Solar DC Wire Size & Voltage Drop Calculator

Find the correct AWG or mm² wire gauge for 12V, 24V, and 48V solar DC circuits. Enter your system parameters and instantly see which conductor keeps voltage drop within your target — calculated from first principles using the resistivity formula.

How it works

Voltage drop in a DC cable is calculated using the conductor's resistivity, the round-trip length (2 × one-way), and the wire cross-sectional area:

Voltage Drop Formula Vdrop = ρ × 2L × I ÷ A
Resistivity (ρ) Copper: 1.724 × 10⁻⁸ Ω·m
Aluminum: 2.82 × 10⁻⁸ Ω·m
Drop % Check Drop% = Vdrop ÷ Vsystem × 100
Round-Trip Length 2L accounts for both the positive and negative (return) conductor runs

Example: 24V system, 20A, 5 m one-way, copper AWG 6 (13.3 mm² = 13.3×10⁻⁶ m²):
Vdrop = 1.724×10⁻⁸ × (2×5) × 20 ÷ 13.3×10⁻⁶ = 0.259 V → 1.08% ✓

Frequently asked questions

Why use 3% voltage drop for solar DC wiring?
The 3% limit is the NEC and industry standard for branch circuits in solar PV systems. Excessive voltage drop wastes energy as heat in the cable — at 3% drop on a 24V/20A circuit you already lose about 14.4 W continuously. For critical loads or long runs, 1–2% is preferred. The 5% maximum is sometimes accepted for non-critical lighting circuits, but never for inverter feeds or charge controller connections where efficiency matters most.
Should I use copper or aluminum wire for solar?
Copper is the default choice for most solar installations: it has lower resistivity (1.724 vs 2.82 × 10⁻⁸ Ω·m), better corrosion resistance, and is easier to terminate. Aluminum is significantly cheaper per metre and is common for long service entrance runs (house to battery bank) where weight also matters. If you choose aluminum, always use anti-oxidant paste on terminals and aluminum-rated lugs — aluminum oxidizes and the oxide layer is highly resistive.
How does system voltage (12V vs 24V vs 48V) affect wire size?
Higher voltage systems carry the same power at lower current (P = V × I). For the same watt load, a 48V system draws ¼ the current of a 12V system. Since voltage drop scales with current (Vdrop = ρ × 2L × I ÷ A), the 48V system can use much thinner wire for the same power and the same percentage drop. This is why larger solar systems (5 kW+) almost always run at 48V — the wiring cost savings alone justify the higher-voltage equipment.
What AWG wire is used for solar panels to charge controller?
This depends on your panel current and run length. A common residential setup — 10A MPPT input, 6 m run, 24V system — calls for AWG 10 (5.26 mm²) at about 2.5% drop. Larger arrays with 30–40A MPPT inputs on longer runs routinely need AWG 4 (21.2 mm²) or AWG 2 (33.6 mm²). Always size for the maximum rated short-circuit current (Isc) of your panel string, not just the operating current. This calculator lets you enter any current value to match your exact equipment.
Does this calculator account for wire ampacity (current-carrying capacity)?
No — this tool calculates voltage drop only. Ampacity (the maximum safe current a wire can carry without overheating) is a separate constraint governed by insulation type, ambient temperature, and conduit fill. Always cross-check the recommended wire size against the ampacity tables in NEC 310.16 (US) or IEC 60364-5-52 (EU). In most solar runs, voltage drop is the binding constraint rather than ampacity, but you should verify both.