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 helps you choose the right cable size every time.
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: 0.0175 Ω·mm²/m, aluminium: 0.0278 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.
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).