I've got a wastewater vent stack that is dead-center behind the master bath vanity. (See attached picture.) Previously, there was a single light fixture. You can still see the box, which wasn't mounted to anything and which was off center. (It was a new work box, and I have NO idea what held it in place while they hung drywall. The sheathing on this house is DennyBoard, and it certainly wasn't affixed to that.)

How should I mount a new box in order to be code-compliant? I am in Texas, and my city has recently adopted the 2011 electrical code. I am a homeowner doing my own work under the homestead exemption -- my city would normally require this work to be done by a licensed electrician. Everything I have done so far has passed without modification.

I can either move the soil stack to the side and install a piece of wood, or I might be able to mount something like a ceiling fan box that would allow me to adjust the position of the fixture horizontally, but I'm not sure how to make that work.

Since the circuit already has the maximum number of fixtures on it, I cannot install two light fixtures with one on either side of the soil stack without running a new circuit back to the panel. I can do that, but I'd rather avoid it.

Installing an old work box is not legal according to the inspector since I have the walls open (drywall had to come down to fix other issues.) I didn't ask if I could in theory mount the box to the PVC pipe somehow, but I intensely dislike that idea anyway. He told me, without mincing words, that it was my job to figure out how to do it and his to tell me if I'd done it right or wrong, and if I wasn't sure I should call a real electrician.

Master Bath Soil Stack

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    You want to put the box next to the pipe, or in front of the pipe? Is there enough depth from the outer face of the drywall, to the pipe, to fit the box? – Tester101 Sep 9 '13 at 17:33
  • I don't like them, but they make 1/2" deep boxes for installing ceiling fixtures on top of a joist. They're only the thickness of the drywall. E.g. amazon.com/Hubbell-8292-Knockout-Ceiling-25-Pack/dp/B005SQNMRO – BMitch Sep 9 '13 at 17:44
  • There isn't much room for a pancake box. (Look at the top plate where the vent stack penetrates.) I would prefer not to go that route anyway, although it is possible in theory, because it'd be difficult to mount without puncturing the vent stack. – Karl Katzke Sep 9 '13 at 19:01
  • "Since the circuit already has the maximum number of fixtures on it". What is the maximum number of fixtures? What if you change from 1 bulb fixture to 3 bulb fixture? What if you plug in an extension cord with multiple receptacles? – Tester101 Sep 9 '13 at 19:11
  • For whatever reason, the city inspector insists that there only be 8 fixtures or outlets on a circuit. – Karl Katzke Sep 10 '13 at 15:03

Throw four 45s at it and get vent out of the way. Mount new work box in center. Use Acme cable stretchers ;-).

How to mount new box, you ask? How about toenailed jack stud floor to ceiling.

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  • I don't know why this was downvoted. This suggestion certainly seems like the "right" way to do it. Though I would be tempted by @longneck shortcut as moving a vent stack out of the way for light seems like more work than I would care to do. – auujay Sep 9 '13 at 18:54
  • Don't know why this is downvoted either; it's the right answer. I just wanted confirmation of that before I got all involved in it, since I tend to overcomplicate things. – Karl Katzke Sep 9 '13 at 19:02
  • Are there any restrictions with the number of bends in a vent, or the overall length of the vent? Are there any concerns; such as inadequate air flow, with a modification like this to the vent system? I'm no fluid dynamics expert, but surely there has to be some difference between a straight pipe and a bent one. – Tester101 Sep 9 '13 at 19:22
  • @Tester101- a fair point, however in the real world this should be fine. The bends should be fine, IPC table 916.1 covers the maximum developed length of a dry vent so it is possible he is already reaching these limits (though it seems unlikely to matter). However the IPC is very explicit about vent diameter, so be sure you don't restrict the airflow by reducing the dimensions of the vent pipe. – auujay Sep 9 '13 at 21:39
  • @tester101 The new net effective length would likely be around 16 inches longer if you moved the vent within the same stud bay, 6 right and 6 back left. – HerrBag Sep 9 '13 at 21:47

Install a new-work box against the stud. Inspect. Install drywall. Once you're all done and passed all your inspections, install an old-work box in the correct location and fix the drywall.

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  • Or remove new box after rough inspection, put up drywall, cut in old work box (ok because wall is now enclosed). – HerrBag Sep 9 '13 at 18:01
  • Nice, even better than having to patch. :) – longneck Sep 9 '13 at 18:52
  • sorry, i had to downvote this suggestion. inspection is not just a necessary evil, it's a process to make sure things are done correctly. doing something with the intent to avoid a decent (though complicated) solution and proper inspections, is a potential recipe for future possible disaster. – alt Sep 10 '13 at 15:32

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