Calculation tool · Salt & Wire

Yacht Wire Cross-Section Calculator

EU Standard (mm²) · AWG Standard · DC / AC

This yacht wire size calculator determines the minimum copper conductor cross-section required for a circuit on board — by voltage drop and by ampacity, with a 10% safety margin — and returns the result in both metric (mm²) and American (AWG / kcmil) sizes. It works for 12V, 24V, 36V and 48V DC systems as well as 110V and 230V AC, with separate handling of the DC return conductor (length × 2) and AC single-cable runs.

The tool is intended for boat owners planning a refit, yard installers, DIY refitters and surveyors who need a fast sanity-check on a marine wire gauge before reaching for a full schematic. Drop limits follow industry convention: 1.5% for sensitive electronics (sonars, AIS, autopilot computers, navigation displays) and 3% for general devices — consistent with ABYC E-11 critical-circuit practice and the figures in ISO 13297 and IEC 60092-352 for recreational craft.

Correct wire sizing on a yacht is not optional. Undersized conductors run hot, accelerate insulation breakdown, cause voltage at the load to sag below equipment minima (electronics reboot, autopilots disengage, fridges lose efficiency), and in the worst case start fires. Over-sized conductors waste copper and money but never hurt safety. Use this marine wire gauge calculator as a first pass; for a final installation, always have the design reviewed against the applicable standard and signed off by a competent marine electrician.

Important notice

This calculator is an auxiliary and indicative tool only — it does not replace professional technical assessment. Results must always be verified by a certified electrician. All electrical installations must be designed and executed by a certified electrician in accordance with applicable standards (PN-HD 60364, ABYC E-11, ISO 13297). Salt & Wire accepts no liability for damages resulting from use of results without specialist verification.

01 — Supply voltage

02 — Load type

03 — Current type

04 — Load and route parameters

Formula: A_min = (ρ × L_DC × I) / ΔU_max · ρCu = 0.0175 Ω·mm²/m · ·

05 — Calculation results

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Yacht wire sizing — frequently asked questions

Expert answers on marine wire gauge, voltage drop, ABYC vs IEC standards, AWG vs mm², ampacity and engine-room derating.

What gauge wire do I need for a 12V yacht installation?

It depends on current and run length. On 12V systems the available voltage drop is small — only 0.36 V at 3% — so cables grow quickly. As a rough guide: a 10 A load over a 5 m one-way run needs about 4 mm² (around 12 AWG) to stay within 3% drop; a 20 A load over 7 m needs roughly 10 mm² (around 8 AWG). Always size for both voltage drop and ampacity (ABYC E-11 Table VI or IEC 60092-352) and add at least 10% margin on continuous loads.

How does ABYC differ from IEC for marine wiring?

ABYC E-11 is the North American small-craft standard (typically applied on US-built and US-flagged yachts). It uses AWG sizing, mandates tinned, stranded copper of Type 2 or 3 flexibility, and ties allowable drop (3% or 10%) to circuit criticality. IEC 60092 / ISO 13297 are the international and recreational-craft equivalents used in Europe; sizing is in mm², ampacity tables differ slightly, and protection coordination follows IEC 60364 logic. The engineering principles match — voltage drop, ampacity, derating, fault protection — but the tables, terminations and conduit rules differ.

How do I calculate voltage drop on a boat?

Use ΔU = (2 × ρ × L × I) / A for DC and ΔU = (ρ × L × I) / A for AC single-phase in a two-conductor cable already counted in L. ρ for copper is 0.0175 Ω·mm²/m at 20 °C (rising to about 0.022 at 70 °C — use the hot value for engine-room runs). L is the one-way run for DC (the ×2 accounts for the return conductor), I is the actual current in amperes, A is the cross-section in mm². Express the result as a percentage of nominal bus voltage and compare to the allowed limit.

Why does marine wiring use stranded copper?

Solid conductors work-harden and fail by fatigue under the continuous vibration, hull flexure and thermal cycling found on yachts. ABYC E-11 therefore requires stranded copper of Type 2 (general) or Type 3 (high-flex) construction, with strand counts increasing with conductor size. Stranding also improves the skin-effect behaviour at higher AC frequencies and makes terminations into crimp lugs reliable. Solid wire of the kind used in dry residential installations is explicitly disallowed.

AWG vs mm² — which standard should I use on my yacht?

Use the standard that matches the equipment, the build standard and the inspector. US-built or US-flagged yachts almost always use AWG to ABYC E-11. Yachts built in Europe and CE-marked under the Recreational Craft Directive use mm² to ISO 13297 / IEC 60092. Mixing is acceptable as long as cross-section equivalence is documented: 1 AWG step is about a 26% area change, so 10 AWG ≈ 5.26 mm² and 4 AWG ≈ 21.15 mm². The physics is identical; the paperwork is not.

What is the maximum acceptable voltage drop on critical circuits?

ABYC E-11 splits circuits into two classes. Critical circuits — navigation lights, bilge pumps, electronics, navigation equipment, panelboard mains — are sized to a maximum 3% drop at the rated load. Non-critical circuits (cabin lighting, general accessories) may be allowed up to 10%. For sensitive electronics on a 12V bus we routinely target 1.5% to keep the supply within instrument tolerances, especially for sonars, AIS and autopilot computers that misbehave below 11.5 V.

Should I oversize wire for future loads?

On main feeders and DC distribution trunks, yes — one or two sizes up is cheap insurance against later additions (windlass upgrade, lithium retrofit, bow thruster). It also lowers conductor temperature, which extends insulation life. On branch circuits, do not oversize past the protective device's interrupt rating window; the overcurrent device protects the wire, so a 30 A breaker on 25 A wire is wrong even if "oversized" sounds safer. Match wire ampacity to the breaker, then upsize for drop, not the other way round.

How long is too long for a DC run on a 12V system?

There is no fixed maximum — only the point where the required cross-section becomes uneconomic or physically impractical. As a rule of thumb, beyond about 8 to 10 m one-way at currents over 20 A on 12V, you should either move the load to 24V, relocate the battery bank, or split the circuit. A 30 A load over 12 m at 3% drop on 12V already wants 25 mm² (3 AWG); the same load at 24V drops to roughly 6 mm² (10 AWG). Voltage choice is part of cable sizing.

Is tinned copper required for marine use?

ABYC E-11 strongly recommends tinned, stranded copper for all onboard wiring, and it is effectively mandatory in saltwater service. Bare copper wicks moisture under the insulation by capillary action; once chlorides reach the strands, corrosion progresses invisibly until you see green powder at a terminal and a resistance that has tripled. Tinned conductors resist this for the design life of the yacht. ISO 13297 does not mandate tinning by name but specifies corrosion-resistant conductors, which in practice means tinned in marine environments.

What about ambient temperature derating in engine rooms?

Tabulated ampacities assume 30 °C ambient. Engine rooms commonly hit 60 °C at running temperature, which derates copper ampacity by roughly 30 to 40% depending on insulation rating (75 °C vs 90 °C vs 105 °C). ABYC E-11 Table VI and IEC 60092-352 publish correction factors. Cables in conduit, bundled in looms, or routed against hot exhausts need further derating. Practically: in any engine-room run, use 105 °C insulation (PVC THWN/SAE J1128 SXL or silicone) and apply the temperature correction before checking against the breaker rating.