How to install box in block wall Q


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Old 05-05-09, 08:59 PM
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How to install box in block wall Q

I need to install a triple gang box recessed in a cinder block wall.The blocks are 16x8 x6 hollow core ,the central web is about 1 3/4 wide

What kind of box?

Where in the block should I place the box?

How should I fasten it?

Thank you
 
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Old 05-05-09, 09:18 PM
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Is this an already built wall?

Or are you going to build this wall?
 
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Old 05-05-09, 09:37 PM
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This is an already built wall,it is the front wall of the house , the box will be on the inside that is plaster finish over the blocks
Thank you
 
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Old 05-05-09, 10:06 PM
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I'll leave the answer to this question to someone in a brick construction area. Maybe they have some more helpful ideas than I would.

I currently live in a mostly wood construction area, so never see any of this. But I did grow up in a brick construction area, and as I recall, this was not fun...

Use a hammer drill and make multiple holes in the hollow part of the brick as square as possible, then use a cold chisel to chip out the rest. Then a big chunk of brick usually breaks away, then run the wire and concrete in the big hole you just made!

Wait for someone else to answer to get better advice on this!
 
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Old 05-06-09, 08:52 AM
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Use a 3-gang "masonary" box , designed to be set in such walls. If you can chisel the opening in the wall for a tight fit , and then fill the seam between the box and the opening with plaster of paris , the box should be secure.

Is this an exterior or interior wall ?
 
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Old 05-06-09, 10:07 AM
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Thank you, it is an exterior wall double cinder blocks and stone finish on the outside, plaster finish on the inside where the box has to be installed
 
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Old 05-06-09, 12:32 PM
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old work CBS

I've done a few of these, but not as wide as a triple. First off, I'm sure you have determined already how to feed the box. If it's an existing exterior block wall, do you have a poured tie above and a slab below that blocks entrance from above or below? Then, you will be forced to feed, exposed, from the outside. Make sure the proposed box knockouts will clear the thickness of the block, unless you rear feed. A typical box will NOT clear the block walls and allow a flush fit. This is very important for conduit installs, not so much for internal cable clamp (NM) installs.
You say inside, it has a "plaster" finish. Meaning, stucco, no drywall, no insulation board, no furring? If so, target the center of a cell for the box. You will need to somehow find the mortar joints and center between them. That might be your biggest issue. You can use the ball end of a ball-pein hammer to make a tidy ~ 1" hole. Just keep hitting it in one spot for awhile. Cut the close-fit rectangle with a jig saw and carbide grit blade. You could also drill corner holes with a 1/2" masonry bit and use a masonry chisel for the edges. Securing the box can be done with stucco patch material, filling the entire column with cement (can't use nm wire here), or hopefully using the hole itself to jam the box in. Make the cutout slightly on the small side, and increase with masonry chisel as required.

edit: I should also mention. In my part of the world, every 4' or so, expect to find a column totally grouted with cement and a vertical rebar. That would be a bad box location!
 
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Old 05-06-09, 03:17 PM
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I would use either a circular saw or a right angle grinder to make an out line for the box. Yes, I said a circular saw either with a diamond blade or a masonry cut off blade. You don't have to go all the way through and of course at the ends because of the circular nature of the blade it will be a shallow cut but it makes a score line that hopefully the break will follow. I'd start breaking in the middle and working outward to the score line.
 
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Old 05-06-09, 08:59 PM
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Thank you for all replies.
The finish it is stucco[my bad] no furrings no insulation board,this is the front wall of the porche that is the main entrance to the house
I need to fit three lights switches in the box,there is a poured tie above but I can get the feeds drilling at an angle through a ceiling rafter and down to the box IF the blocks are hollow
The celing is low, less than 8'
Im not too concern with damaging the stucco as I m planning on laminating the walls with drywall so I could chissel a channel to acomodate the feeds inside a sleeve and run them vertically through the ceiling and along the rafters
I can also chissel away some of the stucco to expose the bloks anf find exactly the centerof the blocks
If I find all this too dificult for my level of knowledge maybe I can frame a section of the wall with small dimention lumber and make life easier .
Regardless I thank you for all the help.
 
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Old 05-06-09, 09:19 PM
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Or use a surface mounted box.
 
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Old 05-07-09, 06:41 AM
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You might want to develop that thought of "laminating the block with drywall". Typically, a 1 x furring strip is nailed up with foam board insulation between on 16" centers. This will give you wireway space. The flush box may need to penetrate the block to clear the back. I don't think you want to attach drywall to exterior block directly for various reasons.
 
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Old 05-07-09, 10:59 AM
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I think the 1x furring should give me enough space to run 3 pieces of emt 1/2''
,about 36'' lenght from the box to the rafters,to protect the 14/3 and two 14/2 NM
feeds

Would I still need a masonry box and where and how should I secure it ?

Thanks again
 
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Old 05-07-09, 01:53 PM
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Although not a code requirement, the idea of 1/2" EMT is a good one. Now, you can consider the use of 3 ganged old-work drywall boxes. They will be held by the sideclamps against the back side of the drywall. Blast a clearance hole in the block. The drywall will now need to be carefully routed or cut to fit the box. The edge of the box needs to be just below (0 to -1/4") of the finished surface of the drywall.

edit: old work boxes are difficult to use with EMT, since they want to be inserted from the front. Use either raw NM or mount the box to the block using a close fit hole or stucco patch, if you do the EMT.
 
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Old 05-07-09, 09:53 PM
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I will , thanks again for all the guidance
 
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Old 05-08-09, 11:12 AM
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Best to set and secure the 3-gang masonary box between two fur-strips set tight against the Left / Right sides of the box.

It the wall-finish is 1/2" SR , and the FS is 3/4" thick , then drill holes is the sides of the box 1/2" + 3/8" = 7/8" back from the front of the box = center of the fur-strips. Fasten the box in place with screws threaded into the FS's.
 
 

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