Wire size for 30 A over 40 ft on a 12 V system

Smallest wire that works

1/0 AWGat 2.44% drop

30 A over 40 ft one-way on a 12 V system

Circuit power30 A × 12 V	360W
Conductor length40 ft out and 40 ft back	80ft
Ampacity alone would allow24.8% drop — legal on heat, not acceptable on drop	10 AWG
Power lost in the cable2.44% of what you are sending	8.8W
— 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	24.8%2.98 V
8 AWG50 A rated, drop too high	15.56%1.87 V
6 AWG65 A rated, drop too high	9.82%1.18 V
4 AWG85 A rated, drop too high	6.16%0.74 V
2 AWG115 A rated, drop too high	3.88%0.47 V
1/0 AWG150 A rated	2.44%0.29 V
2/0 AWG175 A rated	1.93%0.23 V
4/0 AWG230 A rated	1.22%0.15 V

Current: 30 A
One-way distance: 40 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 40 ft at 12 V, ampacity alone would allow 10 AWG. Voltage drop says 1/0 AWG — 2.44%, 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 80 ft, not 40 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

Two different answers, and the bigger one wins. 10 AWG carries 30 A without overheating, but it drops 24.8% over this distance. You need 1/0 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 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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