Size RV battery cable from the complete circuit's current, length and installation conditions, then coordinate its fuse and terminals.
Battery cable sizing is the most consequential wiring decision in a van build. Undersized cables get hot, lose voltage, and can cause fires. Here's how to size them correctly.
The two factors: ampacity and voltage drop
Ampacity is the maximum current a wire can carry without overheating. AWG (American Wire Gauge) ratings reflect this.
Voltage drop is how much voltage is lost across the cable at a given current. At 12V, even a 0.5V drop is a significant percentage of system voltage. Long runs need thicker wire even if the ampacity rating would allow thinner.
Main battery cable (battery to positive bus bar)
This cable carries the sum of all charging inputs and load outputs — your total system current.
Step 1: Identify your peak current. The main contributors:
- 2,000W inverter at full load: ~185A at 12V
- 3,000W inverter at full load: ~275A at 12V
- DC-DC charger (60A): 60A
- MPPT solar (50A): 50A
For most van builds with a 2,000W inverter: peak current ~185–220A (inverter dominates; other sources add current, other loads reduce battery draw).
Step 2: Select AWG based on ampacity + run length:
| AWG | Ampacity (60°C) | Max run at 2% drop / 150A |
|---|---|---|
| 4 AWG | 85A | ~1.5 ft |
| 2 AWG | 115A | ~2.5 ft |
| 1/0 AWG | 150A | ~4 ft |
| 2/0 AWG | 175A | ~5 ft |
| 4/0 AWG | 230A | ~8 ft |
For a 2,000W inverter with a 3-foot main cable run (battery to bus bar): 2/0 AWG is the standard choice.
Inverter cable
The cable from battery positive (or positive bus bar) to the inverter typically carries the highest current in the system. Run it as short as physically possible — mount the inverter close to the battery.
| Inverter size | Cable AWG | Max run length |
|---|---|---|
| 1,000W | 1/0 AWG | 4 ft |
| 2,000W | 2/0 AWG | 4 ft |
| 2,000W | 4/0 AWG | 8 ft |
| 3,000W | 4/0 AWG | 4 ft |
Always use the AWG specified by your inverter manufacturer. Most quality inverters include cable size recommendations in their documentation.
DC-DC charger cable
A 40A DC-DC charger draws 40A from the alternator (starter battery side) and pushes 40–60A to the house battery side. Both cables need appropriate sizing:
- Alternator/starter side: 8 AWG handles 40A continuous with some margin
- House battery side: 8 AWG for 40A, 6 AWG for 60A
Both cables need fuses within 18" of each battery terminal.
MPPT solar controller cables
- PV input cable (panels to MPPT): 10 AWG is standard — handles 30A PV current across typical roof-to-controller runs
- Battery output cable (MPPT to bus bar): 10 AWG for 30A controllers; 8 AWG for 50A controllers
Parallel battery interconnects
If you wire two batteries in parallel (two 100Ah batteries for 200Ah total), the interconnect cable should be as short as possible and sized for the maximum current that could flow between them — typically 2/0 AWG for lithium builds.
A note on welding cable vs battery cable
Both work. Welding cable (SAE J1127) is more flexible, easier to route, and available cheaply at welding supply stores. Marine-rated battery cable (ABYC A-30) is appropriate for RV applications and often used in marine/RV supply stores. Avoid automotive primary wire for high-current battery cables — it's rated for 60°C continuous use, which is marginal for high-current van applications.
Worked planning check
At an illustrative 150A, 0.003 ohms total resistance produces 0.45V drop and 67.5W of heat. Improving one cable while leaving a poor terminal can still leave significant loss. Record cable and connection requirements together in the build schedule.
The figures or dimensions in this example are explicit planning assumptions, not a product test or an installation specification. Replace them with your equipment data and actual measurements.
Planning questions
Can I choose a fuse just from the appliance current?
No. Check cable ampacity under the installed conditions, equipment instructions, terminal limits, DC voltage and interrupt capacity. Protection must coordinate with the complete circuit.
Does a wiring diagram prove the installation is safe?
No. It is a planning record. Actual routing, termination, protection and commissioning results must agree with the design and applicable requirements.
When should I involve an electrician?
Before work that exceeds your competence, particularly mains design, source transfer, bonding, high-current faults and final inspection or testing. Agree the scope and evidence of verification in advance.
Related
- Wiring & safety guide
- AWG wire gauge guide for 12V circuits
- How to crimp battery cable lugs
- RV fuse sizing guide
Use the free electrical planner to organise your daily loads and charging assumptions.
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Jack Kennedy is a self-builder who has built two vans. He created Roam Wired to simplify conversion planning and share free tools, supported by affiliate links.
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