12V Wire Gauge Ampacity Chart by Length

How to read the table

Rows are AWG (4 through 14). Columns are one-way length. Each cell is the largest continuous current that stays at or under 3% of 12 V (0.36 V) for that gauge and length. Find your one-way feet on the top, read down until the amp number exceeds your load, then use that row or thicker. Thicker is smaller AWG number.

Example: 20 A at 10 ft one-way. 10 AWG cell is 14.5 A — too small. 8 AWG is 23 A — that row works. 20 A at 20 ft: 8 AWG is only 11.6 A; 6 AWG is 18.3 A — still short; 4 AWG is 29 A — use 4 AWG or run the calculator if you can live with 5% drop.

Max continuous amps at 3% drop, 12 V, copper, round-trip = 2 × one-way. I_max = 0.36 × 1000 / (R × 2 × L). Not a chassis ampacity or fuse chart. 24 V at 3% roughly doubles these amp numbers (allowable volts double).

When a chart is the right tool

You already know the path is about 10 ft or 20 ft, the load is a catalog number (5 A pods, 15 A fridge), and you want a yes/no without opening a form. Shops tape this grid in the van. It is also the way to see, at a glance, that 14 AWG is a courtesy-light wire, not a light-bar wire, once the bumper is 10 ft of loom from the battery.

The chart is the wrong tool when length is 7 ft or 22 ft, when you need 4% or 2%, when the bank is 24 V, or when current is 47 A. Those four knobs are why the calculator exists. Welding-cable and 120 V extension-cord charts are different metals and voltages — do not cross-read extension cord gauge or welder gauge into a 12 V fridge circuit.

Chart vs calculator vs ampacity

Voltage-drop amps in a cell can exceed what you should fuse on that gauge in a hot engine bay. 4 AWG showing 117 A at 5 ft is a drop number, not permission to fuse 117 A on cheap insulation. After you pick a row, still fuse for the conductor and the accessory, and pick fuse vs breaker on purpose (comparison). If lights are already dim, skip the grid and measure — problems and solutions.

NEC 120/240 V drop uses the same R table but answers a different question (conductor size on a branch circuit). Use that calculator for premises wire. This grid is 12 V accessory copper only, 3% only, lengths listed only.