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Issue #210 January 2008
INTELLIGENT ENERGY SOLUTIONS
Solar-Powering the Circuit Cellar
Part 2: From the Ground Up
by Steve Ciarcia
Start | Problem Solved | Pole Mounts | Under Construction | Success at Last | Sources & PDF
UNDER CONSTRUCTION
To make a long story short, I contacted Sunlight Solar Energy and told them the good news. Rather than follow it with, “I guess you guys have bigger problems than you thought,” I told them that I had just the two professional contractors, Bob Kuca and Floyd Palmer, who knew how to solve the problem and I would arrange to get it done. Off the record, I preferred this solution anyway. Bob and Floyd had worked with me often enough on previous building projects to know that the only resolution I wanted would have to be absolute overkill. For example, if they build a deck for me, it’s designed to support a truck, not people. I finally had my arms around the big torque numbers and their implications, but I also now recognized that the real integrity of the pole mount rested with the concrete. Time for some more overkill.
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| Figure 2—This is a diagram of the concrete footing for the pole mounts. The 8² schedule 80 mounting pole was custom manufactured to hold the POWER-FAB series 225-8-80 mounting rack. |
Since we couldn’t put the pipe in the manufacturer-specified 2 yards of concrete in a 7¢ deep hole, the obvious solution was to put the pipe in a lot more concrete firmly anchored to ledge in the 5.5¢ to 6¢ deep hole that we could dig. Figure 2 is a diagram of the footing. The concept was to dig a large excavation and pour a 5¢ wide by 17¢ long concrete rebar-filled footing securely anchored to the ledge (see Photo 2). Then, the idea was to put the two schedule 80 poles 13¢ apart, steadfastly anchored to the footing with additional rebar and 3¢ diameter 4¢ concrete columns.
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| Photo 2 —The solution to the ledge problem was to call in a couple of friends with lots of expertise and bigger equipment. Here, Floyd Palmer digs a 7¢ × 20¢ hole for the concrete footing. |
A 5¢ × 17¢ wooden form was constructed and set on the ledge in the excavated hole. As I said before, the ledge had an irregular surface. The high/low areas of the ledge varied about 6² to 8² across the length; but more importantly, the surface contained many crevasses and indentations. The net result of pouring concrete onto this surface was like making an interlocking English dovetail joint in furniture. Basically, the only way to move the footing was to move the ledge or break the concrete. To further reinforce the concrete and provide attachment to the column supports, Bob put interlaced structural rebar into the concrete as it was poured (see Photo 3). In total, the 5¢ × 17¢ poured footing required four yards of concrete. If you do the math, this turns out to be an average thickness of 15².
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| Photo 3 —Bob Kuca constructs a 5¢ × 17¢ wooden frame to hold the four yards of concrete and rebar that form the base of the pole-mounted array footing. |
Figure 2 shows the dimensions and construction details of the schedule 80 pipes. (Photo 4 shows them being moved to the hole.) Because they are 750 lb each, typical methods of simply setting a pole in a 3¢ diameter dirt hole, surrounding it with wet concrete, and then straightening it with a couple of external two-by-fours are completely out the window. The pole locations and orientations had to be fixed before any concrete was poured. That was the reason the poles were made with a 20² × 20² 1² thick steel plates welded on the bottom with predrilled mounting holes.
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| Photo 4 —The 15¢ half-inch-thick steel schedule 80 pipes necessary to hold the solar panel racks weigh 750 lb each. While Bob and Floyd might typically just throw something like this over their shoulders, today they decided to use the tractor. |
The technique Bob used to locate the poles on the footing was to set four bolts in the wet concrete where they would go (see Photo 5a). The plywood holding the bolts in the proper pattern also kept them from sinking into the concrete. The plywood was removed after the concrete set. Besides the bolts, more lengths of steel rebar were attached to the rebar in the footing and positioned vertically around the base plate of the pole. Additional welded rebar connected these vertical rods to produce a substantial cage around the poles and firmly bond the two concrete sections, as you can see in Photo 5b. (Circular pieces of rebar were also added during the pour.) While the bolts extended 6² down into the concrete and increased overall structural integrity, their primary purpose was to position and orient the mounting poles, not provide primary structural attachment to the footing. The rebar is what firmly attaches the poles to a concrete footing that can never move.
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| Photo 5a—Before the concrete is set, Bob inserts the plywood form that holds the four mounting bolts for the 15¢8² schedule 80 pipe along with additional rebar that firmly fastens the vertical concrete around the pole to the bottom footing. b—Additional welded rebar is added to the rebar extending up from the footing so the concrete that will be poured around the pole will be firmly secured to the footing. Circular rebar added during the pour and the welded angle iron on the sides of the pipe that inhibit rotation of the pole create a truly immovable array. |
Once the concrete footing was dry, Floyd lowered each pipe onto the pre-located bolts (see Photo 6a). Then Floyd and Bob adjusted the mounting plates so that each pole was perfectly vertical (see Photo 6b). Next a 4¢ tall, 3¢ diameter sonotube was placed around all of the rebar along with plastic conduit for the wiring from each array. Then the whole assembly was filled with about a yard of concrete (see Photo 6c). You’ll note that there is angle iron welded to the sides of the pipe that also got covered with concrete. The rebar connecting the footing to the sonotube-filled concrete keeps the whole structure from bending or lifting from a directed force applied to the top of the pole. The two welded angle irons keep the pole from rotating in the concrete as the result of any side motion of the array in strong winds as well. The end result—this pole ain’t going nowhere.
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| Photo 6a—As Floyd lowers the 750-lb pole into the hole, Bob guides it onto the mounting bolts. b—After the pole is secured in place with the top bolts, the bolts under the base plate are adjusted to make it perfectly vertical. c—After the 3¢ diameter sonotubes are put in place around the bottom of the poles, Bob calls in the cement truck and adds another couple of yards of concrete. Conduit is preinstalled on each pole and extends down into the concrete and out through the side of the sonotube where they all meet in the wiring trench going to the inverters. |
If the yard wasn’t a mess enough already, the final landscape-intensive task was getting the power from the three arrays to the inverters on the garage (see Photo 7a). National electrical code required that the conduits be buried in a trench from the arrays to the house. In order to limit wiring losses, Sunlight Solar Energy chose to use #6 copper wire and run each array in a separate conduit. This was a great idea even if it did involve burrowing under the sidewalk and drilling through 6² concrete walls (see Photo 7b).
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| Photo 7a—If the yard wasn’t a mess already, the wiring trenches definitely finished it off. Here, Bob and Floyd drop in a special cable for my future signal monitoring along with the solar array conduits. Because of the currents involved, a separate conduit was used for each of the three arrays. b—Jason Ross, Sunlight Solar Energy’s on-site foreman, routes the three conduits through the garage wall where the three inverters are located. |
Finally, the whole mess was back-filled with dirt (see Photo 8a) and covered with plastic and crushed stone (see Photo 8b). The end result hardly looked like the disaster area full of construction vehicles that it had been. Now I had two nicely painted pole mounts ready for the arrays.
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| Photo 8a—Once poured and set, Floyd backfills the hole with dirt. b—After the poles were backfilled and the soil tamped down, a little paint and crushed stone made it all look less like a disaster area. The next procedure was to mount the POWER-FAB series 225-8-80 solar racks and SunPower panels. |
The final proof of whether I had succeeded in my dictate about overkill requires a little more back-of-the-envelope math. POWER-FAB says that the pole should be set in 3¢ diameter, 7¢ deep concrete. That’s about 2.25 yards of concrete. At 4,000 lb/yard, that’s about 9,000 lb. Structural integrity depends on the effort it takes to move this 9,000-lb cylinder against the surrounding dirt. I think my construction weighs a bit more.
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