Mounting electrical boxes to horizontal strapping
maine_tyler
| Posted in Mechanicals on
What’s the protocol / product used to mount electrical boxes in a wall with horizontal strapping? Especially if its 2×2 or 2×3 strapping?
I’m sure those more versed in electrical work / service cavity work than I will find this elementary, but I cannot seem to find boxes for such a scenario unless there are mounting tricks that allow standard boxes to work? (code approved)
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Replies
I think we need a little more info here. Are you talking 2x3 on flat, so that you only have 1.5" depth? Is this inside where you will have half inch drywall, or outside where you'll have some other siding depth?
Attaching non-specialty boxes to horizontal members is typically done by adding vertical blocking. Boxes come in a variety of depths, from 1.5" to 3" being the most common (2.5" is usually considered standard, anything less is a shallow box, 3" or more is deep). The shallower the box, the fewer wires and smaller devices you can fit into it. You can compensate for that with wider boxes. A single gang utility box is 2 1/8" deep, but the volume is greater than a 2x3x2.5" box. If you've only got 2" though, that doesn't help you. If that is the scenario you're in, your best bet is probably using 1.5x4" square boxes and raised covers; you can get the covers in single and dual gang. If you've got any triple boxes planned, that could be a challenge.
Tyler,
Trevor has given you good advice. I'd just add:
- There are specialty boxes which have their mounting tabs on the end, although this doesn't help with the problem of depth Trevor brought up.
- There is nothing in our code (and I don't think yours either) that prohibits mounting boxes horizontally.
Also note that the boxes with tabs generally cost an arm and a leg. I wanted to get the utility boxes I mentioned in the side tabbed version, to make my installation easier. Regular ones were $2, the tabbed version was over $10. It seems like anything they don't sell tons of is priced like that, such as box extenders.
Not mounting boxes sideways is a pretty universal, stylistic convention. Only time I've seen sideways boxes is on baseboard retrofits in century homes, or garages where people are less likely to judge you.
Trevor,
I had to buy some red receptacles for our local Community Hall to identify which outlets were on the emergency generator. The only difference between them and the $2 white ones is the colour of the plastic. They were $11. Nuts!
Related: NEC 300.4 (Protection Against Physical Damage) states there needs to be 1 1/4" free space in front of the cable or it needs to be protected by a 1/16" steel plate.
When wiring a 1 1/2" service cavity (2x2 or 2x3 laid flat), can romex cable be used? Or would single conductors have to be run in conduit?
Kevin,
Is the 2"x horizontal strapping attached to sheathing, or directly to the wall studs like a Mooney Wall?.
Malcolm,
My strapping is theoretical, for a house I'm planning. Between the 2x horizontal strapping and wall studs would be either an interior taped OSB air barrier or something like Intello Plus. What would the approved method be in either case?
Tyler and Kevin,
You might want to see these relevant links to previous discussions of this topic:
"2-inch foam on the interior face of the wall: What are my electrical box options?"
"Service Cavities for Wiring and Plumbing"
Be sure to read the comments posted at the end of my article on service cavities. Here are two relevant comments:
Martin Holladay: "Standard wall boxes are 3 inches deep, but shallower boxes are available. The electrical code has requirements on the minimum volume of electrical boxes; volume requirements vary depending on the number of wires entering the box and the number of receptacles or switches. It's certainly possible to use shallower 2 inch or 2.5 inch deep boxes for receptacles and switches.
"Most ceiling boxes are about 2 inches deep, but (as with wall boxes), different depths are available, including shallow boxes that are merely 1/2 inch deep -- just deep enough for the Romex. Obviously, these 1/2-inch boxes can only be used for a limited number of applications."
Floris Keverling Buisman: "The depth of the cavity can be dictated by the fact that the electrical inspector/NEC requires wires to be > 1.25" from edge of the stud it is passing through. If it is less you need a metal plate to prevent shorting out these wires by nails/screws used for sheetrock, picture hanging etc. If you use a 2x2 and INTELLO Plus you can fish the wire behind it and comply with this code w/o metal plates or drilling through studs. Shallow 1.5" boxes would be needed in that case."
14/2 Romex is 0.162" thick, so there is plenty of room for the 1.25" clearance. Since you in the theoretical stage, try to get a bigger service channel than 1.5". It will cause headaches. Sure, you can get 2" deep boxes, but they probably still won't fit without sheathing modification. That 2" doesn't include ground or clamping screws protruding out the back. Even if you manage a 2" box, you are going to run into problems. Take a look a box fill tables and see how many wires and caps you're allowed in there. Then reduce that by at least two or three wires if the box is going to have a dimmer or GFCI receptacle. I had 1 11/16" service channels to work with, and it wasn't too fun, requiring lots of different workarounds. One spot where I needed a tri-gang box, I broke down and cut a section of the sheathing and sealed it with flashing tape to get the required depth. There just aren't the options available for tri-gang as there are for single and double.
Bottom line is, it's possible to do, but has some downsides that aren't worth the extra half or 3/4" of perimeter floor space you save. It will cost more in boxes, take longer to install, and at every single box you have to do a box fill calculation before you put it in to make sure it is big enough so you have to all your devices decided on in advance. Consider if you have to run a plumbing vent on an exterior wall in the kitchen, because that is definitely not going to fit. Even PEX pipes will be a challenge.
Kevin,
if the air-barrier is flexible, like Intel plus, you can do as Martin suggested and tuck the wires behind the strapping. You can also use deep boxes, which as Trevor points makes everything a lot easier. That's the approach many people use with Mooney Walls.
If the air barrier is something solid like OSB, everything becomes a lot more difficult. Unfortunately the code requirement is 1.25" to the edge of the hole, so although the Romex might only be 0.162", the hole necessary to run it is typically at least 5/8". It won't all fit in 2"x material - nor will any usefully deep boxes.
I agree with Trevor. Service spaces need to be sized for something more that just electrical if they are going to be useful. I'd go further: before deciding to include a service cavity you should detail what services are actually going to be run in each wall or ceiling. If you are strapping something that is only going to include a couple of wires and boxes, is it really worth the effort rather than just effectively sealing them when they penetrate your air barrier?
Malcolm,
The code doesn't mention anything about the edge of the hole. This is what it says,
"In both exposed and concealed locations, where a cable- or raceway-type wiring method is installed parallel to framing members, such as joists, rafters, or studs, or is installed parallel to furring strips, the cable or raceway shall be installed and supported so that the nearest outside surface of the cable or raceway is not less than 32 mm (11⁄4 in.) from the nearest edge of the framing member or furring strips where nails or screws are likely to penetrate. "
So you have the additional options of stapling the wire 1.25" away, or securing it with a spacer. The only realistic way to get the cables through the strapping is to notch the back of the strapping before installing it anyway . Drilling a hole through strapping in situ would be an exercise in frustration.
Very helpful advice. Thanks!
Trevor, I'm not very familiar with the NEC, but the link I looked at seems to indicate the setback is to the hole:
https://allstarce.com/wp-content/uploads/2016/12/2017-NEC-Code-Ch-3-Article-300-1.pdf
I'm also not sure whether the setback would be interpreted as being from either face of the strapping the way it is with studs. Then you are really hooped.
Malcolm,
My mistake, the section I was quoting was in reference to cables parallel to, not through, members. If it's a bored hole, the edge must be 1.25" away, as you say. What I am describing is not a hole, however. This is the section that would apply:
"(F) Cables and Raceways Installed in Shallow Grooves.
Cable or raceway
-type wiring methods installed in a groove, to be covered by wallboard, siding, paneling, carpeting, or similar finish, shall be protected by 1.6 mm (1∕16 in.) thick steel plate, sleeve, or equivalent or by not less than 32 mm (11∕4-in.) free space for the full length of the groove in which the cable or raceway is installed"
It wouldn't be interpreted as being from either face, because no fasteners are coming from the other direction.
This is how my house was done, as well as several others by the same builder. All inspected by different electrical inspectors, none of which batted an eyelid.
As an aside, I found that a 7/16" hole was adequate to run a 14/2 wire through. 1/2" was required for a 12/2 and 10./2. 5/8" wouldn't be needed until you got up to 8/3.
Trevor,
Electricians typically only use one bit, a 3/4" one. With that they can easily pull two wires through the hole or a single one of any gauge. If centred on the stud it leaves you the 1.25" protection and a bit of wiggle room.
I was away for a bit and just found all these great responses; thanks!
The vertical blocking seems like the obvious answer to the mounting problem. As for the depth, that is something I am still designing, so its great to think it through. The plan is for intello +, but I hadn't decided on how large of a cavity to use. Interior runs...
"If you use a 2x2 and INTELLO Plus you can fish the wire behind it and comply with this code w/o metal plates or drilling through studs. Shallow 1.5" boxes would be needed in that case."
Is this referring to when running the wires vertically, perpendicular to the strapping? I'm not sure I see its applicability to running parallel to the strapping (horizontal) which is the majority of the time. It wouldn't be necessary anyways to get the 1.25" distance if stapled 'near' the back of the strapping right (but on the underside/topside)?
Unless the idea is to cross over the vertical stud tightly (on the inside of the intello) then make a slight bend to snake behind the strapping for the rest of its run... not sure how it'd be fastened behind the strapping sandwiched by the intello/strapping.
Tyler,
If you are running the electrical cables on the exterior side of the Intello, you'll need to drill through the studs for horizontal wiring runs. The location of these holes won't be critical, however, due to your use of horizontal strapping on the interior of the Intello.
Tyler,
You can use a metal 4" square x 1.5" deep metal junction box with the appropriate depth drywall mud ring. Simply screw the 4" sq box onto the vertical studs and strap the wires on the vertical studs as well. See the attached photos.
William,
That's a great solution. Solves all the problems.
Yes, thanks William that looks like a great solution.
2 questions:
Do you say 'metal' because that is what is available in that size/configuration, or some other reason(s)?
Is there any concern with the 1.25" setback where the wires enter the box? They appear to be rather central in the 1.5" (if a 2x2) cavity. Perhaps that's beyond the level of scrutiny typically expected?
Tyler,
Every house uses several of the square boxes for the range and dryers outlets. the location of the knockout for wires is within the 1 1/4" of the stud face and it isn't seen as a issue.
Tyler,
Plastic boxes and plastic mud rings are also available in the same sizes. So you could use plastic if you wanted to.
As Malcom mentioned there shouldn't be a concern as this is a common product used in houses everyday.
Here is a link for some additional info on this very topic. http://blog.lamidesign.com/2018/05/usa-new-wall-never-ending-question-of.html