1What your device needs Voltage drop
35 mm²
72 % of allowed drop
At most 10 % drop (1.20 V) at 110 A: calculated 25.29 mm², rounded up to 35 mm².
Boat Cable and Fuse Size Calculator, 12 V, 24 V and 48 V DC to ISO 13297
Enter what you connect on board and how the cable runs. You instantly get cable size and fuse to the guide values of ISO 13297: not just by voltage drop, but also by heat, engine room and bundling.
Some inputs were invalid and have been replaced.
On board the hull is not a conductor: the negative always returns in its own cable. That is how we calculated.
2What the cable can take Heat
35 mm² ↑ thicker✓ fine
35 mm²: 48 % load
At 30 °C, 35 mm² carries up to 210 A continuously. With 100 A it runs at 48 %: fine.
3What the installation leaves Installation
35 mm² ↑ thicker✓ fine
35 mm²: 48 % load
Single, free air: factor 1, each cable carries its full table value. 35 mm² runs at 48 %: fine.
4Which fuse fits Fuse (DC)
MEGA 175 A
none fits
In the fuse box at 65 °C it must hold 120 A continuously: MEGA 175 A (holds 125 A there). The cable carries 210 A, so the fuse protects it against short circuit and overload.
Place the fuse in the positive cable no more than 200 mm from the battery. Up to there the cable is unprotected.
Only for DC on board (12, 24, 48 V). Not for 230 V: shore power, generator and inverter output belong in the hands of a qualified electrician.
How to go thinner
- calculate
- With 24 V instead of 12 V only half the current flows for the same power: 10 mm². calculate
- calculate
- calculate
- It does not get thinner here without cutting into function or safety.
Lugs, crimps, fuse holders and switches must handle the same current. Most hot cables are really hot connections.
Working, tables and background
Working with your numbers
- 1 Power with margin: 1,200 W × 1.1 = 1,320 W
- 1 Design current at nominal voltage: 1,320 W ÷ 12 V = 110 A
- 1 Cable length: 8 m × 2 = 16 m (positive out, negative back)
- 1 Allowed voltage drop: 10 % of the nominal 12 V = 1.20 V
-
1
A = L × I ÷ (κ × ΔU) = 16 × 110 ÷ (58 × 1.20) = 25.29 mm², rounded up 35 mm² - 2 Continuous current for heat and fuse: 1,200 W ÷ 12 V = 100 A, at nominal voltage and without margin, because a cable heats up over minutes.
- 2 Table A.1, 105 °C cable column, 30 °C ambient: the smallest cross-section carrying 100 A continuously is 16 mm².
- 3 Factor for Single, free air: 1. Needed: 100 A at 30 °C: 16 mm².
- 4 Fuse: 100 A + 20 % safety factor = 120 A it must hold at 65 °C. Table: MEGA 175 A holds 125 A there. Under ISO 13297 at most the cable's rating: 35 mm² carries 210 A.
- = Result: the largest cross-section of steps 1 to 4, at least 1 mm². 35 mm²
What the result means
- Voltage drop with 35 mm²: 0.867 V (7.2 %). With 12 V at the battery, 11.13 V reach the device.
- Heat in the cable at continuous load: 79 W.
- Copper: 5.02 kg. The copper core is about 6.7 mm across; cross-section is not diameter.
- With 35 mm² the device may be up to 11 m from the battery.
- Cross-check with the real strand resistance (0.527 mΩ/m at 20 °C) and the current without margin (100 A): 0.843 V (7 %). Fine, the margin covers the difference to κ = 58.
What a cable carries continuously (ISO 13297, Annex A, single conductor)
| Cross-section | at 30 °C (105 °C cable) | in the engine room, 60 °C (105 °C cable) | Resistance |
|---|---|---|---|
| 0.75 mm² | 16 A | 13.8 A | 24.7 mΩ/m |
| 1 mm² | 20 A | 17.2 A | 18.5 mΩ/m |
| 1.5 mm² | 25 A | 21.5 A | 12.7 mΩ/m |
| 2.5 mm² | 35 A | 30.1 A | 7.6 mΩ/m |
| 4 mm² | 45 A | 38.7 A | 4.71 mΩ/m |
| 6 mm² | 60 A | 51.6 A | 3.14 mΩ/m |
| 10 mm² | 90 A | 77.4 A | 1.82 mΩ/m |
| 16 mm² | 130 A | 111.8 A | 1.16 mΩ/m |
| 25 mm² | 170 A | 146.2 A | 0.743 mΩ/m |
| 35 mm² | 210 A | 180.6 A | 0.527 mΩ/m |
| 50 mm² | 270 A | 232.2 A | 0.368 mΩ/m |
| 70 mm² | 330 A | 283.8 A | 0.259 mΩ/m |
| 95 mm² | 390 A | 335.4 A | 0.196 mΩ/m |
| 120 mm² | 450 A | 387 A | 0.153 mΩ/m |
Factors for bundling (positive and negative count separately)
| Conductors | Devices | Bundled, in conduit or trunking |
|---|---|---|
| 1–3 | 1 | 1 |
| 4–6 | 2–3 | 0.7 |
| 7–24 | 4–12 | 0.6 |
| ≥ 25 | ≥ 13 | 0.5 |
Your cable by installation and count
| Devices | Single, free air | Bundled, in conduit or trunking |
|---|---|---|
| 1 | 35 | 35 |
| 2 | 35 | 50 |
| 3 | 35 | 50 |
| 4 | 35 | 70 |
| 5 | 35 | 70 |
| 6 | 35 | 70 |
| 7 | 35 | 70 |
| 8 | 35 | 70 |
| 9 | 35 | 70 |
| 10 | 35 | 70 |
| 12 | 35 | 70 |
| 14 | 35 | 95 |
| 16 | 35 | 95 |
| 18 | 35 | 95 |
| 20 | 35 | 95 |
Fuses compared
| Type | Rating | holds at 65 °C | protects the cable |
|---|---|---|---|
| Blade fuse (ATO) | – A | – A | ✓ yesshort circuit only |
| MIDI | 200 A | 122 A | ✓ yesshort circuit only |
| MEGA | 175 A | 125 A | ✓ yesshort circuit only |
Why we calculate this way
Why is voltage drop not enough? Current heats every cable. Above a certain current it gets hotter than its insulation can take continuously, however short it is. Short, thick inverter cables slip through the voltage-drop check and fail on heat.
Why nominal voltage? ISO 13297 states the allowed voltage drop as a percentage of nominal voltage. So here we calculate current and drop with 12, 24 or 48 V. The camper profile instead uses an almost empty battery (10.7 V).
Why a 10 % margin? No device is lossless. An inverter with 1,000 W output draws about 1,050 to 1,100 W from the battery. The margin covers efficiency and disturbances.
Why 3 or 10 %? ISO 13297 allows at most 10 % of nominal voltage, i.e. 1.2 V at 12 V. For important circuits it recommends 3 %: main feeders, navigation lights, bilge pumps and bilge blowers. We also use 3 % for electronics such as plotter, radio and autopilot, because they cut out at low voltage.
Why double length? Current flows out and back. If the device is 3 m from the battery, that is 6 m of cable. On board the negative is always its own cable, because the hull is not a conductor.
What is 58? The conductivity of copper in m/(Ω·mm²). The ISO 13297 formula with 0.0164 corresponds to about 61. We use 58, slightly more cautious, and the cross-check shows the margin covers fine-stranded cable.
Why 30 °C, and what about the engine room? ISO 13297 Annex A gives what a single conductor carries continuously at 30 °C, by insulation rating. In the engine room it assumes 60 °C: a 105 °C cable carries 86 % there, a 70 °C cable only 75 %. There the insulation must be rated at least 70 °C and be oil resistant or protected.
What does installation mean? A cable sheds heat through its surface. On its own in free air that works well. In a bundle, conduit or trunking, cables heat each other, so each may carry less current. On board the calculator only distinguishes these two cases.
What counts? ISO 13297 counts conductors, not devices. Positive and negative of every device in the same bundle that draws current at the same time count separately, so 2 conductors per device. Up to 3 conductors keep the full rating, 4 to 6 conductors 70 %, 7 to 24 conductors 60 %, 25 or more 50 %. Two devices are already 4 conductors. The steps come from the AC part of the standard; we cautiously apply them to DC as well.
What the fuse is for: it protects the cable, not the device. Under ISO 13297 its rating must not exceed what the thinnest cable in the circuit carries. If the cable is not enough, we size it up. Place it in the positive cable no more than 200 mm from the battery. Fuses also weaken in heat: a MEGA 300 A holds only 205 A at 65 °C.
DC only: everything here applies to 12, 24 and 48 V circuits on board. 230 V, shore power and generators follow the AC part of ISO 13297, other protective devices (circuit breakers, RCDs) and other tables.
Cable and fuse for typical loads on board
Calculated for 12 V, negative as a cable, cable alone in free air, outside the engine room (30 °C), cable rated 105 °C. Columns: distance to battery. In a bundle or the engine room it gets more, the calculator above shows how much.
| Load | Current | 0.5 m | 1 m | 2 m | 3 m | 5 m | Fuse |
|---|---|---|---|---|---|---|---|
| LED navigation lights | 0.50 A | 1 mm² | 1 mm² | 1 mm² | 1 mm² | 1 mm² | Blade fuse (ATO) 1 A |
| Bilge pump (up to 5 A) | 5.00 A | 1 mm² | 1 mm² | 1.5 mm² | 2.5 mm² | 4 mm² | Blade fuse (ATO) 7.5 A |
| tigerexped water pump small (11 l/min) | 8.00 A | 1 mm² | 1 mm² | 1 mm² | 1 mm² | 1.5 mm² | Blade fuse (ATO) 15 A |
| tigerexped water pump large (18 l/min) | 15.0 A | 1.5 mm² | 1.5 mm² | 1.5 mm² | 1.5 mm² | 2.5 mm² | Blade fuse (ATO) 25 A |
| Compressor fridge | 5.00 A | 1 mm² | 1 mm² | 1.5 mm² | 2.5 mm² | 4 mm² | Blade fuse (ATO) 7.5 A |
| Chartplotter 9 inch | 2.08 A | 1 mm² | 1 mm² | 1 mm² | 1 mm² | 1.5 mm² | Blade fuse (ATO) 3 A |
| VHF radio (25 W transmit) | 5.83 A | 1 mm² | 1 mm² | 1.5 mm² | 2.5 mm² | 4 mm² | Blade fuse (ATO) 10 A |
| Autopilot (drive up to 7 A) | 7.50 A | 1 mm² | 1 mm² | 2.5 mm² | 2.5 mm² | 4 mm² | Blade fuse (ATO) 15 A |
| Anchor windlass (approx. 80 A) | 83.3 A | 16 mm² | 16 mm² | 16 mm² | 16 mm² | 16 mm² | MIDI 125 A |
| DC-DC charger 30 A | 41.7 A | 10 mm² | 10 mm² | 10 mm² | 16 mm² | 25 mm² | MIDI 70 A |
| Inverter 2000 W | 167 A | 70 mm² | 70 mm² | 70 mm² | 70 mm² | 95 mm² | MEGA 300 A |
Frequently asked questions
What cable size do I need on a boat?
It depends on current, length and installation, not just the device. Following ISO 13297 the calculator checks voltage drop, current rating at 30 °C or in the engine room, bundling and fuse, and takes the largest result. Single conductors on board are at least 1 mm².
How much voltage drop is allowed on a boat?
ISO 13297 allows at most 10 % of nominal voltage, i.e. 1.2 V at 12 V. For navigation lights, bilge pumps, bilge blowers and main feeders it recommends 3 %, which is 0.36 V. We also use 3 % for electronics.
What cable size for an anchor windlass?
A windlass with a 700 W motor draws around 80 A, briefly much more when starting. At 6 m one way on 12 V, 10 % drop and run in free air that is 16 mm² with MIDI 125 A. If the windlass manual calls for a thicker cable, follow it.
What cable size for a 2000 W inverter on a boat?
On 12 V, 1.5 m from the battery, run singly with 105 °C marine cable: 70 mm² with MEGA 300 A. In the engine room or in a bundle it gets thicker, on 24 V much thinner.
Which fuse goes with my boat cable?
Under ISO 13297 its rating must not exceed what the thinnest cable in the circuit carries. At the same time it must hold the continuous current when warm without blowing. The calculator picks a blade (ATO), MIDI or MEGA fuse and sizes the cable up if the two would not match otherwise.
Where must the fuse go?
In the positive cable, no more than 200 mm from the battery. If that is not possible, the cable up to the fuse must be protected along its whole length, for example in conduit or trunking. Exceptions apply, among others, to the starter cable.
Can I use the hull as the negative?
No. The ISO small craft standard does not allow the hull as a current-carrying conductor, the negative always returns in its own cable. Only the engine's own wiring may use the engine block. That is why the calculator always counts out and back.
Do I count positive and negative in the cable length?
Yes. ISO 13297 takes the length from the battery to the device and back. Enter the one-way distance, the calculator doubles it.
What changes in the engine room?
ISO 13297 assumes 60 °C there instead of 30 °C. A 105 °C cable then carries 86 %, a 70 °C cable only 75 % of its table value. The insulation there must be rated at least 70 °C and be oil resistant or protected.
Does boat cable have to be tinned?
The standard requires stranded copper; according to our sources it does not prescribe tinning. Tinned strands corrode more slowly in damp, salty air and are the usual recommendation on board. CCA and speaker wire do not belong in the boat's wiring.
How do I count cables in a bundle?
ISO 13297 counts conductors: positive and negative of each device count separately. Up to 3 conductors keep the full rating, 4 to 6 conductors 70 %, 7 to 24 conductors 60 %, 25 or more 50 %. Two devices in a bundle are already 4 conductors.
ISO 13297 or ISO 10133: which applies?
ISO 13297:2020 covers DC and AC on board together and replaces ISO 10133. The current rating table in Annex A is the same in ISO 10133:2000/2012 and 13297:2000. We use it as guide values.
DC only: everything here applies to 12, 24 and 48 V circuits on board. 230 V, shore power and generators follow the AC part of ISO 13297, other protective devices (circuit breakers, RCDs) and other tables.
The calculator applies to DC, copper conductors and the assumptions stated in the calculation steps. Working on electrical systems requires expertise. Guide values derived from ISO 13297, Annex A (identical in ISO 10133:2000/2012 and 13297:2000). Bundling to ISO 13297 (cautious reading). Fuses from manufacturer data (Littelfuse). ISO and Littelfuse are not affiliated with tigerexped and have neither reviewed nor endorsed this calculator.