Cable sizer
Find the minimum cable size for a given run, current and acceptable voltage drop budget.
Related tools
Worked example
You want to wire a 24V windlass (40A) with an 8 m one-way run, keeping drop within 3%:
- Max allowable drop: 24V × 3% = 0.72V
- Max resistance: 0.72V ÷ 40A = 18 mΩ total
- Total cable length: 8 m × 2 = 16 m — so max 18 ÷ 16 = 1.125 mΩ/m
- Result: 16 mm² cable (1.15 mΩ/m) is the minimum — 25 mm² gives margin
Try 24V / 40A / 8 m / 3% above to verify.
How it works
The sizer calculates the maximum allowable resistance for the circuit, then finds the smallest standard cable size whose resistance per metre is low enough to meet it.
Max resistance = Max drop (V) ÷ Current (A)
Always go one size up if the cable will be bundled with others, as bundling reduces current-carrying capacity.
Frequently asked questions
Should I always use the 3% drop setting?
For safety-critical circuits — bilge pumps, navigation lights, VHF radio, engine starting — yes, always use 3%. For cabin lighting or non-critical accessories, 10% is acceptable. When in doubt, use 3% and size up.
What does "one-way run length" mean?
It's the distance from your fuse or distribution panel to the device — not the total cable length. The calculator automatically doubles it to account for the return conductor.
My cable size seems very large — is that right?
High-current devices over long runs genuinely need large cable. A 30A windlass on a 10 m run needs 16–25 mm² cable. Undersizing is a leading cause of boat fires. If the result surprises you, check your current figure is accurate — peak starting current is often much higher than running current.
Can I use two smaller cables instead of one large one?
Yes — two cables in parallel halve the effective resistance. Use the voltage drop calculator with the "2 parallel runs" option to check. Both cables must be the same size and length, and both must be fused.