The Challenge
At utility scale, DC cabling is a major cost line item. On a 130 MWp plant with 338 DC combiner boxes across 26 transformer areas, the choice of string cabling topology determines total cable length, voltage drop, and procurement cost for the entire site.
Most teams select one topology and apply it uniformly. This study compared all three standard approaches on the same terrain-aware layout.
The Three Topologies
Line String: Each string runs a dedicated positive and negative cable pair directly to the combiner box. Simplest to design, but the return path creates the longest total cable run.
U String: Cables route in a U-shape, covering two parallel rows per string. Both cable ends terminate near the combiner, eliminating the long return run.
Leapfrog String: Cables zigzag across adjacent rows in an alternating polarity pattern. Most complex to install, but produces the shortest total cable path.
Per-String Analysis
PVX.Cad generated all three topology variants from the same layout. Cable routes followed actual site corridors, not bird’s-eye distance estimates.
| Topology | Loop Length (m) | Voltage Drop @ 4mm2 | @ 6mm2 | @ 10mm2 |
|---|---|---|---|---|
| Line String | 191.86 | 1.20% | 0.80% | 0.48% |
| U String | 181.37 | 1.14% | 0.76% | 0.45% |
| Leapfrog | 165.85 | 1.04% | 0.69% | 0.42% |
String current: 21.49 A. Voltage drop calculated per IEC 60287 using the formula: ΔV = I x L_loop x ρ / S.
Aggregate Cable Voltage Calculator
| Topology | Total Loop (m) | Voltage Drop |
|---|---|---|
| Line String | 575.57 | 12.43V (0.83%) |
| U String | 544.10 | 11.75V (0.78%) |
| Leapfrog | 497.56 | 10.74V (0.72%) |
CAPEX at 130 MWp Scale
Combiner box power: 24 strings x 16.12 kW = 386.88 kW. Each 5 MW transformer area contains 13 combiners. The full 130 MWp site has 26 transformer areas and 338 total combiners.
| 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.
The Hybrid Approach
Leapfrog is not always optimal. The zigzag pattern is more complex to install and harder to troubleshoot during maintenance. On ridge-adjacent blocks with access road crossings, the routing may not be feasible.
The engineering team adopted a hybrid: Leapfrog for 73% of inverter blocks, U String for three ridge-adjacent blocks. This captured 96% of the maximum savings while avoiding installation complexity in difficult zones.
Key Findings
- $429,936 saved at 130 MWp by choosing Leapfrog over Line String topology.
- 14% shorter total cable runs with Leapfrog versus Line String.
- Voltage drop reduced from 1.20% to 1.04% at the same cable cross-section (4mm2).
- Cable routes were calculated along actual site trench corridors, not bird’s-eye estimates.
- PVX.Cad auto-generated all three topology variants and computed voltage drop for every string.
- A hybrid approach (Leapfrog + U String) captured 96% of savings while managing installation complexity.
- The $1,272 difference per combiner compounds to $430K across 338 combiners.
Engineering analysis by Mustafa Unal. Designed with PVX.Cad and PVX.View.