What is different about a vehicle
The electrical maths does not change. A 12 V circuit in a van obeys the same NEC ampacity tables and the same voltage drop equation as a 12 V circuit in a cabin. What changes is everything around it.
Vibration. This is the big one. Solid conductors and coarse stranding work-harden under constant vibration until individual strands fracture — inside the insulation, where nothing is visible and the first symptom is a hot connection. Use finely stranded cable for everything, and welding-style or marine-grade cable for the battery runs.
Distance versus voltage. A van looks small until you route a cable. A fridge at the back, a charge controller under a bed and an inverter near the batteries can easily total 15 to 20 ft one way. At 12 V that is enough to force two or three sizes up.
Heat. Cables run behind panelling, above a fridge compressor, or through a sealed battery box in July. The NEC ampacity tables assume 30 °C. A sealed compartment in summer is not 30 °C, and the installation conditions panel is there for that.
Movement and abrasion. Every point where a cable passes through metal needs a grommet, and every run needs securing. Chafe is the most common cause of vehicle electrical fires.
Sizing the circuits in a typical build
| 12 V | 5 ft | 10 ft | 15 ft | 20 ft | 30 ft |
|---|---|---|---|---|---|
| 5 A | 14 AWG | 14 AWG | 12 AWG | 10 AWG | 8 AWG |
| 10 A | 14 AWG | 10 AWG | 8 AWG | 8 AWG | 6 AWG |
| 15 A | 12 AWG | 8 AWG | 8 AWG | 6 AWG | 4 AWG |
| 20 A | 10 AWG | 8 AWG | 6 AWG | 4 AWG | 3 AWG |
| 30 A | 8 AWG | 6 AWG | 4 AWG | 3 AWG | 2 AWG |
| 50 A | 6 AWG | 4 AWG | 2 AWG | 1 AWG | 2/0 AWG |
Roof array to charge controller. Often the longest run in the vehicle. Wire the panels in series where the controller allows it: series raises string voltage and leaves current alone, which makes this run dramatically cheaper. A two-panel series string at 40 V behaves like a 40 V circuit for voltage drop, not a 12 V one — enter the string voltage, not the battery voltage.
Controller to battery. Short, high current, and the one people get wrong. The controller senses battery voltage at its own terminals, so drop here makes it read high, end absorption early, and leave the bank chronically undercharged. Target 2 %.
Battery to inverter. The highest current in the vehicle. A 2000 W inverter on a 12 V bank draws around 185 A. Mount the inverter as close to the batteries as the layout allows — moving it three feet closer usually saves more than a cable size increase costs.
Branch circuits. Lights, fans, pump, fridge. Small currents, long runs, and where dimming shows up.
Cable types worth knowing
Welding cable is extremely flexible and cheap per amp, which is why it is everywhere in van builds. It is generally not listed as a building wire, so it can be rejected in a permitted installation, but in a vehicle that is usually not the governing standard. It does not change the physics — size it identically.
Marine-grade tinned copper costs more and resists corrosion far better in humid or salt environments. In a van that sees coastal weather or condensation, it is money well spent.
Duplex and triplex cable bundles conductors in a single jacket, which is neater and reduces chafe points. Remember that bundled current-carrying conductors count toward the NEC 310.15(C)(1) fill adjustment once there are four or more.
Where to go next
Sizing individual circuits is the last step of a van electrical design, not the first. Work out your daily consumption, then the bank, then the array, then the inverter — each one produces the current figure the next calculator needs.