It appears the ceiling-light junction box is centered on a stud. As a result, the wiring is not passing through the center the three entry holes (the center hole is "blocked" by the wooden stud). Instead, the wiring is coming from a hole closer to the outer edge of the junction box.
I am attempting to install a new lighting fixture which has a circular mounting plate. The mounting plate has a large hole in the center that wires are supposed to feed through. The center hole of the mounting plate fits cleanly around the (unused) center-hole of the junction box. However, the off-center hole that the wires actually come through presses directly into the metal of the mounting plate. If I try to feed the wiring through the center of the mounting plate, when the mounting plate is tightened, it "pinches" the wiring. If I tighten enough to support the weight of the fixture, the metal of the mounting plate begins to cut into the wire - which will cause a short circuit (hot wire can come into direct contact with mounting plate).
Is there a simple fix for this? Are their mounting plates that are compatible with off-center junction-box holes?

 

Welcome to the forums.

It would be helpful to know what type of box you have there. Can you shoot and post a pic for us ?
http://www.doityourself.com/forum/el...-pictures.html

 

Sorry, having trouble getting photos off my phone, but I think I found a couple "close enough" pictures on the web that illustrate my problem. I was able to work through this issue using input from local hardware store (not sure what the policy is on putting in plugs is on this site, but I'd love to credit the franchise in question for being able to give solid, workable solution - contrast to typical "big box" experience). I'll walk-through the problem and solution, and ask for input on any alternate ideas, on the chance someone encounters something similar.

Okay, root problem was with the ceiling junction box (in center of room). The wires did not pass through the center "cut-out", which passed directly over stud. Instead, they passed through the side "cut-out" hole, as illustrated:
PICTURE 1:


This was a problem because the light fixture was shipped with a mounting plate that needed to secure full weight (25 lbs) of fixture to junction box. However, the mounting plate was designed for a j-box with wiring in center hole, as illustrated:
PICTURE 2:


The default approach is to feed the wires from the off-center cut-out of the J-box (RED-X IN PICTURE 1) through the middle of the mounting plate (RED-X IN PICTURE 2). But after doing so, as I tightened the 2 mounting screws (BLACK CIRCLES IN PICTURE 2), the wires became tightly pinched between the two plates.

The center ring of plate fixture actually cut into the wire enough that when I flipped the breaker on, it shorted ... The metal of the plate was cutting hard enough into wiring to making electrical contact with the hot/black wire inside.

The solution from hardware store was pretty clever (I thought). He had me purchase a metal-cutting hacksaw (side note: I'm always happy to add another tool to my meager arsenal), then use that to cut out the offending area of the fixture mounting plate, as illustrated here:


This solved the problem perfectly. The wiring could now feed directly through off-center J-box, through my cutout, and was easily attached to fixture lighting per instructions.

I was set-back $10 for a hacksaw, plus I splurged $25 for a (much needed) clamp set to help with cutting. That said, I'm still curious to know if there was a cleaner/better alternative. After all, the round J-box is clearly designed to let wires pass through one of 3 different holes, so I don't imagine it's *that* unheard of for wires to use one of the non-centered hole. ... yet all the pictures I see for ceiling mounting plates would seem to have same problem - they all only open in the center.

 

That's called a pancake box. It was the only box available in the past when a fixture needed to be installed in the exact center of the room and there was a joist there. That box is mounted directly to the bottom of a joist.

Now they make a box that mounts over the stud. There is plenty of room for the wire to pass thru to the light's canopy.

 

That appears to be a ceiling fan mounting plate. The box you posted is not rated for ceiling fans.

 

I believe he mentioned a 25lb light fixture. Some back plates come with the hook to aid in connecting the wiring before attaching the fixture.