Most of the electrical system for an earth sheltered home is pretty conventional, but there are a few differences.
Good electrical ground is able to dump many amps of electricity into the earth with as little resistance as possible. It is important for occupant safety, and surge protection (including lightning).
Code can be met a few different ways, but most electricians use one (or more) 8 ft grounding rods buried in the earth. Since it is all about conducting electricity into the earth, it all works better if there is moisture in the earth. Most electricians will try to bury one or two rods (separated by at least 6 ft) somewhere near a corner of the house where a downspout is likely to dump water.
If you are putting in an insulating umbrella that will keep the earth under your home dry, you will need to move your grounding rods out much further away from the home than an electrician normally would.
Since many earth sheltered homes have concrete footings full of rebar, it may seem logical to use a “concrete encased electrode” in the foundation (aka Ufer ground). Normally, a concrete encased electrode is a very effective grounding system and the NEC allows it to be used as your only grounding. However, this method is not as compatible with the earth sheltered umbrella and other water proofing measures that will keep the soil around your foundation very dry. Again, dry soil will not conduct electricity as effectively.
There have been cases when lightning strikes actually cause the moisture in the concrete to rapidly expand, crack and occasionally explode
Another popular option is attaching the grounding system to a galvanized well casing (an uber grounding rod). This can work well and should certainly be done if you have a metal well casing. PVC casings cost about 1/3rd of a galvanized casing and never corrode. Obviously it is not worth paying three times the price just so you can use your well as a grounding rod, but some areas still require the metal casings for all or at least the first 20 ft of the well. Find out the rule and costs for your area and plan accordingly.
Grounding is very important to the electrical system of a home and is probably one of the areas where you want to exceed code. I will probably start with the “concrete encased electrode” and also put in a few extra grounding rods and tie onto my well casing (if it ends up with a galvanized casing).
With my construction method (steel covered in shotcrete), I don’t have wood frame walls (or even wood furring strips) that I can run wiring through in a traditional manner. This is probably the reason that electrical contractors all gave me such ridiculously high bids. My walls start with a framework of steel studs and rebar. I will need to run my electrical conduit in through that. There are several types of conduit that the NEC (National Electric Code) will accept for concrete encasement. Some are more expensive or difficult to use than others.
The above chart only tells part of the story. The first two rows, PVC Conduit and Electrical Metal Tube, come in straight 10 ft pieces. They can be bent, but that is much more work than the flexible tubes. There are also a lot more fittings when you have to put at least one every 10 ft, and the cost of fittings really starts to add up. EMT and PVC also take a lot more work to make each connection.
The Liquid Tight Flexible Non- Metallic tubing is really designed for situations, usually industrial or agricultural, where the conduit frequently gets liquid on it. This is really overkil for a residential wiring application. It does come in long flexible lengths, but its fittings are expensive and a take much longer to apply than ENT. The real killer is the price, although I did see some very reasonably priced at the reuse place.
That leaves us with the Smurf tube as the cheapest and easiest overall solution for most of the conduit.
My recommendation is to use a combination of PVC and ENT. The PVC is the cheapest per foot, comes in a wide range of sizes and can be buried directly. You can also use it in straight walls or even bend it a little with some heat. The ENT, with its long flexible coils, is great for longer runs through curved walls.
And the Electrical Metal Tube will look great in my steel Quonset hut garage.
ENT, Smurf Tube
The NEC stubbornly refuses to call it Smurf tube and instead insists on calling it ENT (Electrical Non-metallic Tubing). They cover it in article 362 of the NEC. Here are some of the highlights;
ENT is allowed to be encased in concrete, but it is not considered strong enough to be buried or used in places where the ambient temperature is greater than 122°f (50°c). Because of the temperature limits, it is not allowed to carry more than 600 volts. There are other weird rules about not allowing it to be exposed if the residence is more than 3 floors tall. Also, even though it is flexible, you still need to pay attention to the conduit rules about the number of turns (the degrees can not exceed 360°.
The flexible connections just snap together (no need for glue or any other tools), but I think I would tape the joints before adding the shotcrete.
You want the kind that connects the conduit right into the box (round holes) rather than the kind where you just push the wires through little square trap doors. If there are any little gaps, tape them to keep the concrete out.
The boxes generally assume that you are attaching them to wood. Some come with nails built right in. You will need to figure out a way to properly attach the boxes to the rebar. When I figure it out, I will come back with pics. It needs to be tightly fixed to make the inspector happy, but you will also want it to stay where you put it when the heavy shotcrete is slamming into it.
If you need to save a bit of money by doing the electrical yourself, or if you just want to keep a better eye on what your electrician is doing, you may want to do some reading.
I summarized my favorite Electrical Books here…