Why length determines the cross-section
Every cable has resistance. When current flows through it, voltage drops across this resistance, and less reaches the consumer than leaves the battery. How much less depends on three factors: current, length, and cross-section.
In a 230-volt house installation, this hardly matters. A one-volt loss there is 0.4 percent. In a 12-volt on-board electrical system, the same one volt is more than eight percent — 12 V becomes 11 V, and a device that shuts off below that will do so even with a full battery.
Therefore, on board, a rule applies that contradicts common sense: The consumer alone does not determine the cross-section. The same anchor winch requires a significantly thicker cable in the bow than next to the battery. The connector says nothing about the required wire.
The Double Path
The second point is the most common calculation error of all. The current flows to the consumer via the positive wire and back via the negative wire. Both paths create voltage drop. A consumer five meters from the distribution board has a ten-meter conductor path.
Anyone who calculates the single path will end up with half the cross-section and double the voltage drop. This is not noticeable with lighting but very much so with a compressor refrigerator.
The Formula
FormulaCross-section in mm² = (2 × length in m × current in A × 0.0175) ÷ permissible voltage drop in V
0.0175 is the specific resistance of copper in ohm times square millimeters per meter, measured at room temperature. Warm cables in the engine compartment have slightly more resistance; the calculation is therefore slightly conservative, which is desirable at this point.
What remains is the permissible voltage drop. ISO 13297:2020 specifies two values: 10 percent of the nominal voltage for ordinary circuits, 3 percent for safety-critical equipment — navigation lights, bilge blowers, bilge pumps. At 12 volts, these are 1.2 and 0.36 volts respectively.
Example
A compressor refrigerator draws 8 A and is located 4 meters from the distribution board. It is not a safety-critical consumer, but it is sensitive to undervoltage — so we calculate with 3 percent.
- Conductor path: 2 × 4 m = 8 m
- Permissible voltage drop: 12 V × 0.03 = 0.36 V
- Cross-section: (8 × 8 × 0.0175) ÷ 0.36 = 3.1 mm²
- Selected: 4 mm² as the next standard size
With the 10 percent limit, it would have been 0.93 mm², i.e., 1.5 mm². The difference between the two criteria here is a factor of three — and that's precisely why it must be determined before calculation which criterion applies.
Tables
12 V, 3 Percent Voltage Drop
| Current | 2 m | 5 m | 10 m | 15 m |
|---|---|---|---|---|
| 5 A | 1.5 mm² | 2.5 mm² | 6 mm² | 10 mm² |
| 10 A | 2.5 mm² | 6 mm² | 10 mm² | 16 mm² |
| 20 A | 4 mm² | 10 mm² | 25 mm² | 35 mm² |
| 40 A | 10 mm² | 25 mm² | 50 mm² | 70 mm² |
| 60 A | 16 mm² | 35 mm² | 70 mm² | 95 mm² |
| 100 A | 25 mm² | 50 mm² | 120 mm² | 150 mm² |
Distance measured simply, doubled in the calculation. Rounded up to the next standard size.
12 V, 10 Percent Voltage Drop
| Current | 2 m | 5 m | 10 m | 15 m |
|---|---|---|---|---|
| 5 A | 1.5 mm² | 1.5 mm² | 1.5 mm² | 2.5 mm² |
| 10 A | 1.5 mm² | 1.5 mm² | 4 mm² | 6 mm² |
| 20 A | 1.5 mm² | 4 mm² | 6 mm² | 10 mm² |
| 40 A | 2.5 mm² | 6 mm² | 16 mm² | 25 mm² |
Only for non-critical consumers. Caution: Here, the current carrying capacity wins in several fields — see next section.
24 V, 3 Percent Voltage Drop
| Current | 2 m | 5 m | 10 m | 15 m |
|---|---|---|---|---|
| 5 A | 1.5 mm² | 1.5 mm² | 2.5 mm² | 4 mm² |
| 10 A | 1.5 mm² | 2.5 mm² | 6 mm² | 10 mm² |
| 20 A | 2.5 mm² | 6 mm² | 10 mm² | 16 mm² |
| 40 A | 4 mm² | 10 mm² | 25 mm² | 35 mm² |
| 60 A | 6 mm² | 16 mm² | 35 mm² | 50 mm² |
| 100 A | 10 mm² | 25 mm² | 50 mm² | 95 mm² |
For the same current, the cross-section is halved compared to 12 V. For the same power, the current is also halved — then the requirement is one quarter.
The second criterion: Current carrying capacity
Voltage drop is only one of two limits. The other is current carrying capacity: how much current the cable can continuously carry without the insulation getting too hot. Dimensioning is always based on the larger of the two values.
Over long distances, voltage drop almost always wins. Over short distances with high current — inverters, anchor winches, battery connections — current carrying capacity wins. An example: 100 A over 1 meter mathematically results in 10 mm² based on voltage drop. However, continuously sending 100 A through 10 mm² is already borderline depending on the installation.
Current carrying capacity depends not only on the cross-section but also on the environment. ABYC E-11 explicitly distinguishes between installation inside and outside engine compartments: The same 12-AWG cable with 75 °C insulation may carry 18.8 A outside the engine compartment, but 35 A inside. There are also derating factors for bundled installations.
WarningThe fuse is determined by the current carrying capacity of the cable, not by the calculated voltage drop cross-section. Therefore, if you generously dimension for voltage drop reasons, you must still not increase the fuse — it continues to protect the weakest point in the circuit.
Further exceptions and limits
Aluminum conductor. The formula applies to copper. Aluminum has about 1.6 times the specific resistance and accordingly requires more cross-section.
Fine-stranded boat cable. On board, fine-stranded, tinned copper should be used — because of vibration and corrosion, not because of resistance. The calculation method does not change as a result.
Contact resistances. The calculation only considers the cable. Poorly crimped cable lugs, corroded terminals, and connectors are added and can double the voltage drop, even if nothing is wrong with the cross-section. If you measure and find more loss than calculated, first check the connections.
Common negative line. If the negative path is shared by several consumers, the currents on the common section add up. The cross-section there must match the sum, not the individual consumer.
Typical errors
- Single instead of double cable length. Half the cross-section, double the loss.
- Only one criterion checked. The larger value of voltage drop and current carrying capacity applies.
- 10 percent used for sensitive consumers. Permissible does not mean sensible.
- Cable chosen based on existing stock. "There was already 2.5mm² in there" is not a design criterion.
- Ignored installation method. Bundled and in the engine compartment, the same cable can handle significantly less.
- Negative line made thinner than the positive line. It carries the same current.
- Contact resistances forgotten. They are not in any table but still make a difference.
Cross-check on the finished onboard electrical system
The calculation can be measured. Switch on the consumer and measure the voltage once directly at the battery and once directly at the consumer — under load, not at idle. The difference is the actual voltage drop of the entire circuit including all connections.
If it is significantly higher than the calculated value, the cross-section is rarely to blame. Check terminals, cable lugs, fuse holders, and the main switch. A single corroded connection can cause more loss than ten meters of correctly dimensioned cable.
