Wire size for 10 A over 30 ft on a 12 V system

Smallest wire that works

6 AWGat 2.46% drop

10 A over 30 ft one-way on a 12 V system

Circuit power10 A × 12 V	120W
Conductor length30 ft out and 30 ft back	60ft
Ampacity alone would allow15.7% drop — legal on heat, not acceptable on drop	14 AWG
Power lost in the cable2.46% of what you are sending	2.9W
— Voltage drop by size —
14 AWG15 A rated, drop too high	15.7%1.88 V
12 AWG20 A rated, drop too high	9.9%1.19 V
10 AWG30 A rated, drop too high	6.2%0.74 V
8 AWG50 A rated, drop too high	3.89%0.47 V
6 AWG65 A rated	2.46%0.29 V
4 AWG85 A rated	1.54%0.18 V
2 AWG115 A rated	0.97%0.12 V
1/0 AWG150 A rated	0.61%0.07 V
2/0 AWG175 A rated	0.48%0.06 V
4/0 AWG230 A rated	0.3%0.04 V

Current: 10 A
One-way distance: 30 ft
System voltage: 12 V

Ampacity and voltage drop are two different questions

Ampacity asks whether the wire gets hot enough to be dangerous. Voltage drop asks whether enough of the power arrives at the other end. On a low-voltage DC run they almost never give the same answer, and the bigger one wins.

For 10 A over 30 ft at 12 V, ampacity alone would allow 14 AWG. Voltage drop says 6 AWG — 2.46%, or 0.29 V of the 12 V you started with.

Why the distance doubles

Current goes out on one conductor and comes back on the other. The copper in the circuit is 60 ft, not 30 ft. Forgetting the return leg is how people arrive at exactly half the real drop.

Why it matters more than it sounds

120 W leaves the source. Undersized cable does not just waste a few percent — on a solar charge controller it means the battery never sees absorption voltage, never finishes a charge cycle, and sulphates its way to an early death while every meter in the system reads normal.

What is not in the table

Copper at 75 °C. Aluminium conducts about 60% as well — go up two sizes. Conduit fill, ambient temperature above 30 °C and bundled conductors all derate ampacity further.

Notes

Two different answers, and the bigger one wins. 14 AWG carries 10 A without overheating, but it drops 15.7% over this distance. You need 6 AWG — sized by voltage drop, not by heat.
3% is the NEC informational recommendation for a branch circuit, not a hard rule. On a 12 V DC system it is already generous: 0.36 V of the 12 V you started with.
12 V is what makes this expensive. The same 120 W at 24 V halves the current and quarters the loss — the cheapest way out of a big cable bill is almost always a higher system voltage.
Figures are copper at 75 °C. Aluminium runs about 60% of copper conductivity — go up two sizes. Conduit fill, ambient temperature above 30 °C and bundling all derate ampacity further, and none of that is in this table.

Nearby sizes

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Daily Solar Output by Region, and the Seasonal Swing Behind the AverageAn annual average tells you what the system produces over a year. It tells you almost nothing about December, and December is where the sizing arguments happen.
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Converting kW to Panels, and the Three Ratings That Get ConfusedThree different numbers on a solar quote are all measured in kilowatts and none of them mean the same thing. Knowing which is which makes quotes comparable.