Running sch40 electrical in floor before concrete?

[ManCave]

Hi All,

I have a project where I need to run the gray PVC SCH 40 electrical conduit in the floor of a cement slab before the concrete is poured. I'm running 3/4 SCH 40 and will be using 12 gauge THHN.

I will have circuits that will have 4 or more receptacles (boxes) on the same 20 amp circuit.

If I use a TEE for the vertical stub that comes up through the slab then I suspect I'll damage the insulation on the THHN when I pull it. I don't think I can use two sweeps because I would have no way of spacing apart 2 vertical stubs such that they would fit into the two openings on the box later. Note that the receptacle box will be sitting directly on the slab. I'll have to cut off the excess stub.

How do you guys normally pre-wire for a slab like that where you have multiple boxes in series?

 

[tab2]

I am not an electrician, but I watch a lot of them work. They always use boxes that go in the floor, not on it. Here are the most popular ones I see installed. They cut the top of the box off flush with the slab after it is poured.

880MP MODULINK MULTI-SERVICE RECTANGULAR FLOOR BOX

 

[Aceman]

I've done A LOT of conduit under slabs and I've never put boxes "right above the concrete for receps in series" as you put it. What is the reason for this?

 

[ManCave]

A tent will be sitting on the slab. There will be a receptacle about every 15 - 20 feet around the perimeter of the slab but inside the tent walls. 4 separate circuits.

The "in floor" waterproof boxes are over $200 each. I need 19 of them. Not going to pay that. So I'll use an outdoor box but not an in-floor box. The tent will be taken down before it snows. If need be, I can wrap the boxes in plastic for extra protection from water until the tent goes back up.

I've done A LOT of conduit under slabs and I've never put boxes "right above the concrete for receps in series" as you put it. What is the reason for this?

 

[fasteddie]

Not sure I follow exactly what you're doing here. You are running one 3/4" conduit? Why wouldn't you use a sweep into the bottom of a box and split out both ends of the box to your other boxes?

 

[ManCave]

yes I want to run wire from box A to box B to box C and ending with box D. That can all be done in a single conduit with a TEE wherever it needs to run vertical through the slab to a box.

If I used sweeps I would have to run a separate conduit to each box. That would mean 18 separate conduits and isn't practical. Alternatively, I could run the conduit and then when I came to a box I would need to use a sweep to run a vertical to the box, then run a second vertical from the same box back down into the floor to another sweep and then running horizontal over to the next box in the series. The problem with the latter is you have 2 vertical stubs for each box. The box will expect the stubs a certain distance, say 1 inch, apart. That would be impossible to keep them spaced properly during the pour and be guaranteed the stubs would line up with the holes in the box when it came time to glue the box to the PVC stubs.

Not sure I follow exactly what you're doing here. You are running one 3/4" conduit? Why wouldn't you use a sweep into the bottom of a box and split out both ends of the box to your other boxes?

 

[mike93lx]

If you are ok with boxes above the slab, why not the whole conduit run? Paint it to match the tent and no one will notice them

 

[dscheidt]

It's usual to attach stubs to rebar to keep them in place.

 

[Stuart in MN]

You can't use a tee in an electrical conduit - you need a junction box or conduit body (like an LB fitting) for access. If you try to turn a right angle when pulling the wire it will just saw into the inside corner of that tee.

Edited to add: Also, stubbing schedule 40 PVC up through the middle of a slab will probably result in the conduit getting snapped off at some point...it's typical to transition to rigid steel conduit above grade.

 

[exranger06]

I would install it with sweeps going in each end of the boxes. To ensure they're spaced properly and will fit in the fittings on the boxes, have the stubs come up roughly a foot above the slab, and temporarily connect them to the boxes. After the slab is poured, cut the stubs down to the appropriate height and reconnect to the boxes permanently.

 

[sberry]

2sweeps are not a problem, it's the only way to do it and ideally they would be high enough off the slab not to sit in snow. Cover with a 5 gallon bucket,, ha

 

[Aceman]

Over here, I'd most likely use an FD box with two 3/4" hubs in the bottom. Install say a 4" ****** into the left bottom hole with a factory rigid 90 to that. Do the same for the right hand hole but use a 6" ******. This will allow you to spin the second 90 on after the first one.

Set this entire assembly down into the ground, into the ditch you dug so it'll be sticking up through the concrete. FA's on the other end of both 90's will allow you to attach pvc to this setup. Drive stakes, short pieces of conduit, whatever and tie wire/duct tape this assembly to it to keep it plumb.

Backfill all your ditches taking extra care to keep these recep boxes plumb. At the last minute before pouring concrete, pull your stakes and be mindful of your assemblies. Tune them up and keep them plumb as you pour concrete around them.

https://www.platt.com/platt-electri...oxes/Appleton/FSS-1-75/product.aspx?zpid=2831

 

[sberry]

I would put the adapters in the box and glue them to the stubs. Stub them up, put board between the right thickness and clamp them for the pour. This **** is done all the time. It doesn't need micrometer perfection.

 

[Toomanytools?]

I used these like $40 each not above ground though, so might be a challenge to keep water tight in your application.

 

[Norcal]

I used these like $40 each not above ground though, so might be a challenge to keep water tight in your application.

Those floor boxes are for indoor use if the OP is going to take the tent down then you can’t even pretend they are indoor. Aceman’s solution is the result of real world experience that would be a clean install.

 

[ManCave]

I think Aceman had the best recommendation. I would prefer the box lay on it's side so that it doesn't stick up so high but not sure that's possible. I don't see any boxes that are made to have the conduit attach to a side instead of the end.

 

[alfredeneuman]

I think Aceman had the best recommendation. I would prefer the box lay on it's side so that it doesn't stick up so high but not sure that's possible. I don't see any boxes that are made to have the conduit attach to a side instead of the end.

They're available, but very hard to find....... and expensive
Go here: http://www.cooperindustries.com/con...fs/fittings/fs-fd-singlegang-device-boxes.pdf
Scroll down to FSR and FRL.

EDIT A possible better option is thishttps://www.supplyhub.com/products/...MImreW4eHe3AIVi8JkCh1gIA8qEAQYBSABEgI6AvD_BwE

 

[matt_i]

I've seen work where a unistrut "fence" was built and a single, rigid stub came up from the concrete to feed it. Between the boxes are just straight runs attached to the strut. Then you can use FS-C boxes and not worry about a bunch of underground work.

Having a box right at ground level is also a trip hazard, people are going to find this at night, etc. The strut fence is more visible and while not idiot proof, is easier to pick out as a structure.

 

[GrayFlattop]

I'd use schedule 80 for this application - more robust. Some jurisdictions will require it for anything above grade. May as well do it all in Schedule 80.

 

[Norcal]

I'd use schedule 80 for this application - more robust. Some jurisdictions will require it for anything above grade. May as well do it all in Schedule 80.

Any plastic conduit will not fare well in the application the OP proposes, rigid or IMC are the only ****** materials that has any chance of holding up.

 

[ManCave]

I ended up with metal all weather boxes and rigid coming through the slab

Any plastic conduit will not fare well in the application the OP proposes, rigid or IMC are the only ****** materials that has any chance of holding up.