Why voltage drop matters on 12V boats
In 12V DC systems, even small voltage losses cause equipment to underperform. A 0.5V drop at the load means motors run hot, LEDs dim, and electronics may brown out. ABYC E-11 recommends 3% max for critical circuits — at 12V that is 0.36V. Non-critical circuits may allow up to 10% (1.2V). Understanding how to calculate voltage drop before you run wire saves rework and prevents failures.
The round-trip formula
Voltage drop = 2 × I × R × L. Current (I) in amperes; resistance per meter (R) from wire gauge; length (L) one-way in meters. The factor of 2 accounts for round-trip: current flows out and back through the same conductor. Copper resistance values follow SAE J1128. Use our voltage drop calculator to verify designs.
Step 1: Determine load current
Check the equipment label or datasheet for continuous current. A bilge pump might draw 5–10A; a windlass 50–100A. Use worst-case continuous current, not peak inrush. For multiple loads on one circuit, sum them.
Step 2: Measure one-way length
Measure from battery positive to the load. Add 10–15% for routing. Do not double for round-trip — the formula handles that.
Step 3: Choose wire gauge and get resistance
AWG 14 has ~8.5 mΩ/m; AWG 12 ~5.3 mΩ/m; AWG 10 ~3.3 mΩ/m. Use the AWG sizing calculator to find minimum gauge for your current, length, and max drop.
Step 4: Calculate drop
V_drop = 2 × I × R × L. Example: 10A, 10m, AWG 14 (8.5 mΩ/m). V_drop = 2 × 10 × 0.0085 × 10 = 1.7V. At 12V that is 14.2% — far above ABYC 3%. You need larger wire.
Step 5: Verify against ABYC
For critical circuits (navigation, bilge, safety), keep drop ≤ 3%. For non-critical, up to 10% is acceptable. If your calculation exceeds that, increase wire gauge and recalculate.
Worked example: 12V bilge pump
Bilge pump: 8A continuous. Run: 8m one-way. Target: 3% max. At 12V, 3% = 0.36V. Rearranging: R_max = 0.36 / (2 × 8 × 8) ≈ 2.8 mΩ/m. AWG 10 (3.3 mΩ/m) is close; AWG 8 would be safer. Use the calculator to confirm.
Common mistakes
Using one-way length without the factor of 2. Ignoring temperature (engine rooms run hotter; resistance increases). Mixing AWG and mm² without correct conversion. Sizing for average load instead of peak.
Next steps
After sizing wire, verify fuse protection and check the full marine electrical systems guide for system design.