Wire size for 30 A over 100 ft on a 12 V system

Smallest wire that works

over 4/0

30 A over 100 ft at 12 V exceeds 3% even on 4/0 — split the run or raise the voltage

Circuit power30 A × 12 V	360W
Conductor length100 ft out and 100 ft back	200ft
Ampacity alone would allow62% drop — legal on heat, not acceptable on drop	10 AWG
— Voltage drop by size —
14 AWGnot rated for 30 A — 15 A max	—
12 AWGnot rated for 30 A — 20 A max	—
10 AWG30 A rated, drop too high	62%7.44 V
8 AWG50 A rated, drop too high	38.9%4.67 V
6 AWG65 A rated, drop too high	24.55%2.95 V
4 AWG85 A rated, drop too high	15.4%1.85 V
2 AWG115 A rated, drop too high	9.7%1.16 V
1/0 AWG150 A rated, drop too high	6.1%0.73 V
2/0 AWG175 A rated, drop too high	4.83%0.58 V
4/0 AWG230 A rated, drop too high	3.04%0.36 V

Current: 30 A
One-way distance: 100 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 30 A over 100 ft at 12 V, ampacity alone would allow 10 AWG. Voltage drop says larger than 4/0 — %, or 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 200 ft, not 100 ft. Forgetting the return leg is how people arrive at exactly half the real drop.

Why it matters more than it sounds

360 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

At this length ampacity and voltage drop point at the same wire, which is unusual. Runs over about 20 ft at low DC voltage are normally decided by 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 360 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.
How Many Panels for 1,000 kWh a MonthThe answer ranges from fifteen panels to thirty-four, for the same house using the same electricity. Location does most of that, panel wattage does the rest.
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.