ABYC E-11 Marine Ampacity Calculator

Enter circuit current, one-way run length, system voltage, conductor temperature rating, location, bundle count, and allowable voltage drop. Returns the minimum AWG sized to ABYC E-11 ampacity tables and the 3% / 10% voltage-drop rule, whichever requires more copper.

Critical-load drop limit

Non-critical drop limit

10%

Engine-room derate

0.58×

Copper K constant

10.75Ω·CM/ft

Circuit current (A) Continuous load on the conductor

One-way length (ft) Source to load — calculator doubles for round-trip

System voltage Nominal bus voltage

Allowable voltage drop Per ABYC E-11 §11.6.5

Conductor temp rating Insulation rating (UL Boat Cable / BC-5W2)

Location Engine room applies E-11 §10.4 derating

Bundle count Per ABYC Table VII bundle correction

Minimum AWG

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Enter values

Voltage-drop AWG

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Ampacity AWG

—

Actual drop at chosen AWG

—V

CM required

—CM

ABYC reference

Table V / VI / VII

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Tip ABYC §11.6.5: critical loads (engine, electronics, blowers, bilge pumps) must be sized for ≤ 3 % drop. Non-critical loads (cabin lights, accessories) tolerate up to 10 %. When in doubt, design to 3 % — voltage drop on a starter motor cable robs cranking torque.

Worked example

30 A panel feed, 25 ft one-way run on a 12 V DC boat, 3 % drop allowed, 75 °C tinned copper, general (non-engine-room) location, single conductor.

1. CM required for voltage drop CM = (K × I × L × 2) / (V × Vd%/100) CM = (10.75 × 30 × 25 × 2) / (12 × 0.03) CM = 16,125 / 0.36 CM = 44,792 2. Smallest AWG with ≥ 44,792 CM #4 AWG = 41,740 CM → too small #2 AWG = 66,360 CM → ✓ Voltage-drop AWG = #2 3. Ampacity check (Table V, 75 °C, general) 30 A load → #10 AWG (40 A rated) is sufficient Ampacity AWG = #10 4. Final = max(#2, #10) Voltage drop dominates. Use #2 AWG copper 5. Actual drop at #2 AWG Vd = (10.75 × 30 × 25 × 2) / 66,360 = 0.243 V Vd% = 0.243 / 12 × 100 = 2.0 % ≈ 0.24 V drop, 1.0 % margin under 3 % limit

Sources

ABYC E-11 AC and DC Electrical Systems on Boats (2023) ABYC Standards & Technical Information Reports USCG 46 CFR §183.340 — uninspected boats IEEE 45.1 Recommended Practice — Shipboard Electrical Calder Boatowner's Mechanical & Electrical Manual (4th ed.)

Common mistakes

Using NEC ampacity tables on a boat. NEC residential values undersize for the marine environment — vibration, salt, and engine-room heat all derate copper. Always use ABYC E-11 Tables V/VI/VII.
Forgetting round-trip length. The voltage-drop formula uses 2× one-way length because current returns on the negative conductor (DC) or neutral (AC). A 25 ft run is 50 ft of copper.
Ignoring connector and terminal drop. Each crimped lug on a battery cable adds roughly 0.1 V at full load. On a 12 V starter circuit with four crimps, that's 3 % gone before the wire even contributes.

Warn Engine-room conductors must use insulation rated ≥ 105 °C (ABYC E-11 Table V) and pass through hot-zone derating. Standard residential THHN is NOT marine-rated — use UL Boat Cable, BC-5W2, or equivalent. Tinned copper is required where corrosion is a risk.

FAQ

Why does ABYC require larger wire than NEC for the same load?

The marine environment derates conductor ampacity per ABYC E-11 §10.4 (ambient compensation). Vibration, salt corrosion, and engine-room heat all reduce safe current capacity, so ABYC tables run more conservative than NEC residential values. A circuit that's fine in a dry wall cavity can overheat in a bilge or behind an engine cover.

Single-conductor vs sheathed cable ampacity?

ABYC Table V (single conductor in free air) gives the highest ampacity. Table VI (2-3 conductor sheathed) is lower, and Table VII (4+ conductors bundled) lower still. Bundling derates because heat from each conductor can't radiate away — the inner cores cook the outer ones. Always count every current-carrying conductor that shares a sheath or wire loom.

12 V vs 24 V system — does wire size differ?

Yes. Halving the voltage doubles the current for the same power (P = V × I). Voltage drop scales linearly with current, so a 12 V boat needs roughly 2× the copper of a 24 V boat at the same load and run length. This is why long bow-thruster runs and inverter feeds often drive a 24 V house bank decision on larger vessels.

Where does the K = 10.75 come from?

It's the resistivity of copper at 75 °C expressed in circular-mil units: ρ ≈ 10.75 Ω·CM/ft. The constant lets you compute voltage drop directly from current, length, and conductor cross-section in CM: Vd = K × I × 2L / CM. Aluminum (rare in marine) uses K ≈ 17.

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Marine Toolkit keeps ABYC E-11 ampacity and tank/scope math on the phone, saves each run, and works in the engine room and offshore. Pay once, own it.