Building Homes or Extensions/Electrical Wiring Under Concrete Slab
I am renovating an old carriage house. The original floor was boards laid over dirt. I have removed the flooring and excavated the dirt in preparation to lay a concrete pad assembly consisting of 4" gravel, 2" Insulation, Vapor Barrier, and 4" concrete.
The main service line needs to be run from the meter box outside, under the foundation, under the slab and up into an interior wood-framed wall to the load center. Wires for receptacles need to be run under the slab from the load center and resurface up to the interior brick wall.
I suppose I would get the best results in running the wires through some sort of conduit. I would be very happy to have your advice on the following:
1. Is there a certain type of conduit that is required to contain the electric wires? (i.e. sch 40, sch 80)? Should the conduit be entirely PVC? or is there any advantage/requirement in metallic?
2. I note from some sources that the minimum burial depth should be 4" from the top surface. Is this always the case and therefore I would am I already OK since the concrete is 4" thick?
3. What size conduit should be used for consideration of ease in pulling wires through or maximum size allowable?
4. Any tips or detail that you may have for this task? I am not an electrician and have very little construction experience.
Gary, great questions that imply you have been doing some research.
Conduit is an excellent choice for many reasons. I would use electrician's gray plastic sch 40. I used to insist on EMT (metal) at least through the slab, but have had some bad experiences with rust. Make sure any and all fittings are electrical fittings, not water. Bends require long sweeps, not the abrupt turns allowed with water.
Outdoor runs that do not have a slab over them must be a minimum of 18", most fellas go 24. Under slab, you should be fine.
Conduit size is determined by the size and number of conductors. YOu cannot exceed a total of 360* of bends in any one run. I do believe in going up one size of pipe for heavy wire (6 and larger) or full runs. The number of conductors is determined by code based on wire size, ampacity, with derating based on pipe fill.
YOur local inspector is the final authority on all things. It may well be worth having a visit before you get started so there are no surprises.
There are many internet sources of information, but here is an easy one that may get you started: