Solar Wire Sizing Calculator
Determine the correct AWG wire gauge for any run in your solar array based on current, distance, voltage, and temperature. Use this before purchasing cable to avoid dangerous overheating or efficiency-robbing voltage drop.
About this calculator
Wire gauge selection balances two concerns: thermal safety (the wire must not overheat under maximum current) and voltage drop (resistance losses must stay within an acceptable percentage of system voltage). The resistance of a copper conductor in circular mils is approximately 12.9 Ω·cmil/ft. Voltage drop allowed = voltage × voltageDrop fraction. The required cross-sectional area in circular mils is: cmil = (2 × current × distance × 12.9) / (voltage × voltageDrop). Higher temperatures reduce copper's ampacity, so a derating factor of 0.4% per °C above 25°C is applied. AWG gauge numbers decrease as wire size increases; each step of 3 AWG roughly doubles the cross-sectional area (factor of 1.26 per step). The formula resolves the required cmil into the nearest whole AWG: AWG = ⌈log(cmil) / log(1.26)⌉. Always verify the result against NEC ampacity tables and local code.
How to use
Suppose you have 20 A of current, a 60-foot wire run (one-way), a 48 V system, 3% acceptable voltage drop (0.03), and 35 °C operating temperature. Temperature derating factor = 1 − max(0, 35−25) × 0.004 = 1 − 0.04 = 0.96. Required cmil = (2 × 20 × 60 × 12.9) / (48 × 0.03 × 0.96) = 30,960 / 1.382 = 22,402. AWG = ⌈log(22,402) / log(1.26)⌉ = ⌈10.77⌉ = 11, so you would select AWG 10 (rounding to the next commercially available size).
Frequently asked questions
Why does wire gauge matter so much in a solar panel installation?
Undersized wires create resistance that converts electrical energy into heat, reducing system efficiency and posing a fire hazard. In a solar DC circuit, even a 3% voltage drop across a long run can meaningfully reduce the power delivered to your inverter or charge controller. The NEC requires wire rated for the maximum short-circuit current of the source, plus a 125% safety factor for continuous loads. Using the correct gauge from the start is far cheaper than replacing burned wiring or damaged equipment later.
What is an acceptable voltage drop percentage for solar wiring?
The standard guideline is no more than 3% voltage drop on any single circuit segment, and no more than 5% total from panels to load. For the DC side between panels and charge controller, 1–2% is preferred because every lost volt reduces charging efficiency. On the AC output side of an inverter, 3% is widely accepted. Lower voltage drop requires larger (lower AWG number) wire, which costs more but improves overall system performance. For long runs—over 50 feet—even 1% drop is worth targeting.
How does operating temperature affect solar wire sizing?
Copper's electrical resistance increases slightly with temperature, and its safe current-carrying capacity (ampacity) decreases. The derating factor used here is 0.4% per °C above the 25°C baseline. Wiring in a hot attic, conduit on a sun-exposed roof, or inside a combiner box can easily reach 50–70°C, significantly reducing safe ampacity. NEC Table 310.15 provides official temperature correction factors by insulation type. Ignoring temperature derating is one of the most common mistakes in DIY solar installations and can lead to insulation damage or fire.