Solar Wire Size Calculator
Determine the minimum wire cross-sectional area needed for a solar DC circuit to keep voltage drop within safe limits. Use this when designing or troubleshooting a solar wiring run between panels, charge controllers, or batteries.
About this calculator
Undersized wiring in a solar system causes resistive losses, heat buildup, and potential fire hazards. The required wire cross-section is calculated by balancing Ohm's Law with acceptable voltage drop. The formula used here is: wire area (mm²) = (2 × current × distance × 0.017) / (voltageDrop% / 100 × 12), where 0.017 is the resistivity of copper in Ω·mm²/m, the factor of 2 accounts for the full round-trip length of the circuit, and 12 represents the system voltage. The result gives the minimum conductor cross-section in mm². You then select the next standard wire gauge that meets or exceeds this value. Most solar DC systems target a maximum voltage drop of 2–3% to minimise power loss and comply with NEC and IEC wiring standards.
How to use
Example: Your charge controller draws 30 A, the wire run is 20 ft (≈6.1 m), the system voltage is 12 V, and you allow a maximum 3% voltage drop. Step 1 — Enter Current: 30 A. Step 2 — Enter Distance: 20 ft. Step 3 — Enter Max Voltage Drop: 3%. Step 4 — Apply the formula: (2 × 30 × 20 × 0.017) / (3 / 100 × 12) = (20.4) / (0.36) = 56.7 mm². Select the next standard cable size above 56.7 mm² for a code-compliant, efficient installation.
Frequently asked questions
What is the maximum recommended voltage drop for a solar DC wiring run?
The NEC (National Electrical Code) and most solar design guidelines recommend keeping total voltage drop at or below 3% for branch circuits and 5% for combined feeders. For solar systems, many installers target 2% or less on high-current runs to minimize power loss and heat. Exceeding these limits wastes energy as heat in the wire, reduces charging efficiency, and can void equipment warranties. In battery-based systems, even small voltage drops compound over thousands of charge cycles, significantly reducing long-term energy yield.
Why does wire run distance matter so much in solar installations?
Voltage drop is directly proportional to wire length — double the distance and you double the resistive loss for the same current. Solar panels are often mounted far from inverters or charge controllers, making long runs inevitable. This is why higher-voltage systems (24 V or 48 V) are preferred for large installations: for the same watt output, higher voltage means lower current, and lower current means far less voltage drop for a given wire gauge. Always measure the actual cable route, not straight-line distance, when calculating wire size.
How do I convert wire cross-section in mm² to AWG wire gauge?
American Wire Gauge (AWG) and metric mm² are two separate sizing systems that don't convert with a simple formula, but standard equivalence tables are widely available. As a reference: 10 AWG ≈ 5.3 mm², 6 AWG ≈ 13.3 mm², 2 AWG ≈ 33.6 mm², and 2/0 AWG ≈ 67.4 mm². Always round up to the next larger (lower AWG number) conductor if your calculated mm² falls between standard sizes. Using a wire that is slightly oversized is safe and reduces losses; undersizing is a fire and equipment risk.