At 130 MWp scale, the choice between three standard DC cabling topologies produced a $430K cost difference. The cables connected the same panels to the same inverters on the same terrain. The only variable was the routing pattern.
Most engineering teams pick a topology early and never compare alternatives. This article shows why that default costs money.
The Three Topologies
Line String is the simplest approach. Each PV string runs a dedicated positive and negative cable pair directly to the DC combiner box. The cable follows a straight path from string end to combiner. It is easy to design, but the return path creates the longest total cable run.
U String routes cables in a U-shape, covering two parallel rows per string. Both cable ends terminate near the combiner box, eliminating the long return run. Total cable length is shorter than Line String, but string halves must be carefully balanced.
Leapfrog String routes cables in a zigzag pattern across adjacent rows. Alternating string polarity creates cross-connections that minimize total cable path. It is the most complex to design and install, but produces the shortest total cable length for most terrain configurations.
The Data
PVX.Cad generated all three topology variants from the same terrain-aware layout. Cable routes followed actual site corridors, not bird’s-eye distance estimates. Positive and negative conductor paths were traced along real trench lines, including turns and corridor constraints.
Per-String Comparison (single DC combiner, 24 strings)
| Topology | Loop Length (m) | Voltage Drop @ 4mm2 | Voltage Drop @ 6mm2 | Voltage Drop @ 10mm2 |
|---|---|---|---|---|
| Line String | 191.86 m | 1.20% | 0.80% | 0.48% |
| U String | 181.37 m | 1.14% | 0.76% | 0.45% |
| Leapfrog | 165.85 m | 1.04% | 0.69% | 0.42% |
Leapfrog produces the shortest loop length and the lowest voltage drop across all three cable cross-sections. Line String uses the most cable and has the highest loss. U String falls between the two.
PVX Auto Cable Voltage Calculator (aggregate per topology)
| Topology | Total Loop (m) | Total Voltage Drop |
|---|---|---|
| Line String | 575.57 m | 12.43V (0.83%) |
| U String | 544.10 m | 11.75V (0.78%) |
| Leapfrog | 497.56 m | 10.74V (0.72%) |
All values are calculated per string using real cable route geometry, not averages.
CAPEX at 130 MWp Scale
The per-combiner cost difference scales linearly across the site:
- Combiner box power: 24 strings x 16.12 kW = 386.88 kW
- Combiners per 5 MW transformer area: 13
- Total combiners site-wide: 338 (13 x 26 transformer areas)
| Topology | Per-Combiner CAPEX | Per 5 MW Area | Full 130 MWp Site |
|---|---|---|---|
| Line String | $11,304 | $146,952 | $3,820,752 |
| U String | $10,880 | $141,440 | $3,677,440 |
| Leapfrog | $10,032 | $130,416 | $3,390,816 |
Leapfrog vs Line String savings: $429,936 at 130 MWp.
That is $430K saved by choosing a different cable routing pattern. Same panels. Same inverters. Same terrain.
The Tradeoff: Cost vs Complexity
Leapfrog is not always the right answer. The topology creates cross-row cable paths that are more complex to install and harder to troubleshoot during maintenance. On sites with difficult terrain features (ridges, access road crossings), the zigzag pattern may not be feasible for every inverter block.
The engineering team in this study adopted a hybrid approach: Leapfrog for 73% of inverter blocks where terrain allowed it, and U String for three ridge-adjacent blocks where Leapfrog routing would have crossed access corridors.
This hybrid captured 96% of the maximum possible savings while avoiding installation complexity in difficult zones.
Why This Matters
Most teams select a cabling topology once and apply it uniformly across the site. PVX.Cad generates all three variants from the same layout and computes exact cable lengths along terrain-aware paths. The comparison takes minutes. The procurement savings last the life of the project.
At utility scale, every design decision compounds. A $1,272 difference per combiner box becomes $430K across a 130 MWp site. The tool should show you the options before you commit to one.
Engineering analysis by Mustafa Unal. Full case study data available at pvx.ai/customers.