Wire size for 20 A over 125 ft on a 12 V system

Smallest wire that works

4/0 AWGat 2.53% drop

20 A over 125 ft one-way on a 12 V system

Circuit power20 A × 12 V	240W
Conductor length125 ft out and 125 ft back	250ft
Ampacity alone would allow82.5% drop — legal on heat, not acceptable on drop	12 AWG
Power lost in the cable2.53% of what you are sending	6.1W
— Voltage drop by size —
14 AWGnot rated for 20 A — 15 A max	—
12 AWG20 A rated, drop too high	82.5%9.9 V
10 AWG30 A rated, drop too high	51.67%6.2 V
8 AWG50 A rated, drop too high	32.42%3.89 V
6 AWG65 A rated, drop too high	20.46%2.46 V
4 AWG85 A rated, drop too high	12.83%1.54 V
2 AWG115 A rated, drop too high	8.08%0.97 V
1/0 AWG150 A rated, drop too high	5.08%0.61 V
2/0 AWG175 A rated, drop too high	4.03%0.48 V
4/0 AWG230 A rated	2.53%0.3 V

Current: 20 A
One-way distance: 125 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 20 A over 125 ft at 12 V, ampacity alone would allow 12 AWG. Voltage drop says 4/0 AWG — 2.53%, or 0.3 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 250 ft, not 125 ft. Forgetting the return leg is how people arrive at exactly half the real drop.

Why it matters more than it sounds

240 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. 12 AWG carries 20 A without overheating, but it drops 82.5% over this distance. You need 4/0 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 240 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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