Pole-mounted solar-electric (photovoltaic; PV) arrays are a great option for many sites. You don’t need to worry about the orientation or angle of an existing roof, or about roof penetrations. Pole mounts allow easy, manual adjustment of the array’s tilt angle, or you can choose automatic tracking mounts to optimize energy production. They provide great air circulation to keep PV temperature down and power output up during warm weather, and make clearing snow off the array a simple task in wintry climes.
If you’ve decided that a pole-mounted solar electric array is the best option for your site, here’s all the info you need to secure it in the ground for the next 100 years or so. While groundwork is arguably the least glamorous part of any construction project (it all just gets covered up anyway), it is in many ways the most important. So let’s dig into the dirty work required to set a pole for a solarelectric array. In our next issue, we’ll show you how to have some good, clean fun mounting and wiring the PVs.
Site the Array
PV modules are expensive. To maximize the energy produced by your investment and minimize system costs, always look for the sunniest location on your property to install your array. When PV modules are wired at high voltage (48 VDC nominal or more), pole mounts can be located a couple of hundred feet away from the batteries or inverter with minimal transmission losses and relatively small gauge wire.
The best tool for locating the sunniest spot for your panels is a Solar Pathfinder. It takes less than a minute to set up, and will 
show you when a particular location will be shade-free for every daylight hour, every day of the year. A good solar site should be unshaded from 9 AM to 3 PM, but a larger solar window is always better. Compromises may need to be made. If your site has morning shade but lots of afternoon sun, for example, the site will still be workable, but will likely require a larger array. The photo at right shows the Pathfinder at the location of my arrays. Yearround, there is no shading between 9 AM and 5 PM. In the summertime, the solar window is even larger, with virtually no shading from 7 AM to 7 PM.
Dig the Hole
While working on roofs is never great fun, neither is digging holes. Pole mounts let you skip the roof work, but leave you doing dirt work instead. In some jurisdictions, the building department will require an engineer to specify foundation details that take into account maximum wind loads and soil types for your area. Pole mount manufacturers can also recommend the appropriate hole size for their racks if an engineer isn’t required. See the table for one manufacturer’s specs. Because deep, narrow holes are difficult to dig by hand, and impossible to dig with a backhoe, a tractor-mounted auger is the best tool for the job. That said, augers don’t work well in rocky soil, and may not be able to drill deep enough for taller poles. The type of
ground you’re working with will often dictate the actual dimensions of the pole’s foundation, and the best way to get it dug. Wooden forms can be built if the hole ends up oversized, but I like to avoid this whenever possible, since it adds time and expense to the job. At my place, the soil at the array’s location consists of 3 feet (0.9 m) of loam and then bedrock. A 5-foot-deep (1.5 m) hole would be impossible to dig without a hydraulic rock hammer, so I expanded the size of the hole to 3 by 3 by 3 feet (0.9 x 0.9 x 0.9 m) deep, using the pole and hole sizing table as my guide. I had a friend with a backhoe come up when I was ready to dig the holes for the mounts. The backhoe made quick work of the holes—in twenty minutes it did what would have taken me two days to do with a shovel.
Pipe Selection & Preparation
The diameter of the pole required for your installation will be specified by the pole mount manufacturer, as will the hole dimensions. Pipe sizes will range from 2 inches in diameter for
small arrays, up to 8 inches for large arrays. Keep in mind that pipe sizes go by the inside diameter of the pipe. Make sure to double-check that the outside diameter of the pipe is correct for the pole mount you’re purchasing. Pole mounts are most often designed to rest on schedule 40 steel pipe, but large arrays or tall poles may require thicker walled, schedule 80 pipe.
Consult your mount manufacturer for the right pipe for your array. The appropriate pipe is almost always available locally at wholesale plumbing warehouses or steel suppliers, and is typically sold in 21-foot (6.4 m) lengths. Usually, your supplier will cut the pipe to length for a nominal fee. Aesthetic and practical considerations will affect pole height. In my case, I want the arrays fairly low so I can reach to brush snow off in the winter. Others choose a taller pole to allow people, animals, and lawnmowers to pass underneath unscathed, or to improve solar access. If you require a taller pole than the standard specified height, you will need to have more pole in the ground. For each extra foot (30 cm) that you add above ground, you’ll want approximately 6 more inches (15 cm) in the ground in concrete. If you have to go more than 2 or 3 feet (0.3–0.6 m) higher than what is shown in the table, a larger diameter pole may be necessary—consult your rack’s manufacturer. In windy conditions, a significant amount of stress is put on the pole’s concrete foundation due to the surface area of the array. At a minimum, the pole should be drilled or cut to allow for the insertion of two rebar crosspieces in the concrete. Use 18-inch (0.46 m) pieces of 1/2-inch (13 mm) rebar running perpendicular to the pole (see photo on the next page). If the concrete mixture and curing process is less than perfect, some shrinking of the concrete can occur. The two crosspieces of rebar will keep the pole and the attached array from rotating in the hole during high winds.
Locate the holes for the two rebar crosspieces by measuring up from the base of the pole. For 1/2-inch (13 mm) rebar, I use a 5/8-inch (16 mm) drill bit and a high power AC drill to cut the holes. Run the drill at low speed, and use cutting oil to prolong the life of the bit and keep it running cool.
Position, Anchor & Plumb the Pipe
The 6- or 8-inch poles used for larger arrays are heavy, often weighing a couple of hundred pounds. When positioning the pole, it’s good to have at least two people on site. Instead of just you and your back doing the job, enlist the help of a friend. If the hole has a narrow diameter, insert the rebar crosspieces before you position the pipe in the hole. There may not be sufficient clearance to do it once the pole is lowered into position.
After installing dozens of pole mounts, I’ve come up with a simple and inexpensive method to temporarily anchor the pole in a vertical position while
I’m getting everything lined out for the concrete pour.
Run two, 8-foot (2.4 m) 2 by 4s flat along the ground on either side of the pole. Drill out the ends of each of the 8-footers using a 9/16-inch (14 mm) drill bit. Then cut four, 18-inch (46 cm) pieces of 1/2-inch (13 mm) rebar, and hammer the rebar through the drilled holes, securing the long 2 by 4s to the ground. Cut two shorter lengths of 2 by 4 (18 inches is usually perfect). Get the pole roughly vertical (plumb) and fasten the shorter 2 by 4s to the longer ones, snug against the other two sides of the pole to form a square hole with the pole in the middle. Use screws for easy removal of the lumber once the concrete has cured.
Plumb poles look good. Out-of-plumb ones don’t. Once the pipe is anchored in place, use a 4-foot (1.2 m) carpenter’s level to plumb up the pole in two directions. Place the level vertically on the south-facing side of the pole, plumb it up, and then check the east side of the pole. You’ll need to check for plumb in each direction several times, since the pole will typically move a bit while you’re making your adjustments.
Install Rebar & Conduit
If your local building codes require engineering for the pole mount’s foundation, the size and spacing of the rebar will be provided to you, and you just need to follow the engineer’s specifications. At a minimum, always install rebar crosspieces as described earlier. Use rebar tie-wire to hold the
rebar in place while the concrete is being poured. After the rebar is in place, drill a hole in the bracing and position the PVC conduit for the PV wire run before the concrete is poured.
After the rebar is in place, drill a hole in the bracing and position the PVC conduit for the PV wire run before the concrete is poured. The conduit can be temporarily fastened to the pipe with the same type of tie-wire you used for the rebar. Choose the diameter and type of conduit based on National Electrical Code (NEC) specifications. The conduit will run down the pole to a 90-degree fitting and then out beyond the edge of where the concrete will be poured. Some
installers use Unistrut to clamp the conduit to the pole once the concrete is cured. If you choose this approach, make sure to use spacer blocks or pieces of Unistrut to set the conduit at the correct distance from the pole.
In most locations, electrical inspectors require all aboveground PVC conduit runs to be schedule 80 PVC. This conduit has a thicker wall than the schedule 40 PVC that is typically used in trenches, and will better resist physical damage. Take a look at NEC sections 352.10(F) and 352.12(C) or section 352.2 in the NEC Handbook for more info on this requirement. Some installers prefer to use rigid metallic or EMT conduit for extra protection on exposed, outdoor conduit runs. I’m installing data acquisition sensors on the PV arrays at my place to measure irradiance (the sun’s intensity), wind speed, and ambient and PV module temperatures. This is why you can see a second, smaller, conduit run in the photos. Most systems will simply have a single conduit run coming down the pole.
Estimate the Amount of Concrete
The amount of concrete required to secure the pole for your PV array will vary depending on the size of the PV array, the soil type, the pole’s height, local wind loads, and the dimensions of the finished hole. Deep, narrow holes will require less concrete than wider, shallower ones. You can determine the amount of concrete required by calculating the volume of the hole. The volume will equal the surface area of the hole times the depth. For square holes, the equation is L x W x D (length x width x depth). For round holes, the equation is π x R2 x D (Pi x the radius squared x depth). Pi is a mathematical constant that always equals
approximately 3.14. Finally, a cubic yard of concrete equals 27 cubic feet.
Let’s run through the math required to calculate the amount of concrete required for two different hole sizes. The two holes my friend with the backhoe dug at my place are 3 feet by 3 feet across, and 3 feet deep. Each hole required one cubic yard of concrete (3 x 3 x 3 = 27 cubic feet or 1 cubic yard). If the holes had been round, and were 2 feet in diameter (1 foot; 0.3 m radius) and 5 feet deep, they would have only required 15.7 cubic feet of concrete (π x 12 x 5 = 15.7). To convert this to cubic yards, simply divide by 27 (15.7 ÷ 27 = 0.59 cubic yards)
Get the Concrete
There are a few different options for getting concrete for the pole’s foundation. You can buy bags of ready-mix concrete and mix them on site. For larger holes, this can be both
expensive and labor intensive. A 90- pound (41 kg) bag of ready-mix will make approximately 2/3 cubic foot of concrete. So a 3- by 3- by 3-foot hole would require about 40 bags of readymix (27 ÷ 0.66). That’s a lot of concrete to mix in a wheelbarrow, but definitely doable. However, there are easier ways to go about getting the job done. Many rental yards now have towable, 1-cubic-yard mixers. If you or a friend has a truck that’s set up for towing, this is a great option. The mixer is filled at the rental yard with concrete and water.
The mixing cylinder is trailer mounted and includes an engine that mixes the concrete while in transit, keeping it ready for the pour. This approach can be cost effective, and is definitely labor saving compared to purchasing, hauling, and hand mixing 40 bags of ready-mix. Another option is to have a cement truck deliver and pour the concrete. While this is definitely the easiest way to go about it, it’s usually going to be the most expensive. Many pole mounts only require a cubic yard of concrete or less. Typical commercial trucks are sized to haul 9 yards, and concrete companies will usually hit you with a short load charge if you need fewer than 3 yards delivered. In addition, if your site is a ways out of town, transport charges usually come into play. So unless you’re installing multiple poles, or have the need for additional concrete, calling in a truck usually isn’t your best bet.
My immediate needs for concrete included 2 cubic yards of concrete for the pole mounts (one yard for each hole), and 3 cubic yards for the slab floor of the power shed I’m building—5 yards total. Since my property is only 25 minutes out of town, simply calling in a truck made a lot of sense.
Pour the Concrete
Wooden forms are usually not needed when pouring a foundation for a pole mount. The sides of the hole will contain the concrete, keeping it where you want it. I like to pour the concrete to within an inch or so of the top of the hole, and then backfill over the top of the cured concrete with soil and some grass seed. To my eye, this looks the best. If you want the concrete to be exposed, a simple form built out of 2 by 4 lumber placed at ground level over the hole, and secured with rebar, will allow you to pour and finish the top of the exposed foundation. You can also use a cardboard tube form (Sonotube) or thin plywood bent into a circle. Immediately before you pour the concrete, grab a hose and wet down the
bottom and sides of the hole. This will keep the soil from drawing moisture from the concrete as it cures. When pouring the concrete into the hole, make sure to direct the chute or shovel away from the pole to minimize the chance of knocking the pole out of plumb. Always make sure to double-check the pole for plumb immediately after the pour, since slight tweaking is often required.
Make sure to give the concrete ample time to set up before you begin installing your PV array. The surface area of the installed array will act like a sail in windy conditions, and may move the pole out of plumb if the array is hastily installed before the concrete is sufficiently cured. Ideally, I like to wait five days before I begin installing the array. If you’re working within a tight time frame, a day or two should be sufficient, depending on the ambient temperature during curing.
Finishing Touches
Finally, I like to paint the pole to prevent surface rust and keep it looking sharp for years to come. Talk with the folks at your local hardware store for info on prepping the pole for paint, and the best paint to use. Using galvanized pipe is another solution, costing less time, but more money.
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