Wire size for 40 A over 10 ft on a 12 V system

Smallest wire that works

4 AWGat 2.05% drop

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

Circuit power40 A × 12 V	480W
Conductor length10 ft out and 10 ft back	20ft
Ampacity alone would allow5.19% drop — legal on heat, not acceptable on drop	8 AWG
Power lost in the cable2.05% of what you are sending	9.9W
— Voltage drop by size —
14 AWGnot rated for 40 A — 15 A max	—
12 AWGnot rated for 40 A — 20 A max	—
10 AWGnot rated for 40 A — 30 A max	—
8 AWG50 A rated, drop too high	5.19%0.62 V
6 AWG65 A rated, drop too high	3.27%0.39 V
4 AWG85 A rated	2.05%0.25 V
2 AWG115 A rated	1.29%0.16 V
1/0 AWG150 A rated	0.81%0.1 V
2/0 AWG175 A rated	0.64%0.08 V
4/0 AWG230 A rated	0.41%0.05 V

Current: 40 A
One-way distance: 10 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 40 A over 10 ft at 12 V, ampacity alone would allow 8 AWG. Voltage drop says 4 AWG — 2.05%, or 0.25 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 20 ft, not 10 ft. Forgetting the return leg is how people arrive at exactly half the real drop.

Why it matters more than it sounds

480 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. 8 AWG carries 40 A without overheating, but it drops 5.19% over this distance. You need 4 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 480 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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