Marine Cable Sizing Explained
7 min read · Updated
Cable sizing on a yacht is not about the current the cable can survive — it's about the current it can carry without excessive voltage drop over its actual run length. This article summarises the practical method used by MarinePower.app and by MS Ocean when specifying DC installations.
Two constraints, always
Every marine cable must satisfy two independent constraints: current-carrying capacity (ampacity) under the bundle/insulation/temperature it will experience, and voltage drop over the full run length. In DC systems on a 12V or 24V bus, voltage drop almost always drives the specification.
Measure the true run length
Run length is the total conductor length — positive plus negative — from source to load and back. It is not the straight-line distance. Under-measuring here is the most common error in retrofit work.
Set an acceptable drop
ABYC E-11 recommends 3% for critical circuits (panels, electronics) and 10% for non-critical loads at full current. On lithium 12/24V systems we generally hold to 3% across the whole distribution to preserve controller headroom.
Verify against ampacity
Once the cross-section satisfies voltage drop, verify ampacity in the actual install condition — engine room heat, bundling, insulation type. A cable that is only voltage-drop-sized can still overheat inside a bundle at ambient +50°C.
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