Cable that would be perfectly adequate for a household circuit is very often the wrong size for the same current on a boat. The difference is run length and the effect of voltage drop, and it catches out a lot of otherwise careful DIY wiring jobs, particularly on longer runs to the bow, the stern, or a mast-top light.
This guide covers how to size a marine DC cable properly, why the standard rule is a 3% voltage drop, and what the practical consequences are of getting it wrong.
Household wiring runs at 230V, where a given percentage voltage drop over a cable run represents a fairly small number of volts. A boat's DC system runs at 12V or 24V, so the same percentage drop is a much larger fraction of the operating voltage, and a much larger effect on anything sensitive to voltage, an LED that dims, a pump that runs slowly, an electronic device that resets.
Cable is also sized against total round-trip length, out and back, not just the distance to the fitting. A windlass 15 metres from the battery has a 30 metre circuit, and that number, not the straight-line distance, is what goes into the sizing calculation.
ISO 10133 and general marine practice work to a maximum voltage drop of 3% of nominal system voltage for most circuits, and sometimes a tighter 1-2% for critical circuits such as navigation lights or electronics where a stable supply matters. On a 12V system, 3% is roughly 0.36V; on 24V it is roughly 0.72V.
Cable sizing tables published by cable manufacturers and in ISO 10133 give a cross-sectional area or AWG size for a given current and one-way cable length, calculated to keep drop within that 3% limit. As a working example, a 15A circuit over a 5 metre one-way run on 12V typically needs in the order of 4mm² (roughly AWG 11) tinned marine cable to stay within 3% drop; the same current over a 10 metre run needs a noticeably heavier cable, illustrating why length matters as much as current.
An undersized cable does not usually fail outright; it runs hot and drops voltage under load. Symptoms include dim navigation or interior lights that brighten when the engine is running and the alternator is pushing more voltage, pumps and windlasses that run sluggishly, and electronics that behave erratically under load, resetting or showing low voltage warnings even when the battery itself is healthy.
Left long enough under sustained load, an undersized cable can also overheat the insulation, particularly where it is bundled tightly with other cables or run through an enclosed space, which is a genuine fire risk rather than just a performance issue.
Work out the maximum current the circuit will draw, measure the actual round-trip cable run including any vertical runs and bends, then use a marine cable sizing chart, cross-referencing current and length, to select a size that keeps drop within 3% (or tighter for sensitive circuits). Round up rather than down if you are between sizes.
It is worth sizing generously on circuits likely to be extended later, an inverter feed or a windlass circuit, since re-running heavier cable after the fact is far more work than fitting it correctly the first time.
Simple, short, low-current additions such as a single interior light on an existing protected circuit are reasonable DIY territory provided the cable size is checked against a proper chart. Anything involving the main battery cables, an inverter, a windlass, or long runs to the bow or mast is worth having sized and installed professionally, since the consequences of getting it wrong range from poor performance to a genuine fire risk.
Yes, oversizing is safe and sometimes sensible for future-proofing, though it costs more and can be harder to route and terminate in tight spaces.
Yes, use the actual physical length the cable will follow, out and back, including any vertical runs, not the straight-line distance between the two points.
Tinning protects the individual copper strands from corrosion in the damp, salt-laden environment on a boat, which untinned domestic cable is not designed to withstand.
It is the standard general-purpose figure; some critical circuits, particularly navigation lights and sensitive electronics, are better served by a tighter 1-2% allowance.
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