Running sch40 electrical in floor before concrete?

[MNBobcat]

Hi All,


This seems like it should be easy but I don't have any great ideas.

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 after the concrete is hard.

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

 

[tjkubota93]

Hi All,


This seems like it should be easy but I don't have any great ideas.

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 after the concrete is hard.

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

You should be able to pull thhn through a tee without damage I would think. Maybe rig one up and try it?

 

[orezok]

One thing I would recommend is to NOT place the conduit directly under the concrete. Make sure you bury it at least 4" into the sub grade. If not, you or someone else will come in and saw cut the floor for a future improvement and they will cut the conduit and then you have big problems. This also allows you to run sweeps.

 

[MNBobcat]

Orezok,

Normally that would be good advice but in this case I know the slab will never be saw cut.

 

[Mendonsy]

Could you use something like these instead???
Non-Metallic-Floor-Box-for-Concrete-Pour

 

[MNBobcat]

I could but they aren't weatherproof

Could you use something like these instead???
Non-Metallic-Floor-Box-for-Concrete-Pour

 

[orezok]

Orezok,

Normally that would be good advice but in this case I know the slab will never be saw cut.

You plan to live forever?

 

[RNeumann]

In that situation you would install the box before the concrete is poured and cover it with duct tape or similar.

They don’t make conduit “tees” for a reason........only junction boxes or similar for a 2 in/1 out situation.

 

[Tractor Seabee]

Post some pictures or a diagram of your puzzle, not sure what you want to do. Wire does not bend very well around plumbing fittings and its easy to break through the insulation, even THHN. Sounds like "CODE" is not in your planning. It is the only way I know how to do it. I have done considerable in and below slab conduit building new churches. Below slab PVC is normally run through a sleeve so it is not bonded to the concrete and it can move independently. It shears easily with movement. Each above slab junction box will have an in and an out conduit. Just tie to a block of wood to maintain spacing. PVC is more forgiving than steel.

Ron

 

[techman]

Instead of THHN, you could pull MTW. Much more flexible. The nylon overcoat on THHN is the rigid, tends to crack part of the wire. It is mainly for oil protection and is not needed for home use.

paul