Solar Wire & Cable Gauge Calculator

How to size solar wire correctly

Sizing solar cable comes down to satisfying two separate requirements, and your wire must pass both. The first is ampacity — the cable has to safely carry the current without overheating. The second is voltage drop — the cable has to deliver that current over distance without losing too much voltage as heat. A wire can be thick enough for one and still fail the other, so the correct gauge is whichever requirement demands the larger conductor.

For ampacity, the widely used standard (NEC 690.8) is to size conductors to at least 125% of the maximum continuous current, because solar circuits can run at full output for hours. For voltage drop, the common target is no more than 3% per circuit section (and 5% total from source to load), with 1–2% considered best practice on the panel-to-controller run where every lost volt is lost harvest.

Why higher voltage lets you use thinner wire

This is the single most useful idea in DC wiring. Voltage drop depends on current, not power — and for a given amount of power, higher voltage means lower current. A 3% drop at 12V is only 0.36V, which thin wire blows through over any real distance; at 48V, 3% is 1.44V, far more forgiving. That's why serious off-grid and home systems run at 24V or 48V rather than 12V, and why wiring panels in series (which raises voltage) dramatically cuts the wire size and cost compared with wiring them in parallel. On a long roof-to-controller run, a series string into an MPPT controller might need only 14 AWG where a parallel low-voltage arrangement would demand 8 AWG or thicker.

The voltage-drop formula

Voltage drop on a DC run is calculated as: VD = (2 × length × current × resistance per foot) ÷ 1,000, where the factor of 2 accounts for the current travelling out and back. Divide that by your system voltage to get the percentage. Thicker wire (a lower AWG number) has lower resistance — each step down in AWG roughly halves resistance, so going from 14 AWG to 10 AWG cuts voltage drop by about 60%.

Don't forget temperature and wire type

• Temperature derating. Rooftops get hot, and a hot wire carries less current safely. Ampacity tables are derated for high ambient temperatures and for bundling several conductors in conduit — always apply the correction for your environment.
• Use PV-rated wire outdoors. Exposed runs on the roof and between panels should use UV-stable, sunlight-resistant PV wire (such as USE-2/PV wire). Ordinary building wire (THHN) degrades in direct sun within a few years.
• Copper vs aluminium. Copper has lower resistance for the same gauge; aluminium is cheaper and lighter but needs to be one or two sizes larger for the same performance.

Common wire-sizing mistakes

• Sizing for ampacity only. The wire is safe but drops too much voltage over distance, quietly wasting harvest. Always check both.
• Forgetting the 125% factor. Sizing to the bare maximum current leaves no margin for continuous operation.
• Ignoring the round-trip length. Voltage drop depends on the total there-and-back distance, not the one-way run.
• Using indoor wire outdoors. A false economy that fails within a few summers.

Reading an AWG wire chart

American Wire Gauge (AWG) is counterintuitive: lower numbers mean thicker wire. Each step down in gauge number is roughly a doubling of cross-sectional area and a halving of resistance, so 10 AWG carries far more current with far less drop than 14 AWG. When you read a sizing chart, two columns matter: the ampacity (how much current the gauge can safely carry, usually given at a 75°C terminal rating for solar) and the resistance per 1,000 feet (which drives voltage drop). A good chart also tells you whether a given recommendation was limited by ampacity or by voltage drop — on long runs it's almost always voltage drop that forces a thicker wire than heat alone would require. Remember that published ampacities are starting points: you still derate for high rooftop temperatures and for bundling multiple conductors in conduit, both of which reduce how much current a wire can safely carry.

Frequently asked questions

It depends on the current, the distance and whether the panels are wired in series or parallel. Series wiring raises voltage and lowers current, allowing much thinner wire; parallel wiring at low voltage needs heavier cable. Enter your figures above and the calculator sizes for both ampacity and a 3% voltage-drop target.

Lower is more efficient but needs thicker, costlier wire. 3% per circuit is the common, code-aligned target; 1–2% is best practice on the panel-to-controller run where losses directly reduce energy harvest. On short battery-to-inverter runs, keep the drop especially low because the currents are very high.

Because at low voltage you need high current to move the same power, and high current over distance causes large voltage drop. Moving to 24V or 48V cuts the current for the same power, so the wire can be far thinner and cheaper — one of the main reasons higher-voltage systems are preferred.

Not for outdoor or rooftop runs. Use UV-resistant PV-rated wire there. Indoor runs in conduit can use standard wire of the correct gauge and temperature rating, but always size it for your actual current, distance and ambient temperature.