Running Electrical Wiring Through an Insulated Slab
bongo30
| Posted in General Questions on
We are building a new home and running our electrical underground. We originally planned to bring it into the house through the box frame or foundation wall, but now the contractor suggests to bring it through the basement slab. We are installing rigid mineral wool and poly under the slab. I feel like bringing it through the box frame would be the best way of doing it. Am I overthinking it? Are there any advantages/disadvantages for each method? What is the standard practice? We are in NY, zone 4.
Thank you!
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Replies
B30B30,
Bear in mind that electrical codes where you are may vary. Here the service has to enter the dwelling through a slab or foundation wall. If it comes in through a framed structure it needs mechanical protection - which is 3" of concrete on all sides (effectively a foundation wall). The alternative is that it comes up outside the house in an exposed conduit and then enters through the framed wall into the back of the panel.
Is there a reason you don't want to bring it up through the slab?
Doesn’t the meter need to be on the exterior of the house somewhere?
Seems to me before the build is done you and the builder are going to have many choices to make. This one seems pretty minor in the skim of things. The smart move may be to give him lots of little wins that do not matter to you and save your ammo for bigger battles.
Walta
Meter location aside, Walter makes a point that is worth remembering. When I was much younger, a senior engineer let slide that he intentionally left some errors or overly tight tolerances on drawings prior to design reviews with suppliers, so that he could be sure that they were actually reviewing the design and to let them have some input into minor details, ensuring that they had some ownership while protecting the key elements of the design. He called them "bitch deflectors", which helped make the lesson memorable...he was an excellent engineer.
If you have a meter going at the house, it probably needs to be mounted on an exterior wall somewhere. Your utility company will tell you what their requirements are for placement. If you have a remote meter on a pedestal or pole, then you have more flexibility -- and more safety. The "safety" part is that remote meters will typically also have a main disconnect in them, so the cable coming OUT of the meter then has overcurrent protection. The cable coming IN to the meter will typically have no overcurrent protection (aside from the primary fuse ahead of the utility transformer, but that will be sized to let a LOT through before it blows), so any damage to that cable is going to cause you to have a very bad day.
My recommendation would be to bring the cable up on the exterior somewhere regardless, and to run the underground cable in PVC conduit (some utilities will require at least 2.5" conduit here, so be sure to check before you install anything). Bring the PVC conduit up into a NEMA 3R rated exterior box on the exterior of your home, then go from that box into the house. The reason for the exterior box is that it prevents any water or gas from sneaking into your home through the conduit. The reason for the conduit is to better protect the wire, and to make any future repairs vastly easier to do.
To be sure you know what your local requirements are, you have to ask TWO people. You need to ask your local electrical inspector, but that inspector doesn't have the last word in this case. You will also need to ask your local utility company's service planner, since the utility side is subject to your utility's requirements, and the utility can overrule the inspector in some cases due to their engineered system requirements.
Bill
Whether the meter is on a pole or on the exterior of the house, I prefer to then run under the slab to an interior wall. That allows you to put the panel where you would like it, shortens the wiring runs, and avoids the problems of air-sealing and insulation you get when it is on an exterior one.
This will be a remote meter on a pedestal/H-frame away from the house, then cable buried underground before it enters the house. Good point about the local Code, I will ask. The reason I was hesitant about going through the slab is that I wasn’t sure if it requires any special air sealing to prevent water/gas intrusion? But I guess it’s not any different than going through the foundation wall? I just thought it was easier to air seal when going through the box frame.
There are special grommets made to seal conduits with cables in them, but I almost never see them used -- even in the telecommunications industry where most of my work is! You can use "duct seal", which is a tar/clay-like substance. Canned foam is another option, but some inspectors will frown on it's use.
If you come up to a box on the wall they way I suggested, you still need to air seal the conduit. The conduit is just a pipe, and it needs to be sealed. The downside to going through the slab is you have a higher risk of water coming in through the conduit, with a wall penetration above grade there is much less risk of water coming in. Either way you need the air seal though.
BTW, with a remote meter, you'll have a remote disconnect, so you'll need to use 4 wire cable (or 4 seperate wires) between the pedestal and your breaker panel. Code requires the ground/neutral bond be at the first disconnect, which would be at that panel. If the pedestal is more than around 50 feet or so away from the home, I recommend adding an extra ground rod at the home too and connecting it to the ground busbar at the home's panel.
Bill
Thank you! That was exactly my concern. Bringing it through the slab, will we be inviting potential water intrusion in the future? Has anyone experience water intrusion when going through the slab?
I've had water problems in commercial buildings where conduits enter through the slab. We had one where when we popped out the cap on a 4" conduit, water started flowing in. A LOT of water. It would have drained down the entire manhole system if we hadn't re-capped it. That's probably less of an issue in a residential setting though.
Bill
What your contractor is suggesting is doing it the way it is most commonly done - and it's done that way because it seems to work without causing the problems you are worrying about.
I disagree that the service represents a potential vulnerability to water intrusion. If the hydrostatic pressure is sufficient to force water above the level of the slab, it will also be exerting the same pressure on the underside of the slab, and neither vapour-barriers or concrete are much of an impediment to entry. If that is occurring you have much bigger problems than whether there is a cable or conduit penetration.
The service can be run in conduit, but more commonly shielded waterproof cable is used. They both penetrate the slab much as your drains and waterlines will, so I don't see how this is a concern unique to the electrical. If conduit is used, the top is sealed with putty designed for this purpose. There is no reason to believe there will be sub-soil gasses in the conduit if it runs continuously from it's source to the basement.
Let me add that I mostly agree with Malcolm here in residential settings. In commercial settings, conduits running from the building to utility services commonly tie into manholes, and all the manholes are linked together by duct ("conduit") systems. This means that there can potentially be A LOT of water in the manhole SYSTEM, all of which will drain out wherever it sees a path.
Residential systems usually tie into ground-mounted pedestals or riser poles, where you don't have the flooded manhole system to provide a large volume source of water. With a conduit that just goes to a pole or pedestal, you're pretty much limited to ground water as Malcolm mentioned, which is also an issue for the slab and foundation walls.
As a rule though, I NEVER specify direct bury cable. I ALWAYS use PVC conduit (or HDPE duct) for underground cable runs. The reason is that it doesn't add much to the cost of the project, but it provides a LOT more flexibility and futureproofing. Direct buried XHHW cables are probably more common, but they are much more prone to issues down the road compared to running cable in conduit.
BTW, do not run cable conduits in the same trench with natural gas services. Natural gas DOES follow underground pathways and can enter structures that way, which is why gas services are required by code to enter above grade on new construction.
Bill
Thank you all! This was very helpful. Will check the code and discuss with my contractor.