Use the free Solar Wire Size Estimator below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.
Why Solar Wire Size Deserves Its Own Calculation
Solar panels usually run at low voltage and modest current over long distances: from a roof or a field to a charge controller or inverter. Low voltage makes voltage drop especially painful, because a 2 volt drop is a small fraction of 400 volts but a large fraction of 24 volts. That is why solar installers size wire by voltage drop, not just by ampacity, and why this estimator works backward from your allowed drop percentage to the right gauge.
The estimator takes the circuit current, the one-way run length, the system voltage, and the maximum allowed voltage drop percentage, then recommends the thinnest gauge that keeps the actual drop within your limit. It uses standard copper resistance values per gauge, and it accounts for the full loop, out and back, because current travels both conductors.
How to Use the Estimator
Enter the circuit current in amps. If you only know the panel short-circuit current, multiply it by 1.25 to get the design current, which is the standard safety factor for solar circuits. For a string of panels, use the string current, not the sum of all strings, unless the wire carries combined strings. Then enter the one-way length in metres, from the array to the controller or inverter, following the actual cable route, not the straight-line distance.
Enter the system voltage: 12, 24, or 48 volts for battery systems, or the string voltage for grid-tied systems, which can be several hundred volts. Enter the maximum allowed drop, typically 3 percent for solar runs, or 2 percent if you want a tighter design. The estimator shows the recommended gauge, the actual drop percentage, and the drop in volts.
A Worked Example
Take a 10 amp circuit over a 20 metre one-way run at 48 volts with a 3 percent limit. The allowed drop is 1.44 volts. A 14 AWG wire would drop about 3.31 volts, which is 6.9 percent, far too much. A 12 AWG wire drops about 2.08 volts, still too much. A 10 AWG wire drops about 1.31 volts, which is 2.7 percent, so the estimator recommends 10 AWG. That is three sizes thicker than a naive ampacity-only choice would suggest, and it is exactly why solar runs surprise people.
Now raise the system voltage to 120 volts with the same current and length. The allowed drop becomes 3.6 volts, and even 14 AWG at 3.31 volts passes. Higher system voltage is the most powerful lever in solar design: it cuts the drop percentage without thicker wire, which is why modern string inverters run at high voltage.
Length, Current, and Voltage: The Three Levers
Voltage drop grows with length and current, and shrinks with thicker wire and higher voltage. If the estimator says no gauge in its table meets your limit, you have three honest options: shorten the run by moving the controller or inverter closer to the array, raise the system voltage by rewiring panels in series, or split the run into parallel circuits. Pretending the drop away is not an option, because lost voltage is lost energy, every day, for the life of the system.
Also remember temperature. Copper resistance rises with temperature, and a roof run in summer sun is much hotter than 20 degrees. The estimator uses standard 20 degree resistance values, so treat its result as a best case and add margin for hot runs. Outdoor solar cable also needs UV resistance and the right temperature rating, which are separate from gauge.
Common Mistakes to Avoid
The most common mistake is entering the loop length as the one-way length, which doubles the calculated drop and oversizes the wire, or entering the one-way length when the tool expects the loop, which undersizes it. This estimator asks for one-way length and doubles it internally, so enter the single distance. Another mistake is using the panel rated current instead of the design current with the 1.25 factor, which undersizes protection and wire. A third mistake is sizing for ampacity only and ignoring drop, which is the classic solar error.
This estimator is educational only. It estimates gauge from voltage drop using standard copper values, but it does not approve a solar installation or guarantee code compliance. Solar work involves DC arc risks and specific code requirements. Check your local electrical codes and consult a licensed electrician before doing any electrical work on solar systems.
Frequently Asked Questions
Why 3 percent? Three percent is the common design target for solar DC runs, balancing energy loss against wire cost. Some designers use 2 percent for premium efficiency.
Can I use aluminium wire for solar? Yes, with thicker sizes and proper terminations rated for aluminium, but copper is the usual choice for small solar runs because terminations are simpler.
Does the estimator cover the AC side? No. It sizes the DC run from array to controller or inverter. The AC output side follows normal mains wiring rules.
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