VANPOWERCALC

12V/24V Wire Size Calculator

Wire Size Calculator

Prevent fires in your van: calculate the required wire cross-section in mm² based on amperage and distance.

Enter the one-way distance. The formula doubles it internally (S = 2ρLI/ΔU), so the drop% shown is the total end-to-end loss across both conductors (ABYC convention). Chassis-return wiring: the formula assumes two copper conductors. If you run negative through the frame, this oversizes the cable (safe direction). ABYC E-11 §11.5.2.2 prohibits chassis as a return conductor on marine; SAE J1127 allows it on automotive.

Percentage is computed against nominal system voltage (12/24/48 V), the conservative ABYC convention. Using charging voltage (14.4 V) would understate the required cable size — the unsafe direction.

ABYC = US standard for mobile installs (marine-grade 105°C wire). NEC 60°C = conservative building-wire. IEC = Europe.

Ampacity per ambient temperature around the cable. At 50°C (cabin parked in direct sun, or engine space), capacity drops ~15% for 105°C marine wire. Above 50°C, ABYC doesn't publish data — consult a marine electrician. Ref: ABYC E-11, Inside Engine Spaces column.
Recommended Standard
0 mm²
Recommended fuse
—

Always protect the wire with an appropriate fuse!

Based on Copper at 20°C (ρ = 0.0175 Ω·mm²/m, IEC 60228 convention). Temperature derating applied per selected standard (IEC +15%, NEC +25%, ABYC none — 'outside engine spaces' values used as-is) compensates for elevated operating-temperature resistance.

DC / Single-phase AC formula — not applicable to three-phase systems.

The complete wiring kit

To do it right: correctly sized cable, fuses, terminals, heat-shrink, and a multimeter to verify. Everything you need to wire safely without overheating.

Advertising

Independent and free thanks to your clicks

Links marked with * are affiliate links. If a purchase is made through them, I receive a commission at no extra cost to you. The editorial selection and product evaluation are not influenced by commission rates. Your click helps fund this free tool.

Battery Cable 50mm² Red+Black

50mm² cable for 2000W+ inverter. Red and black, 1m each.

$33 on Amazon*

Solar Cable 6mm² MC4 10m

6mm² solar cable with pre-crimped MC4 connectors, 10m.

$38 on Amazon*

ANL Fuse 250A + Fuse Holder

Mandatory inverter protection. 250A with integrated fuse holder.

$13 on Amazon*

12V Fuse Box

Protects each circuit independently, easy installation.

$41 on Amazon*

Electrical Crimping Tool

Reliable crimp connections on 0.5mm²–16mm² cables, no soldering.

$25 on Amazon*

Heat Shrink Tubing Kit

Insulate and protect electrical connections.

$6 on Amazon*
BLUETTI Digital Multimeter

BLUETTI Digital Multimeter

Clamp multimeter for troubleshooting 12V/24V DC circuits — measures voltage, current, resistance. Fits any vanlife toolbox.

£20 on BLUETTI*

Why calculate wire size?

In DC current (12V, 24V), the current (Amps) is very high for a given power. If the wire is too thin or too long, it will heat up (Joule effect) and cause a voltage drop. This can damage your devices, or worse, cause a fire. You should aim for a maximum voltage drop of 3% for the general circuit, and 1% for critical charging circuits (solar panels, chargers).

Wiring & fuses guide →DC-DC →← Back to Main Power Audit