For 30 amps at 30 feet on a 12 V system (one-way), you need 2 AWG copper at 3% (the ABYC target for critical circuits) or 6 AWG copper at 10% for non-critical loads like resistive accessories.
| Budget | Minimum copper | Volts allowed |
|---|---|---|
| 3% — critical (electronics, nav, pumps) | 2 AWG | 0.36 V |
| 5% | 4 AWG | 0.60 V |
| 10% — non-critical | 6 AWG | 1.20 V |
At 12 V the budgets are brutally small — 3% is just 0.36 V for the whole round trip, which is why marine charts look oversized next to household ones. Distance is one-way and includes the path through the boat or rig, not the straight line; for battery-bank and inverter cabling, hold 2% instead (see the inverter cable preset). Marine installs should use tinned fine-strand cable with crimped lugs per ABYC practice.
Because the absolute budget is tiny: 3% of 12 V is 0.36 V, versus 7.2 V on a 240 V circuit. Same physics, twentieth of the headroom.
Only for loads that don't care — resistive heaters, simple lights. Electronics, radios, fridges and pumps misbehave well before 10%; ABYC's 3% column exists for them.