Marine DC Wire Size Calculator (ABYC 3% / 10% Voltage Drop, Ampacity, Fuse Size)

How it works

circular mils = 10.75 × I × round-trip length (ft) ÷ allowable drop (V) · allowable drop = 3% (critical) or 10% (non-critical) of system voltage · pick the larger of the drop size and the ampacity size (ABYC E-11, 105 °C, × 0.85 in engine spaces, × 0.7/0.6/0.5 bundled) · fuse ≤ derated ampacity of the wire

On a 12-volt boat a few volts lost in the wiring is a big share of the supply — a VHF or a bilge pump on undersized wire simply does not get its rated voltage — so marine practice sizes wire for voltage drop first and heat second. ABYC E-11 allows 3% drop on circuits that matter for safety and navigation and 10% on the rest, calculated on the full round trip, and then checks the wire can carry the current at the temperatures found in engine spaces and bundles. The fuse is chosen to protect that wire, not the device. This calculator does the same steps.

Worked example

A 20 A load 20 ft from the panel on a 12 V critical circuit: 40 ft round trip and a 0.36 V allowance need 10.75 × 20 × 40 ÷ 0.36 ≈ 23,900 circular mils — 6 AWG (13.3 mm²), even though 18 AWG could carry 20 A by heat. Actual drop with 6 AWG is 0.33 V (2.7%); the wire may be protected by anything up to 120 A, so a 25 A fuse near the source suits the load. Allowing 10% instead, the same run would take 10 AWG.

Frequently asked questions