Short answer: Find your current and one-way distance; the chart gives the minimum copper AWG for a 3% drop at 12 V that also meets a conservative ampacity limit. For 24 V the same wire carries the load twice as far.
| Amps | 5 ft | 10 ft | 15 ft | 20 ft | 25 ft |
|---|---|---|---|---|---|
| 5 A | 14 | 14 | 12 | 10 | 10 |
| 10 A | 14 | 10 | 8 | 8 | 6 |
| 20 A | 10 | 8 | 6 | 4 | 4 |
| 30 A | 8 | 6 | 4 | 2 | 2 |
| 40 A | 8 | 4 | 2 | 2 | 1 |
| 60 A | 4 | 2 | 2 | 1/0 | 2/0 |
| 80 A | 2 | 2 | 1/0 | 2/0 | 3/0 |
| 100 A | 1 | 1 | 2/0 | 3/0 | 4/0 |
| 150 A | 1/0 | 2/0 | 3/0 | — | — |
| 200 A | 3/0 | 3/0 | — | — | — |
"—" means over 4/0: shorten the run or raise the voltage. Calculated from NEC Chapter 9 Table 8 copper resistances (round trip). Use the wire size calculator for other voltages and drops.
Safety: These are general estimates, not a design. Follow your equipment manufacturer's instructions and local electrical code, and consult a qualified electrician for anything connected to shore power, a house, or high-current batteries.
For 10 ft one way at 3% drop, 6 AWG; for 5 ft, 8 AWG.
Use the panel string's short-circuit current × 1.25 and your run length; 10 AWG is common for 1–2 panels on short runs.
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