Electrical box too small/incompatible with mounting plate

Question

The mounting bracket for my new wall light fixture is incompatible with the existing electrical box. While it seems reasonable to drill the mounting plate so that the top/bottom mounting screws can be used, the horizontal mounting pins for the fixture extend beyond the cavity of the electrical box onto the plaster wall.

Is it safe to simply remove the plaster behind the pins so that the plate will lay flat against the wall? Is it safe that the grounding screw will be outside of the box on top of the plaster wall?

I'm handy, but I know very little about electrical work.

Answer 1

The inner set of slots should line up with your box screws, no need to drill the mounting. They are slots so the mounting can be rotated, and you can turn it 180 degrees if the end of the slots don't put the mounting screws level (that's why the mounts are off-set like that from the slot positions.)

The ground location out of the box is fine, it's a steel plate connected to a steel box. However, the ground wire coming in should be connected first to the steel box, which it's not. There should be a small threaded hole which takes a 10-32 machine screw in the box already, or you can buy self-tapping grounding screws if you have an odd box without a pre-threaded hole.

Yes, you can poke holes in the plaster for the mounting pins/screws.

Answer 2

The ground wire needs to go to the steel box first.

Somewhere in the back of the box should be a hole tapped #10-32 for a ground screw a lot like that one. Route the ground wire so it makes a "U" around that screw before it comes out to attach to the light.

Why is this a pain?

Because that's not the correct box type for a light. It's for switches and receptacles. A light box looks like this

(except with appropriate mounting flanges for your requirements).

That's why nothing seems to fit. You can see where the diagonal screws create an important role for that adapter plate :)

Answer 3

Those round mounting plates are primarily intended for use with round or octagonal fixture boxes. Some are usable with standard switch/outlet boxes, and some are not. The mounting hole distance of switch boxes is different from the mounting hole distances on any size of round/octagonal box. Most hardware stores will have a variety of mounting plates/straps, and likely some of them will work with what you have. The green grounding screw on the mounting plate is intended for use with the ground wire coming from the fixture itself, when mounting on a metal box. With a plastic box, the fixture ground wire would be directly spliced with the house wiring ground wire.