Size your 12V / 24V / 230V electrical installation
Learn in just a few minutes how to size your electrical cables for your campervan, motorhome or boat. We walk you through the tool step by step and show you how to interpret the results.
This free tool lets you precisely size your electrical cables for any converted vehicle or vessel. Whether you're building the electrical system for your campervan, wiring your motorhome, fitting out your caravan or installing the electrical circuit on your boat, our calculator instantly determines the correct cable size, the right fuse rating and checks the voltage drop on every circuit.
Cable sizing is governed by two fundamental constraints: the thermal limit (the cable must not overheat) and the voltage drop (limited to 3% to ensure your appliances work correctly). Our calculator applies both rules automatically and selects the more conservative size.
The basic formula is I = P / V (current = power / voltage). A 60W fridge on 12V draws 5A. A 2000W inverter on 12V draws 167A. The higher the current, the larger the cable size needs to be.
In a 12V installation, distance matters enormously. Current flows through both the positive and negative cables: for an appliance 3m from the battery, the actual electrical path is 6m. That's why our formula uses 2 × L in the cable size calculation.
The calculator determines the minimum cable size meeting both constraints, then recommends a fuse rated at 125% of the nominal current. The fuse protects the cable (not the appliance): it must always be below the maximum thermal capacity of the chosen cable.
Here are the recommended cable sizes for the most common equipment in a campervan, motorhome or boat (copper cable H07V-K, max 3% voltage drop, 3m length):
| Equipment | Power | Current (12V) | Min. size (3m) | Fuse |
|---|---|---|---|---|
| LED lighting | 20W | 1.7A | 0.75 mm² | 5A |
| Water pump | 40W | 3.3A | 1.5 mm² | 5A |
| 12V fridge | 60W | 5A | 2.5 mm² | 10A |
| MaxxFan ventilator | 35W | 2.9A | 1.5 mm² | 5A |
| Diesel heater | 120W | 10A | 4 mm² | 15A |
| 1000W inverter | 1000W | 83A | 25 mm² | 100A |
| 2000W inverter | 2000W | 167A | 50 mm² | 200A |
| Anchor windlass | 1500W | 125A | 35 mm² | 150A |
| Bow thruster | 3000W | 250A | 70 mm² | 300A |
These values are indicative for a 3-metre cable run. Use the calculator above to get a precise result for your actual cable length.
A campervan electrical installation is typically 12V with one or two leisure batteries (AGM, gel or lithium LiFePO4). Common equipment includes: LED lighting, a Dometic or Indel B fridge, a Shurflo water pump, a Webasto or Planar diesel heater, a MaxxFan roof vent, and a 12V to 230V inverter for mains sockets. Average cable runs range from 2 to 6 metres.
Motorhome wiring can be 12V or 24V depending on the base vehicle. Installations are often more complex than a van: solar panels, MPPT charge controller, DC-DC charger, high-capacity lithium battery, and sometimes a BMS. Cable runs are longer (up to 8-10m), requiring larger cable sizes.
Boat cable sizing follows the same physical principles but with additional constraints: salt water environment, vibrations, and maritime standards. Always use tinned marine-grade cable for corrosion resistance. Boats typically run 12V (small yachts) or 24V (vessels over 12m). Specialist equipment includes: anchor windlass, bow thruster, watermaker, navigation lights.
The size depends on the current (power / voltage) and the cable length. For a 12V fridge (60W) with 3m of cable, you need a minimum of 2.5 mm². For a 2000W inverter, you'll need at least 50 mm². Our calculator accounts for voltage drop (max 3%) and thermal limits to give you the exact cable size.
The formula is: S = (2 × L × I) / (σ × ΔU) where L is the length in metres, I is the current in amps, σ is the conductivity of copper (56 m/Ω·mm²) and ΔU is the allowable voltage drop (3% of the voltage). At 12V, ΔU = 0.36V. You must then verify that the calculated size also meets the thermal rating of the cable.
The fuse should be rated at 125% of the nominal current. For example, a 10A device requires a 15A fuse. Standard ratings are: 5A, 10A, 15A, 20A, 25A, 30A, 40A, 50A, 60A, 80A, 100A. Important: the fuse protects the cable, not the appliance.
At 24V, the current is halved for the same power (I = P/V). This allows smaller cable sizes and reduces losses. 24V is recommended for large installations with high power demands or long cable runs. The downside: fewer appliances are available in 24V.
The sizing follows the same principles as a campervan. In a marine environment, always use tinned copper cable for corrosion resistance, and allow an additional 20-30% safety margin. ABYC and ISO 13297 standards require more conservative minimum sizes.
Voltage drop is the loss of voltage along a cable due to its resistance. The longer or thinner the cable, the greater the drop. It's limited to 3% (0.36V at 12V) so that appliances receive sufficient voltage. Beyond this, a fridge might not start, LEDs may flicker, or an inverter may shut down.
For a campervan, motorhome or boat, always use flexible multi-strand cable (H07V-K or H07RN-F). Vehicle vibrations would quickly break a rigid single-core cable. For marine installations, choose tinned multi-strand cable to resist salt corrosion.
Both protect cables, but in a 12V campervan, fuses are more common and adequate (cheaper, compact). Use an ANL fuse holder for heavy cables (battery to inverter) and a blade fuse box for distribution. Circuit breakers are useful on 230V circuits.
⚠️ Important disclaimer — This calculator is a sizing assistance tool designed to estimate cable sections and fuse ratings. It does not replace the advice of a qualified electrician or the professional inspection of your installation. All electrical work should be carried out in compliance with applicable standards and verified by a certified professional before use.