Why Cable Resistance Matters
Every meter of cable in a solar system loses energy. The losses might seem small—a few percentage points—but they add up. Over 30 years of operation, inadequate cable sizing translates directly to lost revenue.
A system with 3% voltage drop loses 3% of its generated power continuously. That's not a one-time efficiency hit—it's ongoing energy that never reaches the inverter or grid.
Voltage Drop: The Hidden Efficiency Killer
When current flows through any conductor, resistance causes voltage drop. Basic physics: V = I × R.
For DC solar circuits, the calculation is: Voltage Drop = 2 × Current × Resistance × Length / 1000
The factor of 2 accounts for both positive and negative conductors. Length is measured in meters, resistance in ohms per kilometer.
To find percentage drop: divide voltage drop by system voltage, multiply by 100.
Standard practice keeps voltage drop under 3%. Codes may allow 5%, but that's leaving money on the table.
Temperature Makes It Worse
Copper resistance increases about 0.4% per degree Celsius. At 90°C operating temperature, resistance is roughly 28% higher than at 20°C.
This matters because solar systems generate most power during hot weather—exactly when cable resistance peaks. The combination reduces system output when production should be highest.
Material Choices
All solar cables use copper conductors—aluminum doesn't provide adequate conductivity for the current levels and installation methods typical in PV systems.
Tinned vs. bare copper: The tin coating doesn't significantly affect conductivity, but it prevents oxidation. Bare copper connections oxidize over time, increasing contact resistance. In humid or coastal environments, this degradation accelerates. Tinned copper maintains low resistance at terminations throughout the system life.
Sizing Cables Correctly
Cable size must satisfy two requirements:
Ampacity: Can it carry the current without overheating? This depends on conductor cross-section, insulation temperature rating, and installation method (free air vs. conduit, bundling, ambient temperature).
Voltage drop: Even if ampacity is adequate, undersized cables waste power through excessive resistance.
Calculate based on actual installation conditions:
String short-circuit current with appropriate safety factors
Actual cable route length (not straight-line distance)
Temperature derating if ambient exceeds 30-40°C
Additional derating for conduit fill
Common sizes in practice:
2.5-4mm² for residential string wiring
6mm² for larger strings or short home runs
10mm² and up for DC main cables and utility applications
Larger wire costs more upfront but pays for itself in reduced losses.
Insulation Resistance
Cable insulation must prevent current leakage between conductors or to ground. Low insulation resistance causes ground faults, system shutdowns, and gradual energy loss.
Quality cables maintain hundreds of megohms across their operating temperature and voltage range. Standards verify this through testing after environmental aging.
Field testing with a megohmmeter identifies degrading insulation before it fails completely.
Connections Are Critical
Perfect cable performs poorly if connections are bad.
Crimping: Use the right tool and die for the connector and wire size. Poor crimps create high-resistance points that generate heat and waste power. Heat accelerates degradation, eventually causing failure.
Torque: Bolted connections need proper torque. Too loose, resistance increases. Too tight, hardware or components get damaged.
Contact surfaces: Keep them clean. Oxidation and contamination increase resistance. Use contact compound where manufacturer specifies.
Bad connections show up as hot spots during thermal imaging. Catch them during commissioning or routine maintenance, before they fail.
KUKA Cable Design
KUKA solar cables use stranded tinned copper conductors sized appropriately across the product range from 2.5mm² to 240mm².
The insulation materials provide high insulation resistance maintained across operating temperatures. Construction is optimized to minimize dielectric losses.
Available sizes cover everything from residential string wiring to utility-scale DC collection cables.
Practical Steps
During design:
Calculate voltage drop for your specific layout
Target under 3% drop
Account for temperature in hot climates
Consider leaving headroom for potential expansion
During installation:
Use proper crimping tools and technique
Check connections before energizing
Keep cable runs as short as practical routing allows
Follow all torque specifications
During operation:
Thermal imaging identifies problem connections
Test insulation resistance every 5-10 years
Re-torque connections if thermal cycling is loosening them
Track measurements to spot trends
The Bottom Line
Cable electrical performance affects every kWh your system produces for its entire life. Proper sizing, quality materials, and correct installation minimize losses.
Larger cables cost more initially. Over 30 years, the energy savings usually justify it. Quality construction—tinned copper, proper insulation, consistent manufacturing—delivers the performance needed for efficient operation.
Calculate voltage drop for your installation. Size accordingly. Don't leave power on the table.
