Ground Mount Systems: Foundations, Trenching, Access
When the roof is wrong: foundation types against frost depth and soil, the trench run where voltage drop bites, and access for mowing and clearing.
A ground mount removes the constraints a building imposes. Orientation and tilt become design choices rather than inherited facts, maintenance happens at ground level, and the roof is left alone.
In exchange it introduces problems a roof array does not have: foundations, a long conductor run, security, and vegetation.
When the Roof Is the Wrong Place
Shading that cannot be removed. An orientation that produces poorly. A covering near the end of its life, where installing now means removing later. A structure that cannot carry the load. Or a roof area too small for the system wanted.
Any of those makes the ground worth considering where land is available.
The advantages are real beyond avoiding those problems. Tilt can be set for the latitude rather than for the roof pitch; see tilt and azimuth. Rows can be spaced to avoid shading each other. And access for cleaning, inspection and repair is at ground level, which affects the cost of everything that happens after commissioning.
Foundations
| Type | Suits | Constraint |
|---|---|---|
| Driven pile | Soils that accept driving | Rock or very dense material stops it |
| Helical pile or screw | Soils where driving is impractical | Requires the right equipment |
| Concrete footing | Poor soils, high loads | Excavation, curing time, more material |
| Ballasted frame | Where excavation is not permitted or possible | Substantial weight, wind exposure limits |
| Pole mount | Small arrays, tracking mounts | Concentrated load at one point |
Frost depth governs in cold climates: a foundation not carried below it moves as the ground freezes and thaws, and an array that shifts seasonally loosens its own connections.
Soil bearing capacity governs the size. Uplift matters as much as downward load, because a tilted array in wind generates lift, and a foundation adequate in compression can be inadequate in tension.
A geotechnical assessment is normal on anything beyond a small residential array, and skipping it is how foundations get designed from assumptions.
The Trench Run
The conductors between the array and the building are frequently the largest wiring decision in the project.
Voltage drop rises with distance and falls with conductor size. A long run either uses larger conductors, which costs more in material, or accepts a loss that continues for the life of the system. See wire sizing and voltage drop.
Whether the run carries DC or AC changes the calculation and the code requirements. An array with its inverter at the array end sends AC down the trench; one with a central inverter at the building sends DC. Each has consequences for conductor sizing, for protection, and for what has to be disconnected during maintenance.
Trenching also has to satisfy depth requirements and to be routed around anything already buried. Locating existing services before digging is not optional.
Vegetation and Animals
Grass grows into the array, and grass that reaches the lower edge of a module shades it. Row height above ground determines how often this matters and how easily it is managed.
Mowing under and between rows needs clearance and access, so the layout should account for whatever equipment will be used.
Animals shelter under raised arrays, and rodents chew conductor insulation. Skirting around the perimeter is common, and it interacts with airflow beneath the modules, which affects their temperature; see temperature and heat.
Ground cover choice affects soiling, dust, and reflection onto the rear of the modules where they are bifacial; see bifacial panels.
Security and Siting
An array in a field is accessible in a way a roof array is not. Fencing, lighting and monitoring are ordinary considerations rather than paranoid ones, particularly for arrays away from occupied buildings.
Siting is also a permitting question. Setbacks, height limits, visibility from roads or neighbours, and land classification are all set locally and vary considerably by jurisdiction. Check the local requirements before committing to a position, because a location that works technically may not be permitted.
What People Get Wrong About This
The first mistake is designing foundations without a soil assessment. The array’s loads are not evenly distributed and uplift is the load most often underestimated.
The second is not carrying foundations below frost depth in a climate that has one.
A third is sizing the trench run to minimum code rather than considering voltage drop over the life of the system. The loss is permanent.
A fourth is setting rows too low, so that vegetation shades the bottom of every module and mowing is awkward.
And the last is confirming the location after the design rather than before. Permitting requirements are local and they can move the array.
Related
Residential solar covers the wider decision, and series against parallel covers the string layout that row arrangement interacts with.
Common questions
When is a ground mount better than a roof mount?
When the roof is shaded, faces the wrong way, is nearing the end of its life, cannot carry the load, or is simply too small. A ground mount also allows the tilt and orientation to be chosen freely rather than inherited from the building, and it makes maintenance far easier.
What foundations do ground mounts use?
Driven piles where the soil accepts them, helical piles or screws where it does not, concrete footings where conditions demand, and ballasted frames where excavation is not possible. Soil conditions and frost depth decide, which is why a geotechnical assessment is normal on anything beyond a small array.
Does the long cable run to the house matter?
Yes. Voltage drop rises with distance and falls with conductor size, so a long run either needs larger conductors or accepts a loss. On a ground mount set well away from the building, the trench run is often the single largest wiring decision in the project.
Do ground mounts need planning permission?
It depends on where you are. Requirements are set locally and vary by jurisdiction, covering setbacks, height, visibility and sometimes land classification. Check with your local authority before committing to a location, because a position that works technically may not be permitted.
How much maintenance do they need?
Less effort per visit than a roof array, because everything is reachable from the ground, and more attention to the surroundings. Vegetation grows into the array, animals shelter under it, and the surface beneath affects both shading and soiling.
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