Depth of electrical box from outside wall

[Quijote]

I mounted electrical boxes flush with the plywood, but by the time you factor in the siding and the mounting block, the fixture mounting plate is recessed nearly an inch from the mounting block and the machine screws on the plate are far too short.

In trying to find the size of those screws (I guess they are 6-32) so I could get longer ones, I read somewhere that the box should not be recessed more than 1/4". I assume that the siding and mounting block don't count towards that and that they mean 1/4" to the drywall/plywood, but can someone confirm?

I can always throw in a 1/2" extender for good measure.

Thanks

 

[kd3pc]

they make differing depth trim rings that would have prevented this...ideally you want the face of the trim ring flush with the finished wall.

Longer screws are just fine to mount the device, it is done all the time.

 

[justsam]

The screws are 6-32. I would use an extender to make the outlet more secure, and safer. The box should be flush to any combustible material such that if arcing should occur in the box it does not reach combustible material.

 

[Zeke]

Actually, nothing wrong with setting boxes at the plywood level when planning on using extensions. You get more room. And they are strong. I'd seal the extension as well as the plate.

 

[Quijote]

Cool. Thanks guys.

 

[Charles (in GA)]

It is a fire prevention thing (which is the entire purpose of the NEC after all )

Note that the ¼ inch depth figure for how far out a flush cover can sit from a box is for non-combustible materials only. NEC 314.20 tells us this and also tells us that if the box is installed in combustible materials the box shall be flush or protrude from the surface of the material, so the flush cover plate can fit tightly to the box.

Also make sure that boxes in attached garages that are installed in walls that are common with living space, cannot be placed back to back in the same stud bay as boxes on the other side of the wall. (due to this being a fire resistant rated wall)

Charles

 

[Zeke]

It is a fire prevention thing (which is the entire purpose of the NEC after all )

Note that the ¼ inch depth figure for how far out a flush cover can sit from a box is for non-combustible materials only. NEC 314.20 tells us this and also tells us that if the box is installed in combustible materials the box shall be flush or protrude from the surface of the material, so the flush cover plate can fit tightly to the box.

Also make sure that boxes in attached garages that are installed in walls that are common with living space, cannot be placed back to back in the same stud bay as boxes on the other side of the wall. (due to this being a fire resistant rated wall)

Charles

Good info, Charles. Does the NEC allow for approved extensions and supplements like gaskets? That would have been my point. Better to have a solidly mounted box rather than a wobbly one up near the surface. The OP didn't mentioned the type of siding but if vinyl there is not a lot of lateral support for the box.

I think that is why I prefer metal boxes for any receptacles. They don't flex and any sealant won't be moved about.

 

[C96]

It is a fire prevention thing (which is the entire purpose of the NEC after all )

Note that the ¼ inch depth figure for how far out a flush cover can sit from a box is for non-combustible materials only. NEC 314.20 tells us this and also tells us that if the box is installed in combustible materials the box shall be flush or protrude from the surface of the material, so the flush cover plate can fit tightly to the box.

Also make sure that boxes in attached garages that are installed in walls that are common with living space, cannot be placed back to back in the same stud bay as boxes on the other side of the wall. (due to this being a fire resistant rated wall)

Charles

^ ^ ^ ^ This ^ ^ ^ ^
Most people don’t realize it, or abide by it.

 

[Quijote]

Good info, Charles. Does the NEC allow for approved extensions and supplements like gaskets? That would have been my point. Better to have a solidly mounted box rather than a wobbly one up near the surface. The OP didn't mentioned the type of siding but if vinyl there is not a lot of lateral support for the box.

I think that is why I prefer metal boxes for any receptacles. They don't flex and any sealant won't be moved about.

The boxes are metal Hex with flanges. I can unscrew them, add spacers and remount flush, I suppose.

The siding is fiber cement. The mounting block is plastic.

 

[Quijote]

It is a fire prevention thing (which is the entire purpose of the NEC after all )

Note that the ¼ inch depth figure for how far out a flush cover can sit from a box is for non-combustible materials only. NEC 314.20 tells us this and also tells us that if the box is installed in combustible materials the box shall be flush or protrude from the surface of the material, so the flush cover plate can fit tightly to the box.

Also make sure that boxes in attached garages that are installed in walls that are common with living space, cannot be placed back to back in the same stud bay as boxes on the other side of the wall. (due to this being a fire resistant rated wall)

Charles

Also, just to clarify, this is on a detached garage 100'+ away from any habitable building - not that I am ok with it going up in flames.

 

[Charles (in GA)]

Also, just to clarify, this is on a detached garage 100'+ away from any habitable building - not that I am ok with it going up in flames.

In this instance, the code does not address the use of the building.

314.20 In Wall or Ceiling. In walls or ceilings with a surface of concrete, tile, gypsum, plaster, or other noncombustible material, boxes employing a flush-type cover or faceplate shall be installed so that the front edge of the box, plaster ring, extension ring, or listed extender will not be set back of the finished surface more than 6 mm (1⁄4 in.).
In walls and ceilings constructed of wood or other combustible surface material, boxes, plaster rings, extension rings, or listed extenders shall be flush with the finished surface or project therefrom.

314.21 Repairing Noncombustible Surfaces. Noncombustible surfaces that are broken or incomplete around boxes employing a flush-type cover or faceplate shall be repaired so there will be no gaps or open spaces greater than 3 mm (1⁄8 in.) at the edge of the box.