Exterior Electrical Box for Wall Light - Placement/Stucco

Question

My Question: What is the proper depth of the J box for mounting exterior light fixtures? Should the outside edge of the box be flush, inset or protrude past the finished stucco?

My exterior walls are 2x4 stud with 1/2" OSB sheathing then there will be three layers of traditional stucco.

I'm going to place the J boxes (planning on 4" metal octagon) direct to the OSB or I can drill a 4" hole and place through the OSB mounting to a backing stud. I figure this is just one of those detail things you have a chance to get right at this stage! Wiring is NM Romex.

Thanks.

Answer 1

The depth of the jbox is a function of volume which is determined by wire fill. Here's a couple online calculators. They will tell you the cu in required which is labeled for any jbox you can buy. If it's just 1 or 2 14/2 romex cables, any standard lighting jbox should be good.

https://www.constructionmonkey.com/calculations/electrical/boxfill

https://www.ecmweb.com/national-electrical-code/code-basics/article/20886012/box-fill-calculations

How the jboxes are secured depends on your jurisdiction. In my location, all outdoor jboxes (whether new or old work) have to the be secured to studs or attached to cross braces secured to studs. Your location may be different...

The building inspectors here want jboxes flush or just proud with the wall material unless that material is fireproof and wire-safe. Since stucco has metal lathe, that could pose a danger to the wires. Again, that is my local building department's position so your jurisdiction may be different.

EDIT: I did not quote the NEC as I am not electrician. I shared how my local building department (AHJ) interprets the code. I should have been more explicit in recommending you contact your building dept to determine what they require or how they interpret the relevant codes as this will probably be different.

Answer 2

You are limited to ¼" setback, but protrusion is OK

NEC 314.20 limits you to a ¼" setback from the finished surface, given that your stucco is noncombustible:

314.20 Flush-Mounted Installations. Installations within or behind a surface of concrete, tile, gypsum, plaster, or other noncombustible material, including boxes employing a flush-type cover or faceplate, shall be made 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 (¼ in.).

Installations within a surface of wood or other combustible surface material, boxes, plaster rings, extension rings, or listed extenders shall extend to the finished surface or project there‐ from.

I'd probably have the box protrude enough from the wall so that the drainage layer in the stucco job can basically be flashed to the box; this may require the box to protrude a bit from the finish wall, but that's OK by Code.