Campervan Wire Gauge Calculator: Choose the Right Cable Size
Choosing the correct wire gauge is one of the most important safety decisions in a campervan electrical build. Too thin, and the cable overheats — potentially causing a fire. Too thick, and you waste money and space. This guide shows you how to calculate the right size for every cable run in your van.
Wire sizing is a core part of safe campervan wiring. If you want the calculations done automatically, the VanPower calculator generates a complete wire schedule for your system.
Auto-Calculate Your Wire Sizes
Our free calculator generates a complete wire schedule with cable sizes, fuse ratings, and voltage drop for every circuit.
The Two Rules of Cable Sizing
Every cable must satisfy two requirements simultaneously:
- Current capacity — the cable must safely carry the maximum current without overheating
- Voltage drop — the cable must not lose more than an acceptable percentage of voltage over its length
You calculate both, then choose whichever gives you the larger cable size.
Step 1: Calculate the Current
For any 12V circuit, current is calculated as:
Current (A) = Power (W) ÷ Voltage (V)
For a 12V system, use 12V as the voltage. Examples:
| Appliance | Power | Current |
|---|---|---|
| LED light strip | 24W | 2A |
| Diesel heater | 36W | 3A |
| Compressor fridge | 60W | 5A |
| USB charger | 18W | 1.5A |
| Inverter (800W) | 800W | 66.7A |
| Water pump | 48W | 4A |
For inverters, calculate the current on the 12V DC side, not the 230V AC side. An 800W inverter draws approximately 800 ÷ 12 = 66.7A from the battery (plus ~15% for inverter inefficiency, so ~77A in practice).
Step 2: Check Current Capacity
Every cable size has a maximum safe current rating. These ratings assume the cable is running in free air at ambient temperature. If the cable is bundled with others or enclosed, derate by 20-30%.
| Cable Size (mm²) | Max Current (Free Air) | Typical Use |
|---|---|---|
| 0.75mm² | 6A | Signal wires, sensors |
| 1.0mm² | 10A | LED lights, small loads |
| 1.5mm² | 15A | Lighting circuits, fans |
| 2.5mm² | 21A | Fridge, water pump, heater |
| 4.0mm² | 27A | High-current accessories |
| 6.0mm² | 37A | DC-DC charger, solar MPPT |
| 10mm² | 50A | Large DC-DC charger |
| 16mm² | 70A | Small inverter |
| 25mm² | 95A | Medium inverter |
| 35mm² | 120A | Large inverter |
| 50mm² | 160A | Battery interconnects |
| 70mm² | 200A | Very large systems |
Choose a cable whose current rating exceeds your calculated current by at least 25%.
Step 3: Calculate Voltage Drop
This is where most DIY builders go wrong. At 12V, voltage drop matters far more than at 230V because you are working with a much lower base voltage. A 3% drop at 230V is 6.9V — your appliance barely notices. A 3% drop at 12V is only 0.36V — and some appliances (especially LED drivers and sensitive electronics) will malfunction below 11.5V.
The Voltage Drop Formula
Voltage Drop (V) = (2 × Length × Current × Resistivity) ÷ Cross-Section
Where:
- Length = one-way cable length in metres (the formula multiplies by 2 for the return path)
- Current = amps drawn by the load
- Resistivity = 0.0175 Ω·mm²/m for copper
- Cross-Section = cable size in mm²
The 3% Rule
The widely accepted maximum voltage drop for a 12V system is 3%, which is 0.36V. For critical circuits (battery charging, inverter feeds), aim for under 1% (0.12V).
Example Calculation
A compressor fridge drawing 5A, cable run of 4 metres one-way, using 2.5mm² cable:
Voltage Drop = (2 × 4 × 5 × 0.0175) ÷ 2.5 = 0.28V
That is 0.28 ÷ 12 = 2.3% — acceptable.
Now try the same fridge with 1.5mm² cable:
Voltage Drop = (2 × 4 × 5 × 0.0175) ÷ 1.5 = 0.47V
That is 3.9% — too high. You would need to use 2.5mm² or thicker.
Always Use One-Way Length
The formula already accounts for the return path by multiplying by 2. Measure the one-way distance from your fuse box to the appliance — do not double it yourself.
Quick Reference: Common Cable Runs
Here are pre-calculated cable sizes for typical campervan circuits, assuming a maximum 3% voltage drop and typical cable lengths:
| Circuit | Current | Typical Length | Minimum Cable |
|---|---|---|---|
| LED ceiling lights | 2A | 3m | 1.0mm² |
| Reading light | 1A | 4m | 0.75mm² |
| USB sockets | 3A | 3m | 1.0mm² |
| Water pump | 5A | 5m | 2.5mm² |
| Compressor fridge | 5A | 3m | 1.5mm² |
| Diesel heater | 3A | 4m | 1.5mm² |
| Diesel heater glow plug | 10A | 4m | 4.0mm² |
| DC-DC charger (30A) | 30A | 2m | 6.0mm² |
| Solar MPPT to battery | 20A | 1.5m | 4.0mm² |
| Inverter (800W) | 80A | 1m | 25mm² |
| Inverter (2000W) | 190A | 0.5m | 50mm² |
| Battery interconnects | 200A+ | 0.3m | 50-70mm² |
Inverter Cables Are Critical
Inverter cables carry extremely high currents. A 2000W inverter draws approximately 190A at 12V. Undersized inverter cables will overheat and can cause fires. Always use the manufacturer's recommended cable size and keep the cable run as short as physically possible — ideally under 1 metre.
mm² vs AWG
The UK and Europe use mm² (cross-sectional area) for cable sizing. The US uses AWG (American Wire Gauge), where confusingly, a lower number means a thicker cable. If you are buying from US suppliers or reading American van build guides, here is the conversion:
| mm² | AWG | Approx Current |
|---|---|---|
| 0.75mm² | 18 AWG | 6A |
| 1.0mm² | 16 AWG | 10A |
| 1.5mm² | 14 AWG | 15A |
| 2.5mm² | 12 AWG | 21A |
| 4.0mm² | 10 AWG | 27A |
| 6.0mm² | 8 AWG | 37A |
| 10mm² | 6 AWG | 50A |
| 16mm² | 4 AWG | 70A |
| 25mm² | 2 AWG | 95A |
| 35mm² | 1 AWG | 120A |
Always buy cable rated for automotive or marine use. Standard domestic cable (twin-and-earth) is not suitable for 12V DC circuits in a vehicle because it uses solid-core conductors that fatigue and break under vibration.
For more on cable types and where to buy, see our cable sizes guide. For understanding how voltage drop impacts your system, see voltage drop explained.
FAQ
What if my calculated cable size falls between standard sizes?
Always round up to the next standard size. If your calculation says you need 3.5mm², use 4.0mm². There is no downside to using a slightly thicker cable — it just costs marginally more and gives you a safety margin.
Do I need to account for temperature?
The current ratings in this guide assume an ambient temperature of 30°C. If your cables run through areas that get very hot (near the engine, inside insulation against a sun-facing metal roof), derate the current capacity by 15-20%. In a UK campervan, this is rarely an issue for most cable runs.
Should I use the same cable size for positive and negative?
Yes. The positive and negative cables carry the same current. Both must be sized identically. Some builders use a thinner negative thinking it is just a "return" — this is wrong and dangerous.
Can I use household cable in a campervan?
No. Household cable (flat twin-and-earth) uses solid copper conductors. Solid conductors fatigue and snap under the vibration of a moving vehicle. Use flexible, stranded cable rated for automotive or marine use — often labelled as "tri-rated" or "auto cable" in the UK.