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Cable cross-section calculator for campervans and RVs
Undersized cable is the most common mistake in a self-build — and the one with the most unpleasant consequences. Here is how the cross-section is calculated, why 12 volt systems follow different rules than household wiring, and what the numbers come out at for typical loads.
Open the calculator Plan the whole system
Why cross-section matters so much at 12 volts
Power is voltage times current. Because the system voltage is low, the current has to be correspondingly high: 1000 watts means a good 4 amps at 230 volts, but around 83 amps at 12 volts. That is why campervans need conductor sizes nobody ever runs in a house.
Two effects make thin cable a problem:
Voltage drop. Every cable has resistance. Whatever voltage is lost along the way never reaches the load. At 230 volts, 0.5 V goes unnoticed — at 12 volts that is more than 4 per cent, and a compressor fridge or water pump answers with poor starting behaviour or cut-outs. This is why low-voltage systems are calculated far more strictly.
Heating. The lost voltage does not disappear, it becomes heat in the cable. Inside a trunking run behind the panelling, bundled with other cables and without airflow, that is not a theoretical risk.
The formula
The minimum cross-section follows from cable length, current, system voltage and the voltage drop you are prepared to accept:
- A
- cross-section in mm²
- L
- one-way cable length in metres (not the sum of both conductors)
- I
- current in amps
- U
- system voltage, i.e. 12 V or 24 V
- δU
- permitted voltage drop as a fraction — 0.02 here, i.e. 2 %
- γ
- conductivity of copper, 56 m/(Ω·mm²)
Where the factor 2 comes from
Current flows in a loop. The length of the negative conductor adds to that of the positive one — the current covers the distance twice. That is why the one-way length is doubled. Using the combined length of both conductors and multiplying by 2 sizes the cable twice as large as needed.
Why 2 per cent and not 3
Values between 2 and 5 per cent are all in circulation. This calculator deliberately uses the strict 2 per cent: at 12 volts that is a mere 0.24 V of headroom. Being more generous saves copper and buys you starting problems on compressors and pumps. For lighting alone, more would be defensible — for anything with a motor, less so.
From the calculated value to a cable you can actually buy
The calculator adds 12 per cent margin to the result and then rounds up to the next standard size: 1 · 2.5 · 4 · 6 · 10 · 16 · 25 · 35 · 50 mm² and so on. It always rounds up — one size too large costs money, one size too small is dangerous.
Worked example
Compressor cool box, 4 m cable run, 5 A at 12 V
A = (2 · 4 m · 5 A) / (0.02 · 12 V · 56) = 40 / 13.44 = 2.98 mm²
plus 12 % margin → 3.33 mm² → next standard size: 4 mm²
Typical values in a campervan
All figures for a 12 volt system, calculated with the formula above. Length is the one-way run from source to load.
| Load | Length | Current | calculated | recommended |
|---|---|---|---|---|
| LED lighting | 5 m | 2 A | 1.49 mm² | 2.5 mm² |
| Compressor cool box | 4 m | 5 A | 2.98 mm² | 4 mm² |
| Water pump | 3 m | 10 A | 4.46 mm² | 6 mm² |
| Solar controller to battery | 2 m | 20 A | 5.95 mm² | 8 mm² |
| DC-DC charger | 4 m | 30 A | 17.86 mm² | 25 mm² |
| Inverter 1000 W | 1.5 m | 90 A | 20.09 mm² | 25 mm² |
| Inverter 2000 W | 1.5 m | 175 A | 39.06 mm² | 50 mm² |
The table is no substitute for doing the sum — one extra metre of cable run already shifts the result. For your own figures: open the calculator.
The fuse belongs to the cable, not to the appliance
The most widely misunderstood point in vehicle electrics: a fuse protects the conductor. Not the inverter, not the fridge — the cable. So it has to be chosen to blow before the conductor is thermally overloaded.
Which means: determine the cross-section from current and length first, then pick the fuse to match the cross-section — not the other way round. And it belongs as close to the battery as possible, because everything between battery and fuse is unprotected.
Also worth watching: isolator switches and battery protection devices have their own maximum currents. If that is below the fuse rating, the component gives up before the fuse does. The planner checks this chain automatically.
Series or parallel — the biggest lever on solar cable
With more than one solar panel, the wiring decides the cross-section you need. In series the voltages add up while the current stays the same — thinner cable, lower losses. In parallel the currents add up and the cable has to grow. With an MPPT controller series wiring is therefore usually the better choice. A PWM controller cannot make use of the higher voltage.
Rule of thumb: if the sum produces more than 10 mm² for a solar run, it is worth revisiting the panel wiring or the position of the controller first — that is almost always cheaper than even thicker copper.
Common mistakes
- Only counting the positive cable. The return path through negative counts just as much — hence the factor 2.
- Using continuous rather than peak current. Inverters and compressors draw a multiple of it on start-up.
- Sizing the cable to the fuse instead of the other way round. The order is: current → cross-section → fuse.
- Assuming the vehicle chassis as return path. In a self-build a properly run negative cable is the more dependable solution.
- Aluminium instead of copper. Aluminium conducts considerably worse; the formula above is for copper.
- Confusing flexible and solid conductors. Vehicles need finely stranded cable — solid conductors fracture under vibration.