Solar Wire Size Calculator
Find the correct wire gauge for panel-to-inverter, inverter-to-battery, and load circuits.
By Syed Mubashir Ali · Last updated
The two checks every run needs
Every cable run in a solar system must pass two independent checks, and the correct size is the larger of the two results. The first is voltage drop: can the cable carry the current without losing too much voltage along the way. The second is current-carrying capacity: can the cable carry the current without overheating in how it is actually installed.
This tool does both, but its current check assumes cable clipped direct in 30°C air (IEC 60364-5-52 method C). Real capacity depends on installation method (cable in free air carries more than cable in conduit, which carries more than cable buried in insulation) plus grouping with other cables, ambient temperature, and the insulation type. Look up the ampacity tables for your cable and your installation method, and take the larger of the two sizes. When in doubt, the larger size is the answer.
System voltage is a design decision
If you are still choosing system voltage, the cable calculator makes the case for going higher. A 12V system moving the same power as a 48V system carries four times the current, which means roughly four times the copper for the same voltage drop, or accepting four times the loss. The price difference in cable alone often covers the cost of 48V equipment.
The trade-off is component availability and safety margins. 12V has the widest range of cheap components and is the most forgiving to work on. 48V needs more care and respect. For anything above a small weekend setup (a cabin, an RV running full-time, a home backup) 24V is the minimum worth considering, and 48V is usually the right answer.
How to Use Solar Wire Size Calculator
- 1For each cable run, enter either the power in watts or the current in amps.
- 2Enter the cable run length in metres: one-way length; the voltage drop calculation accounts for the return path.
- 3Set the system voltage and the maximum voltage drop you will accept as a percentage.
- 4Calculate to get recommended wire sizes from the standard range, 1.5mm² up to 35mm².
- 5Treat the result as a voltage drop check only, and verify against current-carrying capacity and your local wiring rules before installing anything.
- 6Run the calculation for the longest run first. That is usually the array-to-controller cable, and it is the one most likely to need upsizing. Shorter runs often pass on the minimum size, so start where the problem is.
Frequently Asked Questions
What voltage drop should I allow?
Commonly 3% for the array-to-controller run and 2–3% elsewhere, though rules vary. On low-voltage DC systems drop matters far more than people expect: the same 1V drop that is trivial at 230V is over 8% at 12V, which is why 12V systems need surprisingly heavy cable over short distances.
Does this check current-carrying capacity?
It sizes for voltage drop. Current-carrying capacity is a separate and equally important check, and it depends on installation method, grouping, insulation type and ambient temperature. Always verify the chosen size against the ampacity tables for how the cable will actually be installed.
Should I enter the one-way or round-trip length?
One-way. The calculation accounts for current flowing out and back, so entering double the length would size the cable twice as heavily as needed.
Does it cover fusing and protection?
No. Overcurrent protection sized to the cable and the source, correct DC-rated devices, and isolation are all essential and separate. DC arcs do not self-extinguish the way AC does, which is why DC-rated equipment matters and AC devices must not be substituted.
Can I use this for the AC side of a grid-tied system?
The voltage drop arithmetic is general, but an AC grid connection is subject to wiring regulations and usually must be carried out by a qualified electrician with the appropriate notification. Do not treat this as design authority for that part of the system.
Why does 12V need such thick cable?
Because voltage drop is a percentage, and the math is unforgiving at low voltages. A 10-amp load over 5 metres at 12V with a 3% limit needs roughly 6mm² of copper, which looks absurd until you realise the same run at 48V would need about 1.5mm². This is the single best argument for 24V or 48V system design: higher voltage means thinner cable, lower cost, and easier installation.
Should I oversize the cable beyond the recommendation?
One size up is cheap insurance and common practice, especially on the array run. Copper is a one-time cost; voltage drop is a daily efficiency loss for the life of the system. But do not oversize wildly: terminations, glands and breakers all have maximum cable sizes, and wrestling 35mm² into a controller rated for 16mm² helps nobody.
About Solar Wire Size Calculator
The Solar Wire Size Calculator recommends cable sizes from the standard range for each run in your system, based on load, run length, system voltage and the maximum voltage drop you will accept. It runs entirely in your browser.
Voltage drop is the constraint that surprises people on low-voltage systems. Drop is a percentage of system voltage, so a loss that is negligible at 230V is severe at 12V, which is why a modest 12V array can need cable that looks absurdly heavy for the current involved. Raising system voltage is often cheaper than upsizing copper.
This sizes for voltage drop only. Current-carrying capacity is a separate check that depends on how the cable is installed (in conduit, in insulation, grouped with others, in high ambient temperature) and the correct size is the larger of the two results.
Protection is out of scope and is not optional: overcurrent devices sized to the cable, proper isolation, and DC-rated equipment throughout. DC arcs do not self-extinguish as AC does, so AC-rated devices are not a substitute. Anything touching the AC side generally requires a qualified electrician.
Buy proper solar cable, not whatever is cheapest. Solar DC cable is double-insulated, UV-resistant, and rated for the outdoors for decades; ordinary flex is not, and it degrades. The cable is the hardest part of the system to replace later (it is buried in conduit, clipped to a roof, or run through walls) so this is the wrong place to save money.
This calculator runs entirely in your browser. Nothing you enter leaves your device.