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I will be installing new light fixtures in two bathrooms that look like this:

new light fixture

Each fixture will be replacing a light box. Meaning it is a simple chrome plated box about 30" wide with a plastic diffuser and sat on top of a mirror.

The challenge: The old light is connected to the power cable and there is no work box, it was simply screwed to the wall and wired up. The wire comes through the wall via a hole as big as the cable.

I plan on using an old work box to not only contain the cable, but attach the new light fixture to it. Old Work Box

My question is: How much weight can the old work box hold? (As it will be secured to the sheet rock and not a stud).

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Any fixture I've seen has an overlap around the opening for the box, so the fixture will actually be resting on the drywall. At that point, it becomes a question of how strong the drywall is –  Niall C. Sep 19 '12 at 14:46
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You really should be asking how much weight the wall will hold, no the box. The box will be stronger than the wall. –  GdD Sep 19 '12 at 14:47
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The product description at Home Depot says "Not intended for fixture support in ceilings", but nothing about walls –  Tester101 Sep 19 '12 at 14:52
    
The wall is in good shape. I figured I would ask due to the two small ears that will be behind the sheet rock. –  Carl B Sep 19 '12 at 14:52
    
@Tester101: and thus my question. –  Carl B Sep 19 '12 at 14:53
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2 Answers

up vote 4 down vote accepted

From what I could find, it doesn't look like these types of boxes are listed for any type of fixture support.

When looking at a box that attaches to a stud/joist using nails (B520P), it states.

Suitable for fixture support up to 50 lbs.

enter image description here


With the the old work version(B618R), it states.

Not listed for fixture support in ceilings.

enter image description here


A box like this (BH614R), states.

Listed for wall fixture support up to 10 lbs.

enter image description here

The last page of this catalog shows some round boxes, and the weight they are rated to hold.

Based on this information, I would say the box you want to use will support less than 10 lbs.

The best options in your situation, would be to move the fixture location to allow you to fasten the box to a structural member, or add a structural member to mount the box to.


Update:

I contacted a company that manufactures these, and here is their response.

[Link to product omitted]

As shown on the link above this item is not listed for fixture support. We do have several other boxes that are rated for fixture support.

[Link to other product omitted]

The link for boxes rated for fixture support, pointed to boxes that attached to structural members.

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It says, [Link to other product omitted] –  Carl B Sep 19 '12 at 18:41
    
+1 Great info - For those of us with aging eyes, the very small text at the end of the update says "The link for boxes rated for fixture support, pointed to boxes that attached to structural members." –  bib Sep 19 '12 at 21:00
    
I will have to check the weight. I can't really move it. It will need to be centered over the sink and the studs are not wood the're metal studs. But great info! –  Carl B Sep 21 '12 at 10:35
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The ears on an old-work box (or the molly-like metal flanges on some) are pretty small contact points behind 1/2" drywall (or even 5/8"). It might be strong enough, but you don't want a lighting fixture with glass shades tumbling on Junior, Rover or Grandma.

What about cutting the hole for the box, inserting a piece of furring or lattice, about twelve inches long horizontally along the top of the hole the box will go in. You can hold it with a string tied around the middle as you position it. Then screw through the face of the drywall to cinch the brace to the wall. Dothe same on the bottomof the hole.

Now you can mount the box using screws into the new bracing. Touch up the screw holes with taping compound or spackle and a bit of paint.

P.S. This is still not a strong structural support, but should be able to hold a moderate weight sconce. I can't give you a real weight limit. For a seriously heavy unit, you should have a real brace tied into the studs.

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