I recently bought a new fixture for my back patio light. (Just replacing an old halogen motion-detector light with a new LED motion-detector light.)
Unfortunately, the mounting bracket for the new light has screw holes which are 3 1/2" apart, while the existing junction box has screw holes which are 2 3/4" apart. (And I can't use the old mounting bracket with the new light, because the way it attaches to the light is completely different.)
First off, is it the new light which is unusual for expecting 3 1/2", or is it the house that's unusual for having 2 3/4" holes?
How does one normally solve this problem?
Is it possible to buy a light fixture that works with 2 3/4" mounting holes? Unfortunately, none of the fixtures I can find online or in the store say what distance of mounting holes they expect… it isn't until you open the box that it says what it's expecting.
Update: In the end, I didn't have enough confidence to DIY this, so I called an electrician. I'm not sure how he solved the problem, but he solved it.
Best Answer
The 2 3/4" and 3 1/2" sizes are both common. Many fixtures have screw slots for both.
A cross strap is a common way to adapt the larger size down to the smaller size. Screws to 3 1/2 box and 2 3/4 fixture mounts to strap bracket.
Some 2 3/4" boxes, particularly the metal styles, can allow a strap bracket to be mounted to the box and let the ends overhang the box so that screws on 3 1/2" centers can clear the outside of the box. In this case you can drill or open some holes under the bracket to allow the fixture mounting screws to go through the bracket and into the wall/ceiling.
You may want to also investigate the possibility to see if you can drill 2 3/4 mounting space holes in the fixture itself and forgo the use of any strap bracket itself.