod to attach chandelier to strap.