How to Install Electrical Wiring in a Monolithic Dome
Tying electrical conduits and boxes to the rebar during shell construction is the most common way to embed electrical circuits in a Monolithic Dome. Wires, electrical outlets, switches and more are added after construction. In this dome, the shell will be four inches thick, so spacers are used to ensure the boxes are flush with the shotcrete.
Electrical wiring is more than power for lights, air conditioning, and toasters. It’s also phone calls, surfing the web, fire alarms, and a surround sound theater—any system where an electrical circuit is needed. With proper planning, wiring a Monolithic Dome—whether it’s a home, school, storage, or storm shelter—is straightforward.
Like most conventional construction, the majority of wiring is inside the interior walls, the space between floors, or the attic space above various rooms. The main electrical and communication panels are placed inside the structure with cables wired like spokes from the central location to the terminal points. The installation of these wires and cables is standardized across all construction methods and well understood.
The only difference in Monolithic Dome construction is when an electrical box or other interface is required on—or in—the dome wall. Maybe it’s a power outlet, a light fixture, an ethernet connection, or a fire alarm. It doesn’t matter. The installation method is the same. Hint: it uses conduits.
Sharp corners are avoided when transitioning from the dome shell to the—future—interior walls. In this example, the top conduit on the right leads from one wall—marked with the water line stub in the floor—to an isolated shell location near the bottom right. Another conduit leads from box to box around the shell. After construction, a steel tape “fishes” wire through the empty conduits.
Conduits
An electrical conduit is just a pipe or tube made of plastic or metal. After construction, an electrician “fishes” wires and cables through these tubes.
The conduits are connected to electrical boxes. There are many types of electrical boxes, such as an octagonal box for a light fixture, a rectangular box for two-gang power outlets, or a round box for a smoke detector. Like conduits, electrical boxes are typically plastic or metal.
Conduits and electrical boxes may be surface mounted to the shell or attached to the rebar during construction and embedded in the concrete.
Complex conduit designs are possible and often preferred depending on the structure’s purpose. This large free-span dome required many connections in the shell. The conduit was added to ties embedded in the foam layers in this example.
Design Considerations
When designing a structure, minimize the number of electrical boxes in or on the dome shell. It’s not a requirement. It’s completely possible to crisscross the shell with electrical circuits and boxes. In some cases, this may be precisely what’s required—maybe it’s the next-generation Vegas sphere.
Typically, the interior wiring takes care of most electrical needs, with only a few power outlets, switches, and fixtures on the dome. In fact, there are often spaces inside a dome—a bathroom or hallway, for example—that could be enclosed with a drop ceiling, which creates an “attic” space above, ideal for running utility lines and HVAC ductwork.
Include a qualified electrician in your construction process. Sometimes, you will be required to have the electrician perform all aspects of electrical installation. In this case, minimizing the number of conduits and boxes reduces the time the electrician must spend on-site during dome construction. In other instances, an electrician may specify what conduits and boxes to use and allow the construction crew to install them.
Two light switches in a junction box near the door splits into two conduits leading to boxes for the lights.
How to Embed Conduits
Embedding conduits and boxes into the concrete shell is easy during dome construction—everything is tied to the rebar grid before applying shotcrete. We usually embed plastic (PVC) conduits because they easily bend and conform to the curved walls.
Begin construction of your Monolithic Dome normally by inflating the Airform and layering on the polyurethane foam with embedded rebar hangers. Clean up the mess from the foam application and begin hanging the horizontal steel rebar.
Mark the interior wall layout on the floor, especially where the walls meet the dome. You must provide a path for the wires from these future interior walls into the dome structure, so accurately locating the walls is crucial. Remember that wiring into attic spaces gives more leeway, so note where attic spaces will likely meet the dome wall.
Find where the electrical boxes are needed in relation to the interior wall layout. For electrical outlets in a room, for example, you may just need to mark two or three places on the dome wall between the interior wall layout and where the box goes up the required height above the floor. It usually doesn’t need to be exact.
On the other hand, you may want a light fixture exactly there in the middle of the room. Locate the fixture as accurately as you can on the floor in relation to the rest of the interior walls. Use a laser plum bob to project a dot on the dome above. Mount the box at that location. Note: the laser dot will be on the foam or a piece of rebar. Remember to adjust it a few inches to match the surface of the yet-to-be-applied shotcrete.
Start attaching conduits to the boxes and the rebar. The wire used to tie the rebar works well. Plastic zip ties work, too.
A line of electrical boxes with spacers to push each box into a position flush with the finished shotcrete. That conduit sticking out to the right intersects with a future conventional wall constructed into the extended augment. Note how boxes are covered with tape to preserve their integrity until the shell is complete.
Four conduits begin in the lower-left corner as sweeps stick out from the dome shell, intersecting a future interior wall.
A conduit should lead from the wiring entrance location—typically in a nearby wall or attic space—over to the box. Use conduit sweeps to curve out from the dome wall into the internal wall. Don’t make sharp corners with the conduit because the electrician needs to push a fish tape—a steel wire puller—into the conduit from the box to the conduit exit. The narrow steel band must have a smooth path to follow. Once the draw wire is snaked through the conduit, the electrician uses it to pull the electrical wires back through the conduit to the box.
Remember to tape over the boxes and conduit entrances to prevent shotcrete from impacting concrete in places you don’t want concrete to go. Also add a flag of some sort, sticking out from the box to help identify the locations after applying shotcrete.
Finish attaching the reinforcing steel rebar grid and then apply shotcrete. It’s important to clean around the boxes after every shotcrete application. Having taped and flagged boxes helps a lot. You don’t want to lose the location of one of these nodes.
There are no embedded circuits in this Monolithic Dome shop. Instead, a simple electrical system was surface-mounted after construction.
A lonely but well-used, four-gange electrical outlet mounted to the concrete shell using a masonry drill and concrete anchors.
Surface Mount
Not every dome needs electrical systems wired inside the dome shell. Shops, storages, and many other structures may use surface-mounted hardware. Sometimes, you miss a location during rebar installation and need a method of adding an electrical circuit after construction.
Nothing is more straightforward than adding conduits and boxes to the concrete shell. A hammer drill, concrete bits, small concrete tapping screws, and u-shaped straps make short work of the job. We often use metal conduits for surface mounting, but it depends on the needs of the structure. A storage building or shop will want a conduit that can be hit with equipment without breaking. Inside a home, you may want something that blends in.
Alternatively, swag chains work well when adding a light fixture or other electrical needs after construction. Drill an anchor into the dome wall. Wire an electrical box to the interior wall. Thread the electrical cable through a decorative chain from the box to the anchor and down to the hanging fixture.
When installed correctly, the embedded conduits are completely embedded, and the boxes are flush with the concrete. A steel wire puller or “fish tape” is pushed through the conduit from one box or sweep to the other, the proper wires are pulled back through, and the electrician can finish adding the circuits, fixtures, and outlets.
Wiring
The rest is straightforward electrical installation—electrical panels, pulling wires, and installing fixtures.
Don’t forget about other wiring needs—for example, the fire alarm system. A smoke detector is usually required at the apex of the dome. It needs a box and a conduit, or you will be mounting it on the surface. What about speakers for your home stereo? Two or more boxes and more conduits. Wired ethernet? More boxes. More conduit.
The bottom line is that with a good plan, adding electrical circuits in a Monolithic Dome is a straightforward process.
This is a closer view of the switch box and the exterior light. All of this conduit will be completely buried in the concrete.
This image has two locations where conduit has a sweep 90 that will drop into wall cavities or attic spaces.