How does the cable calculator work?
The cable calculator determines the minimum cable cross-section for your 12V or 24V installation. A cable that is too thin causes voltage drop, heat build-up and, in the worst case, fire. This tool gives a well-founded minimum cable size based on standard values; check it against the specifications of your cable and devices.
The formula
The minimum cross-section is calculated as: cross-section (mm²) = (2 × ρ × L × I) / Vdrop. Here ρ is the resistivity of the material (copper: 0.0179 Ω·mm²/m, aluminium: 0.0294 Ω·mm²/m), L is the cable length in metres, I is the current in amps and Vdrop is the maximum allowable voltage drop in volts. The factor 2 is needed because the current flows through the cable both ways — to the appliance and back.
Rounding to standard sizes
The calculated cross-section is rounded up to the nearest standard cable size (e.g. 1.5 / 2.5 / 4 / 6 / 10 / 16 / 25 mm²). The comparison table shows the actual voltage drop for each standard size so you can compare at a glance.
Copper versus aluminium
Copper conducts better than aluminium and therefore requires a smaller cross-section at the same current. Aluminium is lighter and cheaper but needs a thicker cable. In camper van builds, copper is used almost exclusively because of its better conductivity and tighter bending radius.
Sources and standards
The calculation uses the resistivity of copper (0.0179 Ω·mm²/m at 20°C) and aluminium (0.0294 Ω·mm²/m). These are the reciprocals of the conductivity values used in practice: κ = 56 for copper and κ = 34 for aluminium. They sit slightly above the resistivity of a pure conductor (IEC 60228 works out at roughly 0.01724 Ω·mm²/m for copper), and that is deliberate: it adds a small margin, and anyone working the formula out by hand arrives at the same cross-section the calculator recommends. Current-carrying capacity per cross-section and the temperature correction follow IEC 60364-5-52 (cable installation and current-carrying capacity), table B.52.14, assuming XLPE or EPR insulation as used in battery and marine cable. For 230V circuits the tool applies a 2.5 mm² floor as its own safety margin. These standards were written for fixed low-voltage installations; for a camper we use them as a guideline.
Frequently asked questions
Why is there a factor 2 in the formula?
The current flows from the battery to the appliance (outbound) and back again through the negative wire (return). Both conductors cause voltage drop, so the total cable length is twice the one-way distance.
What percentage of voltage drop is acceptable?
For 12V systems, 3% is a common guideline (0.36 V). For longer runs or sensitive equipment (such as LED lighting), 1–2% is preferable. In 24V systems, 3% is less critical because the absolute loss is smaller relative to the supply voltage.
When should I choose aluminium instead of copper?
Aluminium is rarely used in camper van builds. It is lighter and cheaper, but its resistivity is about 60% higher, so you need a significantly thicker cable. Aluminium also corrodes more easily at copper contact points, which can cause transition resistance.
What is the difference between mm² and AWG?
mm² is the European standard for cable cross-section, AWG (American Wire Gauge) is the American standard. Thinner cables have a higher AWG number. For example: 2.5 mm² ≈ 14 AWG, 4 mm² ≈ 12 AWG.
Why does temperature matter for cable calculations?
At higher temperatures, copper resistance increases by about 0.4% per degree. In an engine bay (60°C), resistance is ~16% higher than at 20°C, meaning you need a thicker cable.
Should I enter the one-way or round-trip cable length?
Enter the one-way length (from source to device). The calculator automatically doubles this because current flows through two conductors (positive and negative).
What cable size do I need for a 12V camper van circuit?
It depends on current, cable length and how much voltage drop you accept. With copper at 20°C and a 3% limit, a 5A LED circuit over 4m one way lands on 2.5mm², a 6A compressor fridge over 5m on 4mm², and a 20A circuit over 5m on 10mm². Above roughly 30A the cable's current carrying capacity takes over from voltage drop: a 100A inverter feed at 12V needs 35mm² even at 1.5m. Enter your own numbers above instead of copying a chart, because length and temperature change the answer.
Does this calculator work for 24V and 48V systems?
Yes. A higher system voltage means less current for the same power, so the cable gets thinner. The same 20A over 5m needs 10mm² at 12V but 6mm² at 24V. Select your system voltage at the top and the calculation adjusts, including the voltage drop limit and the current capacity check.
Wire size, conductor size, cross-section: are these the same thing?
Yes, they all describe the same measurement. Cable size, wire size, cross-section and conductor size all refer to the area of the copper core, expressed in square millimetres (mm²). Wire diameter is the odd one out: that is the width of the strand in millimetres, not its area. For 12V and 24V systems you always work in mm², because resistance depends on the area and not on the diameter. American tables use AWG (American Wire Gauge) instead, where a lower number means a thicker cable.