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.
Last updated: September 2026
Formula below · 2 sources (nrel.gov, Wikipedia) · Updated Sep 2026
Compare with similar
About this calculator
Wire gauge selection balances two concerns: voltage drop (resistance losses must stay within an acceptable percentage of system voltage) and thermal safety (the wire must not overheat under the current it carries). The required copper area in circular mils is: cmil = (2 × current × distance × K) / (voltage × voltageDrop), where distance is the one-way run in feet and K is copper resistivity in ohm·cmil/ft at the operating temperature you enter: K = 12.9 × (234.5 + T) / (234.5 + 75), i.e. 12.9 at 75 °C and about 10.8 at 25 °C (copper resistance rises about 0.39% per °C). Copper areas (NEC Chapter 9, Table 8): 14 AWG = 4,110 cmil, 12 = 6,530, 10 = 10,380, 8 = 16,510, 6 = 26,240, 4 = 41,740, 3 = 52,620, 2 = 66,360, 1 = 83,690, 1/0 = 105,600, 2/0 = 133,100, 3/0 = 167,800, 4/0 = 211,600. The calculator returns the smallest standard gauge whose area meets the requirement — it always rounds up to the larger wire. It also checks ampacity: the conductor must carry 125% of the entered current using the conservative 60 °C column of NEC Table 310.16 (14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A, 8 AWG 40 A, 6 AWG 55 A, 4 AWG 70 A, 3 AWG 85 A, 2 AWG 95 A, 1 AWG 110 A, 1/0 125 A, 2/0 145 A, 3/0 165 A, 4/0 195 A), and it never returns smaller than 14 AWG. Sizes 1/0 and larger are shown as text (for example "1/0 AWG"). Ampacity must still be corrected for hot locations, conduit fill and the conductor insulation rating, and PV source circuits must use the NEC 690.8 design current; have the final size confirmed by a licensed electrician.
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. K = 12.9 × (234.5 + 35) / 309.5 = 11.23 ohm·cmil/ft. Required area = (2 × 20 × 60 × 11.23) / (48 × 0.03) = 26,957 / 1.44 = 18,721 cmil. 8 AWG (16,510 cmil) is too small, so the result is 6 AWG (26,240 cmil). The ampacity check needs 1.25 × 20 = 25 A, which 6 AWG (55 A) covers. Using 10 AWG here would give about 5.4% voltage drop instead of 3%.
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. This calculator corrects copper resistance for the operating temperature you enter (about 0.39% per °C); the ampacity check uses the 60 °C column for a 30 °C ambient, so apply the NEC Table 310.15(B)(1) correction factors for hotter locations. 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.