Rainscreens and Junction Box Mounting Blocks
Are most people forgoing junction boxes for exterior wall lights these days, such as seen in this video with Ben Bogie:
It’s a rainscreen set-up and he’s using a mounting block, but I’m not sure I get it. Is it just that the fixture has an integral box with a weather tight wire insertion point?
When it comes to rainscreens and mounting blocks what’s more common, or preferable: cutting in a box, or just stubbing out some wire as shown in the video? If most fixtures don’t need the box, certainly easier not to cut one in…
The video can also be seen here: https://www.finehomebuilding.com/2020/10/23/exterior-pvc-trim-wire-and-pipe-penetrations
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Most fixtures are likely to need a box, and either way a box gives you a lot more flexibility. Remember that code does not allow you to just stuff a connection into a wall cavity -- there always has to be an enclosure. If the fixture is RATED to be used without a box, which would apply to something like a recessed can that has a sort of integrated "box", then you're OK, but in all other cases you need to put a box and mount the fixture to the box.
I would put in some kind of mounting block. This gives you something to flash, and provides a flat surface to mount the fixture (which is especially helpful in cases like lap siding that would otherwise set the light at an angle). Put the box up first on, or better yet, recessed within, it's mounting block, then flash the block like a very small window. Once the siding is all up and done, come back and mount the light fixture to the box. If you ever want to change out the light fixture for something else, it's a quick and easy process that doesn't require messing with the siding again.
Bill
Like Bill said, you need a box (most likely). I use blocks made of Boral, but they go over the box that's already flashed to the sheathing.
I use these boxes from Arlington: http://www.aifittings.com/catalog/siding-mounting-blocks/non-metallic-fr-series-device-and-fixture-box/FR420F
I'm trying to find a better photo, but in the attached one, you can see three installed with the block (above the windows and door) and one installed pre-block (right side of picture).
Additionally, the wire itself is sealed to the sheathing with a carefully applied piece of Zip stretch tape.
Thanks Patrick and Bill.
The pictures are pretty small on my phone... Are the electric boxes recessed into your sheathing? You mention flashing the wire itself so I'm assuming not. How is that possible? Are the boxes thinner than your rainscreen?
For some reason this issue is really confusing to me, and the more I read the more confused I get.
Are some people-- like the setup Ben bogie has going, and what wooba gooba shows in a linked thread-- surface mounting junction boxes to siding/trim blocks?
I never thought of junction boxes as something you'd want to see protruding way out past your siding. What am I missing?
I would love not to cut a box into the sheathing and rather just penetrate the sheathing/WRB with a wire. Is the key a shallow box?
> Are the electric boxes recessed into your sheathing? You mention flashing the wire itself so I'm assuming not. How is that possible? Are the boxes thinner than your rainscreen?
The boxes have adjustable rings to get to the thickness you want. I bring it to about the thickness of the block mounted over it. They are *not* recessed into the sheathing.
> I never thought of junction boxes as something you'd want to see protruding way out past your siding. What am I missing?
I agree with not wanting them to protrude.
> Is the key a shallow box?
Yea, you don't really need a deep box for a light, usually. The canopy tends to afford you some extra room, practically speaking, and the smallest boxes are sufficient for a single 14-2.
I attached some more pictures. They're heavily cropped, but hopefully will help.
With one 14-2 you could use a pancake box. Some of those other methods(that flashed in plastic one looks great) would be better but a pancake is an option.
If you use a 1-1/2" deep octagon box, you can recess it into a piece of 2x framing lumber and use that as your mounting block. There are many ways to do it depending on what you need, and how thick of a block you need. Patrick shows one of the special boxes with a built-in flashing plate, I've always site built things and used "regular" electric boxes. Both ways work. The advantage to site built is that you don't need to order in any specialty items. The advantage to the purpose-built box like Patrick shows is that it installs quick.
Bill
Tyler,
A combination of Joe and Bill's methods works well to0. A site-made mounting block with a recess deep for a pancake box. It's more weatherproof than a full depth box, but you still have the advantage of block, which is much easier to flash to the siding.
Ok, I think it's starting to make sense. Thanks all, and for those close-ups Patrick.
In the u.s. can this wire be NM or does it need to be UF? Similarly, does the box need to be exterior rated (and with attention paid to how the wire enters the box in terms of water sealed) or does it essentially get treated as interior, with reliance on the flashing of the mounting block.
I'll be doing a receptacle as well, so will need a deeper box in that case. I have seen some suggestions to run the wire out, make a small drip loop, then come into the bottom of the box a short distance away (as opposed to straight in the back). Any thoughts on that? Seems like technically the wire needs to be UF or a conduit system in this case.
> In the u.s. can this wire be NM or does it need to be UF?
This is up to the local AHJ as NM-B has to be used in a "normally dry" location. Lots of people use NM-B for this, but you'll also find stories of inspectors not being okay with it.
> I'll be doing a receptacle as well, so will need a deeper box in that case. I have seen some suggestions to run the wire out, make a small drip loop, then come into the bottom of the box a short distance away (as opposed to straight in the back).
I can understand the wisdom of the drip loop, but in that case UF is absolutely more appropriate, but still may not be sufficient if the AHJ deems it subject physical damage.
If you want to go with a surface mounted box, I would again try to seal the cable to the sheathing and enter the box directly in the back.
For outlets, I actually prefer to use an Arlington box that gets cut into the sheathing: http://www.aifittings.com/catalog/inbox/
The question of whether NM-B is Ok is a bit of a grey area. If technically illegal it is the norm.
UF is for direct burial, and stands for "Underground Feeder". You don't need UF for places that might just get dripped on, such as an above-grade receptacle fed through a wall. NM-B should be fine for this application, as far as the wire's rating goes...
There IS a catch though. NMB is NOT supposed to be used where subject to physical damage. Leaving a "drip loop" exposed would count as "exposed to physical damage", and really isn't permitted. That doesn't mean it might not get by inspection, but it's not a good idea to do. The reason it's not a good idea is that it doesn't take much of an impact with something that has an edge on it (a shovel, etc.), to cut through the jacket into the conductors inside. UF cable isn't all that much more durable in this regard. I strongly prefer to feed the wire into the back of the box for this reason, or use an LB to take the wire from the side/top/bottom of the box into the wall. The bast connector to use is a cable gland here, which has a rubber grommet that will seal the cable to the box.
Bill
Thanks Bill. I was wondering about the connector- I'll look for a cable gland.
For the 'drip loop': just to be clear, if I were to do that (not saying I am) it would be behind the siding in the rain screen gap. I assumed that would be considered protected from damage. No?
I understand the acronym for UF, but what do people use when above ground and in damp or wet conditions, yet protected from damage? Always need conduit? UF seems easier in those situations, at least to someone like me inexperienced dealing with conduit.
Either way, in my case it sounds like rear entry with NM-B will be the way to go.
Now that I think of it, I'm not sure how I would be able to use a cable gland or other protruding clamp from behind if I have little to no clearance between the back of the box and the sheathing.
That would be one benefit I see to coming into the side or bottom.
I see the boxes Patrick linked just have a integral strain relief.
UF cable needs physical protection where exposed the same way NMB does. Those two cable types aren't really very different physically. The usual way to provide that physical protection is to either conceal the cable within a structure, or sleeve the exposed portion the cable with conduit (there are fittings made to transition the cable to conduit directly without the need for a box).
If you will have the cable concealed in the rain screen gap, then you should be fine. I had envisioned a loop coming out of a wall, exposed, then making a loop before entering the bottom of a surface mount box. A loop like that would NOT be OK.
I think you can get a right angle cable gland for NMB cable. I know you can for SJ/SO cable (flexible cord, like an extension cord). I just don't think I've ever used a right angle NMB cable gland myself. I'm sure such a connector could be made though, since the only real difference between the SJ/SO cable glands and the NMB cable glands is that the NMB cable glands have a rubber grommet with an oval shaped hole in it instead of a round hole. You could probably even swap the grommets between the connectors, although that would technically void the UL listing of the connector since you would be effectively modifying it.
Bill