How to Use This

Critical loads vs. non-critical: ABYC E-11 sets two voltage drop limits. Use 3% for anything where reduced voltage causes problems — bilge pumps, navigation electronics, panel feeds, autopilot, MFDs, refrigeration. Use 10% for everything else — cabin lights, fans, USB outlets, accessories.

Engine room vs outside: Wires inside an engine room (or anywhere the ambient temp regularly exceeds 50°C) need to be derated for ampacity. Toggle the dropdown if your wire run is in a hot location.

The "limited by" line tells you what really matters. Most marine runs are limited by voltage drop, not ampacity. That's why short runs use thinner wire than the gauge ratings suggest, and long runs use thicker wire than you'd expect from current alone.

What ABYC E-11 Actually Says

ABYC Standard E-11 ("AC and DC Electrical Systems on Boats") is the rule book that marine surveyors, insurance companies, and competent boat builders all reference. The relevant rules for wire sizing:

  • Round-trip length is what counts. A 20-foot run from battery to bilge pump is 40 feet of total wire (positive + negative), and that's what's used to calculate voltage drop. This calculator doubles your one-way input internally.
  • 3% voltage drop for critical loads, 10% for non-critical.
  • Tinned copper marine wire is required (UL 1426 type BC-5W2 or equivalent) for any wet location. Standard automotive wire is not sufficient.
  • Stranded wire only. Solid wire fails from vibration in marine environments.
  • Insulation rated for at least 105°C for general use.

This calculator uses the standard resistance values for stranded copper conductors and the ABYC E-11 ampacity tables for 105°C insulation, both inside and outside the engine room.

Where to Buy Marine-Grade Wire

You need tinned copper, stranded, 105°C insulation, marine-grade. Ancor is the standard. Pacer and Cobra also work. Don't substitute automotive wire — the bare copper corrodes inside the insulation in salt air.

→ Marine Wire Selection on Dupree Products

Common Mistakes

Sizing for ampacity only. This is the #1 mistake. A 14 AWG wire is rated for 20A, but for a 25-foot run at 20A you actually need 10 AWG to keep voltage drop under 10%. The wire wouldn't melt — it would just deliver 9V to a 12V appliance.

Using one-way distance somewhere it shouldn't be. This calculator already doubles for round trip. If you're getting unusably thick recommendations from another calculator, check whether you're feeding it round-trip length when it expects one-way (or vice versa).

Forgetting fuses. Wire size determines fuse size, not the other way around. The fuse protects the wire from overheating, so the fuse rating must be at or below the wire's ampacity rating. ABYC requires fuses on every positive lead within 7 inches of the battery (with a few specific exceptions).

Mixing AC and DC sizing. This calculator is for DC systems only. AC wire sizing follows the same physics but uses NEC tables and different voltage drop conventions.

Cross-References

Found a bug or disagreement with the calculation? Email braden@goodkit.io. We use this calculator on Yoto and update it when ABYC publishes errata.