Understanding Cable Size Selection for Solar Systems
Selecting the correct cable size is one of the most important safety and efficiency decisions in any solar installation. A cable that is too small causes voltage drop and heat buildup, reducing power delivery to your loads or inverter. A properly sized cable ensures minimal losses, better performance, and long-term system reliability.
1. What Is Voltage Drop?
Every wire has some electrical resistance. As current flows, a small amount of voltage is lost across the length of the cable. This is called voltage drop. It reduces the voltage available at the load end and can cause inefficiency or malfunction of connected equipment.
For example, if your solar charge controller receives only 11.5 V from a nominal 12 V battery due to cable losses, charging performance drops sharply. In high-current systems, even a 0.5 V loss can waste a significant amount of energy as heat.
2. Allowable Voltage Drop in Solar Wiring
For DC solar systems, the general recommendation is to keep total voltage drop below:
- • 2–3% for main charging or inverter cables
- • 3–5% for low-current signal or lighting circuits
- • 1–2% for long runs where efficiency is critical
These limits are not just about efficiency—they also prevent excessive heat generation. Overheating can degrade insulation and shorten the life of your wiring.
3. The Cable Sizing Formula
The formula used in this calculator follows Ohm’s law and basic electrical principles:
Area (mm²) = (2 × L × I × ρ) ÷ ΔV
Where:
- L = one-way cable length (m)
- I = current (A)
- ρ = resistivity of conductor (0.0175 Ω·mm²/m for copper)
- ΔV = allowable voltage drop (V × % drop / 100)
This gives the required cross-sectional area in square millimeters (mm²). The calculator then converts this to the nearest AWG (American Wire Gauge) size.
4. Copper vs. Aluminum Conductors
Most solar wiring uses copper due to its higher conductivity and better durability. However, aluminum cables can be cheaper and lighter for large distances. Aluminum has about 60% of copper’s conductivity, so you need a larger size to carry the same current.
If using aluminum, multiply the copper cable area by approximately 1.6 to get the equivalent cross-section.
5. Cable Length and Efficiency Loss
Cable length has a direct and quadratic effect on losses. Doubling the length doubles the resistance—and thus doubles the voltage drop. For long-distance runs between solar panels and inverters, consider using a higher system voltage (e.g., 24 V or 48 V) to reduce current and cable size requirements.
For instance, a 10 m 12 V run carrying 30 A might need 16 mm² copper wire. The same power at 48 V would only carry 7.5 A—requiring just 4 mm² cable for the same loss percentage.
6. AWG to mm² Conversion Table
The calculator internally uses the standard AWG conversion chart. Here are a few common examples for quick reference:
| AWG | mm² | Ampacity (approx.) |
|---|---|---|
| 10 AWG | 5.26 mm² | 30–35 A |
| 8 AWG | 8.37 mm² | 40–55 A |
| 6 AWG | 13.3 mm² | 60–80 A |
| 4 AWG | 21.2 mm² | 85–100 A |
| 2 AWG | 33.6 mm² | 110–150 A |
7. Temperature and Installation Conditions
Cable current ratings assume standard conditions (30 °C ambient, open air). In hot or enclosed environments—such as conduits on rooftops—derating is necessary. Higher temperatures increase resistance and lower ampacity.
Always check manufacturer datasheets for derating factors when cables are bundled, buried, or exposed to sunlight.
8. Practical Tips for Real Installations
- • Keep high-current runs (battery to inverter) as short as possible.
- • Use tinned copper lugs and crimp connectors to minimize contact resistance.
- • Choose fine-stranded flexible wire for mobile or vibration-prone setups (e.g., RV or boat solar).
- • Apply heat-shrink and cable management for safety and longevity.
- • Verify voltage drop on both positive and negative conductors (the calculator already accounts for round trip).
9. When to Use Parallel Cables
For very high current loads, one cable may not be practical or available in large enough size. In such cases, you can use parallel cables—two or more cables of the same gauge carrying equal current. For example, two 10 mm² wires in parallel provide the same total capacity as one 20 mm² wire.
This approach reduces heating and increases flexibility, but all cables must be the same length and routing to ensure even current sharing.
10. Summary
The correct wire gauge is vital for solar system safety, efficiency, and compliance with standards. Using the SolarMathLab Cable Size Calculator, you can quickly find the minimum cable cross-section needed for your current, length, voltage, and voltage drop target. Always round up to the next larger size or AWG, especially for long runs or hot environments.
Remember: oversized cables cost a bit more, but undersized cables waste energy, overheat, and pose a fire risk. In solar systems, efficiency and safety start with the right cable size.