Solar cables carry both DC current from panels to inverters and AC current from inverters to the grid. Understanding resistance differences between DC and AC applications ensures accurate voltage drop calculations and proper conductor sizing for each portion of the photovoltaic system.
Fundamental Difference
DC Resistance: Direct current distributes uniformly across the conductor cross-section. DC resistance depends solely on conductor material, cross-sectional area, length, and temperature. This straightforward relationship makes DC resistance calculation and measurement relatively simple.
AC Resistance: Alternating current creates time-varying magnetic fields inducing eddy currents within conductors. These effects cause current to concentrate toward conductor surfaces—the skin effect—increasing effective resistance compared to DC measurements.
Skin Effect Mechanism
Current Distribution: At higher AC frequencies, electromagnetic induction causes current to flow primarily in a thin layer near the conductor surface. The conductor center carries minimal current, effectively reducing the cross-sectional area carrying current and increasing resistance.
Skin depth—the depth where current density falls to 37% of surface value—decreases with increasing frequency. At 50-60Hz power frequency, skin depth in copper exceeds 8mm, meaning skin effect minimally affects conductors under 16mm diameter.
Frequency Dependency: Higher frequencies create stronger skin effect. At power frequency (50-60Hz), skin effect marginally affects typical PV cable sizes. At kilohertz frequencies from inverter switching, skin effect becomes more pronounced.
Practical Impact on Solar Systems
DC Circuit Considerations: Solar panel to inverter wiring carries DC current where skin effect doesn't occur. Voltage drop and power loss calculations use DC resistance values at operating temperature.
For a 6mm² copper conductor:
DC resistance: approximately 3.39 Ω/km at 20°C
Temperature coefficient: 0.00393 per °C
AC Circuit Considerations: Inverter to grid connection carries AC current where skin effect may increase resistance slightly. For typical conductor sizes and power frequency, the increase remains modest—typically 2-5% for conductors under 50mm².
Larger conductors used in utility-scale installations experience more significant skin effect, with AC resistance potentially 10-15% higher than DC resistance for conductors above 95mm².
Calculating Resistance Differences
Small Conductors (≤16mm²): Skin effect at 50-60Hz is negligible. AC and DC resistance differ by less than 1-2%, within measurement uncertainty. Designers can use DC resistance values for both AC and DC portions of small-scale systems.
Medium Conductors (25-50mm²): Skin effect creates 2-5% resistance increase at power frequency. This modest difference rarely affects conductor sizing decisions but should be considered in precise voltage drop calculations for long AC cable runs.
Large Conductors (≥70mm²): Skin effect becomes significant, with AC resistance 5-15% higher than DC values. Utility-scale installations with large conductors require separate AC and DC resistance values for accurate system design.
Temperature Effects
Both AC and DC resistance increase with temperature following the same temperature coefficient for copper (0.00393 per °C). Skin effect doesn't significantly change with temperature, so the ratio between AC and DC resistance remains relatively constant across operating temperature ranges.
Measurement Considerations
DC Resistance Measurement: Four-wire (Kelvin) measurement eliminates test lead resistance, providing accurate DC resistance values. Testing at known temperature allows correction to standard reference temperatures.
AC Resistance Measurement: AC resistance measurement requires specialized equipment applying alternating current at specified frequency while measuring voltage drop. The measurement captures combined effects of DC resistance and skin effect.
Standard Specifications
Cable Data Sheets: Manufacturer specifications typically provide DC resistance values at 20°C or 90°C. For solar applications where DC circuits predominate, DC resistance values serve most design calculations.
Some manufacturers provide both DC and AC resistance for larger conductor sizes where skin effect becomes significant.
Design Standards: IEC and NEC standards primarily reference DC resistance for photovoltaic cable specifications. AC resistance specifications appear mainly in utility-scale system designs using larger conductors.
Conductor Design Impact
Stranding Benefits: Stranded conductors minimize skin effect compared to solid conductors of equivalent cross-section. Individual strands have smaller diameter where skin effect is negligible, even though total conductor area may be large.
KUKA CABLE uses tinned stranded copper conductors optimizing both DC and AC performance while providing installation flexibility.
Proximity Effect: When multiple AC conductors run parallel, magnetic field interaction creates additional resistance increase beyond simple skin effect. This proximity effect affects AC cable grouping but not DC solar circuits.
Practical Design Guidance
DC System Sizing: Use DC resistance values for all calculations in solar panel to inverter wiring. Apply appropriate temperature correction for operating conditions.
AC System Sizing: For small systems (residential/commercial) with conductors under 50mm², DC resistance values provide adequate accuracy. For utility-scale systems with large conductors, use AC resistance values accounting for skin effect.
Voltage Drop Analysis: DC circuits: Calculate using DC resistance at maximum operating temperature. AC circuits: Use AC resistance for large conductors; DC resistance acceptable for small conductors with minimal error.
Inverter Output Considerations
High-Frequency Components: Modern inverters produce high-frequency switching components superimposed on power frequency AC. These high-frequency components experience stronger skin effect, creating additional losses in conductors.
Quality solar cables maintain low AC resistance even at higher frequencies, minimizing losses from inverter-generated harmonics.
Conclusion
DC resistance governs most photovoltaic cable design since solar panels generate DC power. AC resistance becomes relevant for inverter-to-grid connections, primarily affecting large conductors in utility-scale systems. Understanding when each resistance type matters ensures accurate voltage drop calculations and proper conductor sizing.
KUKA CABLE's stranded tinned copper conductors provide optimal performance for both DC and AC portions of solar systems, ensuring efficient power transmission throughout the installation.
Contact KUKA CABLE technical team for DC and AC resistance specifications for your solar cable requirements.
