Gauge is set by voltage drop, not just current
Current alone tells you what a wire can carry without overheating. But on a DC run the deciding factor is usually voltage drop over distance: I is the current in amps, L the one-way length, ρ the resistivity (copper 0.0172, aluminum 0.0285 Ω·mm²/m), and the factor of 2 covers the return conductor. Bigger cross-section means a lower AWG number and less drop.
Copper AWG for a 3% drop at 12 V
| Current | 2 m | 5 m | 10 m | 20 m |
|---|---|---|---|---|
| 10 A | AWG 14 | AWG 12 | AWG 10 | AWG 8 |
| 20 A | AWG 12 | AWG 10 | AWG 8 | AWG 6 |
| 50 A | AWG 8 | AWG 6 | AWG 4 | AWG 2 |
| 100 A | AWG 4 | AWG 2 | AWG 0 | AWG 00 |
Distances are one-way; the sizing already accounts for the return leg. Lower numbers are thicker wire — every drop of 3 AWG steps roughly doubles the cross-section. Solar and marine wiring (NEC / ABYC) commonly lands in the AWG 6 to 2 range for 20 to 80 A runs.
Where DC sizing goes wrong
- Counting length one way. Always use the full loop, out and back. Halving the distance halves the calculated drop and undersizes the cable.
- Treating aluminum like copper. Aluminum needs about two sizes larger for the same drop.
- Rounding to the exact result. If the math lands on AWG 8, fit AWG 6. A ~20% margin covers connector resistance and warm ambient temperatures.
Common questions
Does the cable length mean one way or the round trip?
The round trip. Current has to flow out and back, so the voltage drop is set by twice the one-way distance. A device 5 meters away needs sizing for 10 meters of cable.
What voltage drop should I allow?
Keep the main run under 3% and the whole circuit under about 5%. On a 12 V system, 3% is only 0.36 V, which is why low-voltage DC runs need thicker cable than people expect.
Can I use aluminum instead of copper?
Yes, but go up about two AWG sizes. Aluminum has roughly 1.65 times the resistivity of copper, so it needs more cross-section to carry the same current at the same drop.
