Every kilowatt generated by a utility-scale solar array has to travel through the collection system before it reaches the inverter and ultimately the grid. That collection system, the network of underground conduit, wire, and combiner connections that gathers DC power from string circuits and routes it to the inverter pads, is one of the largest and most labor-intensive electrical scopes on any solar construction project. Yet underground collection system installation receives far less public attention than modules, trackers, or substations, even though it runs underneath every acre of a utility-scale project and must be executed correctly for the system to perform as designed.
The collection system is where conduit trenching, conduit installation, wire pulling, splicing, termination, and grounding all converge in a scope that must be coordinated with every other construction activity on the site. Getting the underground collection system right, on schedule and within the design specifications, requires careful planning, disciplined sequencing, and a field crew that understands the technical requirements of solar electrical work at scale.
What the Underground Collection System Includes
The collection system on a utility-scale solar project begins at the module string leads on each tracker row and ends at the DC input terminals of the inverter. Between those two points, the system typically includes string wiring connecting individual module strings to combiner boxes or string inverters; homerun conduit runs carrying wires from the combiner boxes to the inverter pads; underground conduit infrastructure including PVC conduit, HDPE conduit, or direct burial cable depending on the system design; pull boxes and junction boxes at routing changes and wire segment transitions; grounding conductors tied into the site’s grounding grid; and in some designs, medium-voltage cable connecting pad-mount transformers to the collection substation.
The volume of underground work on a large utility-scale project is substantial. A 100 MW project might involve hundreds of miles of conduit, thousands of circuit connections, and millions of feet of wire. The electrical scope rivals the mechanical scope in labor hours and material cost, and a failure to sequence or execute it correctly affects every other phase of the project from tracker commissioning through energization.
Trenching: The Foundation of the Collection System
Underground collection system installation begins with trenching, and trenching must be sequenced correctly relative to other civil and mechanical work to avoid conflicts that cost time and money to resolve.
Rough grading must be complete before collection system trenching begins in any given area of the project. Trenching in areas that have not been graded risks having the trench cross areas that will be disturbed again during final grading, requiring the trench to be reopened and the conduit to be relocated or protected during subsequent grading passes. The civil crew must complete final grade in each zone before the electrical trench crew moves through.
Trench depth is specified by the electrical design and must meet the minimum cover requirements of the National Electrical Code (NEC) for the type of conduit and wiring method used. For rigid metal conduit the minimum cover is typically 6 inches. For PVC conduit in areas not subject to vehicular traffic, 18 inches is typical. For direct burial cable and HDPE conduit in areas that may be subject to damage, 24 inches is common. The trenching crew must maintain consistent depth throughout each run and must verify final depth before conduit is placed and backfill begins.
The trench route must avoid conflicts with the pile grid, with drainage structures, and with any other underground utilities that were installed during civil work. Pre-trench locating and coordination with the civil as-built records is essential. A conduit run that encounters an unexpected drainage culvert or a mislocated pile after the trench is open creates a routing conflict that requires engineering input to resolve.
Our post on Grading and Civil Work for Utility-Scale Solar: Setting the Stage covers how grading and drainage work on a solar site sets up subsequent construction phases, including the sequencing of civil work that must precede underground electrical installation.
Conduit Installation: Material Selection and Field Execution
The conduit specified for underground collection system installation depends on the design engineer’s selections based on voltage, ampacity, depth of burial, soil conditions, and the project specification. Common options include Schedule 40 or 80 PVC conduit for most underground residential and commercial runs, HDPE conduit where flexibility for directional boring or longer runs is needed, and rigid galvanized steel conduit (RGS) for exposed above-grade portions and where additional mechanical protection is required.
PVC conduit in underground solar collection systems must be properly solvent welded at every joint. A poorly made joint that allows soil water infiltration into the conduit run becomes a water trap that can damage wire insulation over time and create ground fault conditions that are difficult to locate and repair after the system is energized. The crew pulling conduit must verify that every joint is properly made and that conduit is seated fully before backfill begins.
Conduit bends must be fabricated or installed with radii that meet the minimum bend radius requirements for the conductors being pulled. Pulling wire through conduit with bends that are too tight damages wire insulation and increases the pulling tension beyond the conductor’s rated tension limit. For long conduit runs with multiple bends, the pulling tension calculation must confirm that the conductor can be pulled without exceeding its rated limit, and pull boxes must be located at intervals to reduce pulling tension on extended runs.
Conduit must be properly supported above grade at inverter pads and transition points, with supports spaced within the code-specified interval for the conduit type. Underground conduit must be installed with a minimum of one inch of sand bedding below and six inches above before the trench is backfilled with native soil. In areas subject to vehicular traffic, concrete encasement may be required.
Wire Pulling: Planning for Long Runs at High Volumes
Wire pulling on a utility-scale solar collection system is a high-volume operation that benefits significantly from planning and equipment investment. A project with hundreds of homerun circuits, each potentially hundreds of feet long, requires an approach to wire management, reel staging, and pulling equipment that is fundamentally different from commercial electrical work.
Reel staging should be planned at the start of the pull sequence to minimize the distance reels must be moved during the installation. Reels are heavy and moving them with forklifts or cable trailers through an active construction site takes time. Identifying staging locations for each zone of the project before work begins and delivering reels to those locations in the sequence they will be used reduces handling time and improves crew productivity.
Pulling equipment for long runs should include a power cable puller capable of generating the required tension for the run length, conductor size, and number of bends involved. Manual pulling of long large-conductor runs is both physically demanding and risks exceeding conductor tension limits when resistance is underestimated. Tension monitoring during the pull allows the crew to verify that the pull is proceeding within safe limits and to stop if an obstruction or excessive friction is encountered.
Wire identification and labeling must be established before pulling begins and maintained consistently throughout the installation. Each circuit must be labeled at both ends with a consistent identifier that matches the project’s circuit numbering system. Wire pairs must be maintained together through the conduit system and must not be mixed with conductors from other circuits. Proper labeling at the time of installation prevents the far more time-consuming process of tracing and identifying unmarked circuits at commissioning.
The National Fire Protection Association (NFPA), through its National Electrical Code (NFPA 70), establishes the installation requirements for underground wiring methods in solar collection systems, including conduit fill limits, minimum conductor ampacity, and grounding requirements. More information on NEC requirements for solar electrical installations is available at nfpa.org.
Grounding System Integration
The grounding system for a utility-scale solar project is installed in conjunction with the collection system and must be planned as an integrated scope, not an afterthought. The grounding grid connects all metallic structures, equipment enclosures, and conduit systems to a common ground reference that limits touch and step potentials in the event of a fault and provides the fault current path required for protection system operation.
Ground conductor installation in the collection system typically involves a bare copper conductor run in the collection system trench alongside the conduit, connected at regular intervals to ground rods and to the structural ground network at pile foundations, inverter pads, and combiner box mounting structures. The grounding design must ensure continuity from every metallic structure to the main grounding grid and ultimately to the substation ground grid.
Grounding connections at equipment must be made with listed connectors and must be verified to be mechanically secure before trench backfill. A grounding connection that fails after backfill creates a ground fault condition that may not be detected until the system is energized, and locating a failed grounding connection under a completed project site is expensive and disruptive.
Our post on Substation and Interconnection Infrastructure covers the substation grounding requirements and how the project’s collection system grounding ties into the main substation ground grid, completing the grounding system that protects the entire project from equipment to grid connection.
Sequencing Underground Collection System Installation With Other Scopes
The sequencing of underground collection system installation relative to tracker installation, module installation, and above-grade electrical work is one of the most important coordination decisions in the utility-scale solar construction schedule.
Generally, underground conduit should be installed after final grade is established in each zone and after tracker piles are driven in that zone, but before tracker motor wiring and string wiring begins. This sequence avoids having underground conduit cross areas that will be disturbed by pile driving and allows the string wiring crew to work toward already-installed homerun conduit rather than working around an incomplete underground system.
Backfill and compaction of the collection system trench must be completed before heavy equipment returns to the area for any purpose. A partially filled trench creates a collapse hazard for both equipment and personnel and can damage conduit if heavy equipment crosses an unsupported area.
The Federal Energy Regulatory Commission (FERC), through its interconnection technical requirements for utility-scale solar projects, establishes the electrical system standards that the collection system must meet as a condition of interconnection approval, including grounding requirements and protection coordination standards that affect how the collection system is designed and installed. More information on FERC’s interconnection technical requirements is available at ferc.gov.
Our post on Construction Phasing in Utility-Scale Solar: Prep to Energization covers how each construction phase, including underground electrical work, is sequenced relative to the other scopes in the project timeline and what the dependencies between phases mean for the project schedule.
Testing and Verification Before Backfill and After Pull
Two verification steps in the underground collection system installation are critical and cannot be deferred to after the project is otherwise complete.
Pre-backfill inspection should confirm conduit depth, joint integrity, conductor routing, and grounding connections before soil is placed over any portion of the underground system. Correcting a problem after backfill requires re-excavation that is time-consuming and disruptive. A brief inspection before backfill begins in each zone is the most efficient way to verify that the work meets the specification.
Post-pull testing including megger testing of conductors for insulation resistance and continuity testing of ground conductors should be completed before the system is energized. Insulation defects in the wire, conduit joints that allowed water infiltration during construction, and grounding continuity failures are all conditions that are far more efficiently detected and corrected before energization than after.
Our post on Electrical Safety During Solar Commissioning covers the electrical safety protocols that govern the commissioning phase when the collection system transitions from a construction scope to a live electrical system, including the testing and verification activities that confirm the system is ready for energization.