Wire size for 40 A over 150 ft on a 12 V system

Smallest wire that works

over 4/0

40 A over 150 ft at 12 V exceeds 3% even on 4/0 — split the run or raise the voltage

Circuit power40 A × 12 V	480W
Conductor length150 ft out and 150 ft back	300ft
Ampacity alone would allow77.8% drop — legal on heat, not acceptable on drop	8 AWG
— 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	77.8%9.34 V
6 AWG65 A rated, drop too high	49.1%5.89 V
4 AWG85 A rated, drop too high	30.8%3.7 V
2 AWG115 A rated, drop too high	19.4%2.33 V
1/0 AWG150 A rated, drop too high	12.2%1.46 V
2/0 AWG175 A rated, drop too high	9.67%1.16 V
4/0 AWG230 A rated, drop too high	6.08%0.73 V

Current: 40 A
One-way distance: 150 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 150 ft at 12 V, ampacity alone would allow 8 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 300 ft, not 150 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

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 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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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.