How the calculation works
Every wire has resistance, and current through resistance costs volts. The loss grows with three things: more amps, more length (counting both directions of the circuit), and thinner copper. Wire sizing means choosing the gauge whose resistance keeps that loss inside your target percentage of system voltage.
The factor of two matters: a "15 foot run" is 30 feet of wire, because current travels out on the positive and back on the negative. Forgetting the return path is the most common reason home-built systems end up one or two sizes thin.
Low-voltage systems feel the loss hardest. Losing 0.36 V is 3% of a 12 V system but only 0.75% of a 48 V system, which is why higher-voltage designs can use dramatically thinner and cheaper cable for the same power. If your wire size keeps coming out enormous, that is usually the system telling you to raise the voltage.
Worked example
A charge controller sits 15 feet from the battery bank and pushes 30 A at 12 V. Target drop 3% (0.36 V):
One size thinner (8 AWG) would lose 4.7% and shave real charging power off every sunny hour; one size thicker (4 AWG) drops the loss under 2% for a few dollars more. Copper is cheap compared to the panels feeding it, so when in doubt, round thicker.
Common mistakes
- Using one-way length in the math. The circuit is a loop; double the distance before calculating. This tool does it for you.
- Sizing only for ampacity, not for drop. A wire can be safe (not overheating) and still waste 8% of your harvest. Ampacity is the safety floor; voltage drop is the performance ceiling. Both must pass.
- Aluminum numbers on copper wire, or the reverse. This calculator is for stranded copper, the standard for solar and battery wiring. Aluminum needs roughly two sizes larger for the same job.
- No fuse or breaker on the battery side. Every conductor leaving a battery needs overcurrent protection sized to the wire, placed close to the battery. The fuse suggestion above is a starting point (1.25 x continuous current, rounded up); confirm against your wire's ampacity and local code.
Frequently asked questions
What wire size do I need for a 100 W panel?
A single 100 W panel pushes roughly 6 A at 12 V nominal. For runs up to about 15 feet, 10 AWG (the standard pre-made solar cable size) keeps the drop around 2 to 3%. Longer runs or paralleled panels need the math above, since amps add up fast.
Does voltage drop matter on the panel-to-controller run with MPPT?
Less than elsewhere, because MPPT controllers accept a wide input voltage and convert whatever arrives. But drop is still lost power. Best practice: wire panels in series where possible to raise voltage and cut current, then the same watts travel on thinner wire.
Can I just use the thickest wire everywhere?
Electrically yes, practically no: heavy gauge costs more, fights you in conduit and terminals, and many devices only accept lugs up to a certain size. Size each run for its own current and length; the battery-to-inverter run is usually the thick one, panel strings the thin ones.
Is this calculator valid for AC household wiring?
The physics is the same, but AC branch circuits are governed by electrical code rules that go beyond voltage drop. Use this tool for the DC side of solar and battery systems; have AC wiring sized and installed per your local code.