Solar Wire Size Calculator (AWG)
Determine the correct wire gauge based on current, voltage, and distance
How to Use This Calculator
- Enter the maximum current (amps) the wire will carry — use your breaker or controller rating.
- Input the one-way cable distance from source to load in feet.
- Select your system voltage (lower voltages like 12V need thicker wire over distance).
- Choose the maximum allowable voltage drop (3% is standard for solar circuits per NEC guidance).
- The calculator recommends the smallest AWG that meets both voltage-drop and ampacity requirements.
Wire Sizing Formula
Required Circular Mils (CM) = (2 × 12.9 × Amps × Distance) ÷ Allowable Voltage Drop (V)
where Allowable Voltage Drop = System Voltage × Drop%, and K = 12.9 for copper.
Select the smallest AWG whose circular-mil area ≥ required CM and whose ampacity ≥ current.
| AWG | Circular Mils | Ampacity (75°C Cu) | Ohms / 1000 ft |
|---|---|---|---|
| 14 AWG | 4,107 | 20 A | 3.077 |
| 12 AWG | 6,530 | 25 A | 1.932 |
| 10 AWG | 10,380 | 35 A | 1.221 |
| 8 AWG | 16,510 | 50 A | 0.769 |
| 6 AWG | 26,240 | 65 A | 0.491 |
| 4 AWG | 41,740 | 85 A | 0.308 |
| 2 AWG | 66,360 | 115 A | 0.194 |
| 1/0 AWG | 105,600 | 150 A | 0.122 |
| 4/0 AWG | 211,600 | 230 A | 0.061 |
Solar Wire & Cable
Frequently Asked Questions
Why does voltage drop matter in solar?
Excess voltage drop wastes energy as heat and can prevent charge controllers and inverters from operating correctly. Keeping drop under 3% ensures efficient charging and protects equipment.
Can I use aluminum wire instead of copper?
Aluminum requires a larger gauge (typically two sizes up) and special connectors. For residential solar, copper PV wire (USE-2) is standard and recommended.
What is the 12.9 constant?
12.9 is the K factor (resistivity) for copper in ohm-circular-mils per foot, used in the standard voltage-drop calculation per NEC Annex C.