How does the cable calculator work?
The cable calculator determines the minimum wire gauge 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, aluminum: 0.0278 Ω·mm²/m), L is the cable length in meters, 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 aluminum
Copper conducts better than aluminum and therefore requires a smaller cross-section at the same current. Aluminum is lighter and cheaper but needs a thicker cable. In RV builds, copper is used almost exclusively because of its better conductivity and tighter bending radius.
Sources and standards
The calculation is based on the resistance formula from IEC 60228 (conductor specifications) and IEC 60364-5-52 (cable installation and current-carrying capacity). For fixed AC installations, the NEC (National Electrical Code) in the US prescribes minimum wire sizes per Table 310.16; in Europe, NEN 1010 prescribes a minimum of 14 AWG (2.5 mm²). The specific resistance of copper (0.0175 Ω·mm²/m at 68°F / 20°C) and aluminum (0.0278 Ω·mm²/m) are standard values from IEC 60228. Temperature correction per IEC 60364-5-52 table B.52.1. For RV installations in the US, ABYC E-11 (AC and DC Electrical Systems on Boats) and NEC Article 551 (Recreational Vehicles) apply. AWG (American Wire Gauge) is the US standard for wire sizing.
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 aluminum instead of copper?
Aluminum is rarely used in RV builds. It is lighter and cheaper, but its resistivity is about 60% higher, so you need a significantly thicker cable. Aluminum 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 wire gauge, 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).
Do I need GFCI protection in my RV?
Yes. NEC Article 551 requires GFCI protection for all 120V receptacles in RVs. Use GFCI breakers or GFCI outlets in wet areas (kitchen, bathroom). This protects against ground faults that could cause electrostatic shock.