Solar Panel Wire Size and Fuse Size Guide (With Examples and Chart)

Wire and fuse sizes are the safety backbone of a solar system. A wire that is too thin overheats and wastes power, and a fuse that is too big will not protect it. This guide gives you simple rules and worked examples so you can understand how installers choose them.

Safety first. Solar DC circuits can cause fires and serious shock. Use this guide to learn and to check your numbers, then have the final design and installation done or reviewed by a qualified electrician, following your local electrical code.

The two jobs: protect the wire, then limit the loss

Every circuit needs two checks:

• Current capacity (ampacity): the wire must safely carry the current without overheating, and the fuse must protect that wire.
• Voltage drop: the wire must be thick enough that little power is lost along its length.

Choose the size that passes both checks. Voltage drop usually decides long runs, and ampacity decides short, high-current runs such as battery to inverter.

Step 1: Find the current

Circuit	Current to use
Panel or string to charge controller	Isc (short-circuit current) of the panel, times the number of parallel strings
Charge controller to battery	Controller's maximum output amps
Battery to inverter	Inverter watts / (battery volts x inverter efficiency)

Step 2: Size the fuse

Solar panel strings. A common rule is to take the panel's Isc and multiply by 1.56. That comes from two 125% safety margins (1.25 x 1.25). Then round up to the next standard fuse size, without going above the panel's maximum series fuse rating printed on its label.

Battery and inverter circuits. Multiply the maximum continuous current by 1.25, then round up to the next standard fuse size.

Standard sizes include 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, 125, 150, 175 and 200 A.

Step 3: Choose a wire that the fuse protects

The wire's ampacity must be equal to or higher than the fuse rating. This is the key rule: the fuse protects the wire. The chart below shows typical copper wire ampacities (75 C column). Real values drop with high heat, bundled cables and long conduit runs, so ask an electrician when in doubt.

AWG size	Approx. mm2	Typical ampacity
14	2.1	20 A
12	3.3	25 A
10	5.3	35 A
8	8.4	50 A
6	13.3	65 A
4	21.2	85 A
2	33.6	115 A
1/0	53.5	150 A
2/0	67.4	175 A

Values are general guides based on common copper-wire tables. Always check the cable's own datasheet and your local code.

Step 4: Check the voltage drop

For copper wire, the minimum cable size in mm2 is:

mm2 = (2 x length x current x 0.0172) / allowed voltage drop in volts

• Length is one-way distance in metres. The 2 covers the round trip.
• Allowed voltage drop: a common target is 3% or less for panel wiring, and 1% to 2% for battery and inverter cables.

Worked example 1: panels to charge controller

Three 400 W panels in series. Imp 10.81 A, Isc 11.4 A, array Vmp 111 V. The cable run is 20 m one way.

• Fuse or protection: 11.4 x 1.56 = 17.8 A, so a 20 A device (if the panel label allows it). With a single string you often need no fuse, but a disconnect is still recommended.
• Voltage drop at 3%: allowed drop = 111 x 0.03 = 3.33 V. Cable size = (2 x 20 x 10.81 x 0.0172) / 3.33 = about 2.2 mm2.
• Ampacity: needs at least 17.8 A, which 12 AWG (25 A) satisfies.
• Choice: 10 AWG (about 5.3 mm2) PV cable gives a comfortable margin for heat and future changes.

Worked example 2: battery to inverter

A 3,000 W inverter on a 48 V battery bank, 93% efficient. The cable run is 3 m one way.

• Current: 3,000 / (48 x 0.93) = about 67 A.
• Fuse: 67 x 1.25 = 84 A, so use a 100 A DC fuse (the next standard size above 90 A gives more room for surges).
• Wire: the cable must carry at least 100 A, so 2 AWG (115 A).
• Voltage drop check: 2 AWG is about 0.5 milliohm per metre. Round trip 6 m gives 3 milliohm. Drop = 67 x 0.003 = about 0.2 V, which is 0.4% of 48 V. That is excellent.

The same 3,000 W on a 12 V battery would pull about 270 A, which needs very thick cable. This is why larger inverters use 24 V or 48 V.

Try the wire and fuse calculator

Choose your circuit, enter your numbers, and the calculator gives the fuse size, the minimum cable by ampacity and by voltage drop, and a PASS or FAIL check. You can export the result as a PDF.

Wire and Fuse Size Calculator

Fuse size, minimum cable and voltage drop for solar circuits

Circuit

Which circuit? Panel string to charge controller Charge controller to battery Battery to inverter

Your numbers

Result

Checks

Fuse: Isc x 1.56 for panel strings, 1.25 x continuous current for battery and inverter circuits, rounded up to a standard size. Wire sizes use typical copper 75 C ampacities. Heat, bundling and conduit lower real ampacity. Estimates only. Have your design reviewed by a qualified electrician and follow your local code.

Where to put fuses

• Battery positive: place a fuse as close to the battery terminal as practical, so the whole cable is protected.
• Parallel panel strings: when you have three or more strings in parallel, each string usually needs its own fuse or breaker, typically in a combiner box.
• Disconnects: add a switch or breaker so you can safely isolate the array, the battery and the inverter.

Choosing the right parts

• Use DC-rated fuses and breakers. AC parts are not safe for DC, because DC arcs are harder to extinguish.
• Voltage rating: the fuse's DC voltage rating must be higher than your highest system voltage, including the cold-weather Voc of your strings.
• PV cable outdoors: use sunlight-resistant solar PV cable, and matching connectors from the same maker.
• Battery cable: use flexible, fine-stranded copper cable with crimped lugs, not just clamped wire ends.
• Colors: red for positive and black for negative, and keep connections tight to avoid heat.
• Ground and bond: follow your local code for earthing the frames and the system.

Common mistakes

• Choosing a fuse by guesswork. A fuse larger than the wire's ampacity will not protect the wire.
• Ignoring voltage drop on long panel runs, which wastes harvested power.
• Using AC breakers on DC circuits.
• Skipping a fuse at the battery. Batteries can deliver huge short-circuit currents.
• Forgetting that hot roofs and enclosed conduit reduce a cable's safe capacity.

Next steps

Our calculator shows your array current, inverter DC current and suggested fuse sizes. To work out how to wire your panels, read Series vs Parallel Solar Panels, and to size your storage see How to Size a Solar Battery Bank.

Try it yourself: Use our free solar system calculator to get your array current, inverter DC current and suggested fuse sizes.

Disclaimer: This article is for general information and education only. Wire and fuse sizes depend on temperature, installation method, equipment ratings and local codes (such as the NEC, the Philippine Electrical Code or IEC-based standards). The numbers here are examples and not a certified design. Have your system designed or reviewed by a qualified electrician or solar installer before installing.