How ABYC sizes boat wiring
ABYC standard E-11 sizes DC conductors by voltage drop over the round-trip length (positive and negative together). Critical circuits such as navigation lights, electronics, bilge pumps and panel feeders are limited to 3% drop; general lighting and other non-critical loads may drop up to 10%. The wire must also have enough ampacity.
Example: 20 A, 15 ft away, 12 V, 3%
Round trip is 30 ft; allowed drop is 0.36 V. CM = 10.75 × 20 × 30 ÷ 0.36 = 17,917 CM. The next size up is 6 AWG (26,240 CM). At 10% drop, 12 AWG would be enough.
Marine wire sizes
| AWG | Circular mils | Ampacity (105°C, outside engine room) |
|---|---|---|
| 16 | 2,580 | 25 A |
| 14 | 4,110 | 35 A |
| 12 | 6,530 | 45 A |
| 10 | 10,380 | 60 A |
| 8 | 16,510 | 80 A |
| 6 | 26,240 | 120 A |
| 4 | 41,740 | 160 A |
| 2 | 66,360 | 210 A |
| 1/0 | 105,600 | 285 A |
| 2/0 | 133,100 | 330 A |
| 4/0 | 211,600 | 445 A |
Inside engine spaces, ampacity is reduced (we apply 85%). Always use tinned, stranded marine-grade (UL 1426 / SAE J1127) cable.
Frequently asked questions
Why does ABYC use round-trip length?
Current flows out on the positive and back on the negative, and both wires add resistance. Use the full loop length.
What wire size for a 12 V bilge pump?
A bilge pump is a critical load (3%). A 5 A pump 20 ft away needs about 12 AWG; a 10 A pump at the same distance needs 8 AWG.
Can I use household or automotive wire on a boat?
No. Marine wire is finely stranded and tinned to resist vibration and corrosion.