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 (3% drop, ρCu @ 30 °C, two-conductor loop): a 10 A load over a 5 m one-way run needs about 6 mm² (around 9 AWG); a 20 A load over 7 m needs roughly 16 mm² (around 4 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 — more for engine-room runs where ambient temperature derates ampacity.
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?
For both DC and single-phase AC in a two-conductor cable: ΔU = (2 × ρ × L × I) / A, where L is the one-way run length (cable distance from source to load — the ×2 accounts for the return conductor that carries the same current back). ρCu = 0.0179 Ω·mm²/m at 30 °C (standard run), rising to 0.0206 Ω·mm²/m at 70 °C — use the hot value for engine-room runs where ambient reaches 60 °C. I is the actual current in amperes, A is the cross-section in mm². Express ΔU as a percentage of nominal bus voltage and compare to the allowed limit (1.5% for sensitive electronics, 3% for general devices, up to 10% for non-critical loads under ABYC).
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. The rule is simple: wire ampacity must always be ≥ breaker rating, because the overcurrent device protects the wire. You can have a wire larger than the breaker requires (upsize for voltage drop, future loads, or thermal derating), but you cannot have a 30 A breaker on a wire rated for only 25 A. Sizing order: (1) determine continuous load, (2) size wire ampacity for ≥125% of continuous load (ABYC / IEC convention), (3) check voltage drop and upsize wire if needed, (4) select breaker to protect the wire.
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 35 mm² (2 AWG). The same 360 W load on a 24V bus draws only 15 A and needs roughly 10 mm² (8 AWG) — voltage choice is a major part of cable sizing, because both current halves (P = U·I) and the available drop voltage doubles when you go from 12V to 24V, so the required area drops by a factor of 4.
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. This calculator handles 60 °C engine-room ambient with a ×0.71 ampacity factor (IEC 60364 Table 52.D1, 90 °C insulation) when you select the "Engine room" environment.
Does this calculator comply with ABYC's 125% continuous-load rule?
Yes — select "Continuous" in the Load duration toggle. ABYC E-11 §11.13.1.1.5 requires that conductors carrying current for three hours or more (continuous loads — refrigerators, autopilots on long passages, navigation lights at night, inverters under sustained load) be sized to carry at least 125 % of the rated load. When you select "Continuous (≥3 h, ABYC 125%)" the calculator multiplies your current by 1.25 when checking ampacity, picking a wire whose derated ampacity meets the higher figure. For short-duty loads (windlass, bilge pump cycles, instrument flashes) keep "Intermittent" — the 10 % margin is sufficient and avoids unnecessary copper.