4mm², 6mm², or 10mm² DC cables: Which cable cross section is best for PV systems?

Selecting the appropriate DC cable size is not just a technical requirement - it is a critical factor that directly impacts the performance, safety, and overall cost efficiency of a PV system.

• Performance: Choosing the correct cable cross-section helps minimize power losses during transmission, ensuring that most of the generated energy is effectively delivered to the inverter and other system components. This is especially important in large-scale systems or installations with long cable runs. If the cable size is insufficient for the current, voltage drop will occur. For example, a 3% voltage drop means a 3% loss in energy output. Over a typical system lifespan of 25 years, this represents a significant cumulative loss.
• Safety: Undersized cables can become overloaded, leading to overheating and potential damage to the insulation. Excessive heat accelerates insulation aging and increases the risk of electrical faults or even fire hazards. DC cables that consistently operate at elevated temperatures are unlikely to achieve their expected service life of 25–30 years. In many cases, premature failure can occur, resulting in costly replacements - especially after the system has already been installed.
• Cost efficiency: Oversizing cables improves safety but increases material and installation costs. On the other hand, undersized cables may reduce upfront costs but lead to higher energy losses and potential maintenance or replacement expenses in the long run. Therefore, selecting the optimal cable size is essential to achieving the right balance between performance, safety, and total investment cost.

2.1 Cable cross section

The fundamental difference between 4 mm², 6 mm², and 10 mm² DC cables lies in the conductor cross-sectional area, which refers to the surface area of the metal conductor (typically copper) inside the cable:

• 4 mm² DC cable: Conductor cross-section of 4 mm²
• 6 mm² DC cable: Conductor cross-section of 6 mm²
• 10 mm² DC cable: Conductor cross-section of 10 mm²

The larger the cross-sectional area, the greater the cable’s ability to conduct electrical current. This directly impacts several key technical factors:

• Current-carrying capacity: Cables with larger cross-sections can carry higher current without overheating. This makes them suitable for systems with higher power output or higher string currents.
• Voltage drop: A larger cross-section reduces the electrical resistance of the conductor, thereby minimizing voltage drop along the cable. This is particularly important in PV systems, where excessive voltage drop can significantly reduce overall system efficiency. For the same current and cable length, a 10 mm² cable will exhibit substantially lower voltage drop compared to a 4 mm² cable.
• Energy loss and system efficiency: Lower resistance results in reduced power loss (in the form of heat). Therefore, selecting an appropriate or slightly larger cable size helps improve long-term system efficiency and performance.

FAQ with HELU: What factors determine a cable’s current-carrying capacity?

2.2 Current-carrying capacity of DC cables

Current-carrying capacity (ampacity) refers to the maximum current a cable can continuously carry under specified conditions without exceeding the permissible temperature limit of its insulation. This is a critical parameter in PV system design to ensure both safety and cable lifespan.

The conductor cross-section (mm²) is the primary factor influencing ampacity. A larger cross-section results in lower electrical resistance and reduced heat generation, allowing the cable to carry higher current without overheating.

For our SOLARFLEX®-X H1Z2Z2-K DC cable, the typical current-carrying capacities are as follows:

2.4 Weight and flexibility considerations

In addition to electrical parameters such as ampacity and voltage drop, the cable cross-section also directly affects the weight and flexibility of DC cables, which in turn influences installation and handling.

In principle, as the cross-section increases:

• The amount of conductive material (copper or aluminum) increases, leading to higher cable weight
• The overall cable diameter becomes larger, which means reduced flexibility and larger minimum bending radius

Take, for example, our SOLARFLEX®-X H1Z2Z2-K and SOLARFLEX®-X H1Z2Z2-K NTS (rodent-resistant version) DC cables:

2.5 Installation environment and its impact on DC cable cross section

The selection of DC cable size (4 mm², 6 mm², 10 mm²) is not determined solely by current and voltage drop. It is also indirectly influenced by the installation environment, which can dictate the required cable construction (sheath, protective layers, armoring). These factors, in turn, affect heat dissipation, cable weight, and ultimately the appropriate cable size.

In environments with a high risk of rodent damage, such as industrial sites or outdoor installations, it is recommended to use armoured DC cables. For example, the SOLARFLEX®-X H1Z2Z2-K NTS (rodent-resistant version) includes an armour layer. However, this added protection increases the cable’s outer diameter and weight, which can reduce heat dissipation. As a result, it may be necessary to increase the cable cross section (e.g., from 6 mm² to 10 mm²) to maintain safe operating conditions.

For direct burial installations, the soil thermal resistivity (K x m/W) becomes a critical factor. Soil with high thermal resistivity (≥ 2.5 K x m/W), such as dry or sandy soil, has poor heat dissipation characteristics. In such conditions, using 4 mm² or 6 mm² cables may lead to overheating. Therefore, a larger size such as 10 mm² is often required to ensure safe and reliable operation.

Selecting the appropriate DC cable cross section (4 mm², 6 mm², or 10 mm²) plays a crucial role in ensuring both the performance and safety of a solar PV system. Each cable cross section has distinct characteristics in terms of electrical capacity, cost, and flexibility, making it suitable for different installation conditions and system scales.

3.1 Applications of 4 mm² DC cable

For small-scale systems, especially residential rooftop installations, 4 mm² DC cables are commonly used for short connections, typically under 10-15 meters. At this distance, current levels remain moderate (around 25–30 A), and voltage drop can be effectively kept below 2%. Thanks to their lightweight structure, high flexibility, and lower cost, 4 mm² cables are ideal for compact system layouts where the inverter is located close to the PV modules. However, when the cable length increases to around 30–40 meters, voltage drop becomes significant, making this size unsuitable for longer runs.

3.2 Applications of 6 mm² DC cable

For larger residential systems or standard commercial installations, 6 mm² DC cables are the most commonly used option. They are well-suited for medium cable runs (typically 20–50 meters), where higher current levels and stricter voltage drop control are required. Compared to 4 mm², the larger cross-section significantly reduces electrical resistance, minimizing energy losses and ensuring stable system performance. This makes 6 mm² the best balance between cost and performance, and the standard choice in most PV installations.