Wire size for 20 A over 25 ft on a 12 V system

Smallest wire that works

4 AWGat 2.57% drop

20 A over 25 ft one-way on a 12 V system

Circuit power20 A × 12 V	240W
Conductor length25 ft out and 25 ft back	50ft
Ampacity alone would allow16.5% drop — legal on heat, not acceptable on drop	12 AWG
Power lost in the cable2.57% of what you are sending	6.2W
— Voltage drop by size —
14 AWGnot rated for 20 A — 15 A max	—
12 AWG20 A rated, drop too high	16.5%1.98 V
10 AWG30 A rated, drop too high	10.33%1.24 V
8 AWG50 A rated, drop too high	6.48%0.78 V
6 AWG65 A rated, drop too high	4.09%0.49 V
4 AWG85 A rated	2.57%0.31 V
2 AWG115 A rated	1.62%0.19 V
1/0 AWG150 A rated	1.02%0.12 V
2/0 AWG175 A rated	0.81%0.1 V
4/0 AWG230 A rated	0.51%0.06 V

Notes

Two different answers, and the bigger one wins. 12 AWG carries 20 A without overheating, but it drops 16.5% 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 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.

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 25 ft at 12 V, ampacity alone would allow 12 AWG. Voltage drop says 4 AWG — 2.57%, or 0.31 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 50 ft, not 25 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 sulfates its way to an early death while every meter in the system reads normal.

What is not in the table

Copper at 75 °C. Aluminum conducts about 60% as well — go up two sizes. Conduit fill, ambient temperature above 30 °C and bundled conductors all derate ampacity further.

Nearby sizes

Backup Generator vs Solar Battery — Cost per Hour of Outage CoveredA battery is cheaper to own and runs out. A generator is cheaper to run and never does. Which one wins is decided by how long your outages actually last.
Solar Cost per Watt — the Only Unit That Makes Two Quotes ComparableTwo quotes for "a solar system" are not comparable. Two quotes at dollars per watt are, and the conversion takes one division.
Solar Payback Period — the Four Inputs, and the One That Decides ItPayback is one division with four inputs. Three of them are knowable to within a few percent. The fourth — what your utility pays for exports — swings the answer by more than a decade.
Home Solar Battery Cost — Priced per Usable Kilowatt-HourStorage is the worst-value component of a solar system on pure energy arithmetic, and often the right purchase anyway. Which of those is true depends on why you are buying it.