Undersized wire is one of the few van build mistakes that's genuinely dangerous, not just expensive. Too-thin wire on a high-draw circuit heats up, which is a fire risk, and it also causes voltage drop that makes your appliances underperform or trip out. Here's how to size it correctly instead of guessing.
The Two Numbers That Determine Wire Gauge
Wire gauge depends on exactly two things: amp draw and wire run length (there and back — round trip, not one-way).
Amp Draw = Watts ÷ Volts
A 1,000W inverter on a 12V system draws roughly 83 amps continuous — before accounting for surge loads, which can spike 2–3x higher momentarily.
Quick Reference Wire Gauge Chart (12V, 3% Voltage Drop)
| Amps | Up to 5 ft | 10 ft | 15 ft | 20 ft | 25 ft |
|---|---|---|---|---|---|
| 10A | 16 AWG | 14 AWG | 14 AWG | 12 AWG | 12 AWG |
| 20A | 12 AWG | 10 AWG | 10 AWG | 8 AWG | 8 AWG |
| 40A | 8 AWG | 6 AWG | 6 AWG | 4 AWG | 4 AWG |
| 80A | 4 AWG | 2 AWG | 2 AWG | 1/0 AWG | 1/0 AWG |
| 100A | 2 AWG | 1/0 AWG | 1/0 AWG | 2/0 AWG | 2/0 AWG |
Remember: lower AWG number = thicker wire. This chart targets a 3% voltage drop, which is the accepted standard for 12V systems — go higher than that and you'll notice dimmer lights and underperforming appliances even on wire that's technically "rated" for the amperage.
Why Wire Run Length Matters as Much as Amps
A 40A circuit run 3 feet from battery to fuse block needs a much smaller wire than the same 40A circuit run 20 feet to a rear cabinet. Voltage drop compounds with distance, which is why generic advice like "use 8 AWG for your inverter" is often wrong — it depends entirely on your specific van's layout.
Always measure the full round-trip distance: positive wire out, negative wire back. A run that's 8 feet from the battery to an appliance and 8 feet back is a 16-foot run for sizing purposes, not 8.
Common Undersizing Mistakes
- Sizing to average draw instead of continuous/surge draw. Inverters and compressor fridges have startup surges 2–3x their running wattage — size for what the circuit needs to handle, not its average.
- Ignoring ambient heat. Wire bundled tightly in a hot engine bay or enclosed panel derates its ampacity — go one size up if wire runs through high-heat areas.
- Using automotive-rated wire for high-draw circuits without checking insulation type. Not all 12V-rated wire is created equal; marine-grade tinned copper wire holds up far better in a van's vibration and moisture environment than standard automotive wire.
Fuse Sizing Follows Wire Gauge, Not the Appliance
Your fuse or breaker should protect the wire, not just the appliance. Size the fuse to the wire's safe ampacity rating, not to what the device draws — this is what actually prevents a wiring fire if something shorts.
Frequently Asked Questions
What gauge wire do I need for a 2000W inverter? A 2000W inverter on 12V draws roughly 166A continuous. At most reasonable run lengths (under 10 ft), that requires 1/0 to 2/0 AWG wire — always confirm against a voltage-drop chart for your exact distance.
Can I use household wire (Romex) in a van? No — household wire isn't rated for the vibration, flexing, and moisture exposure of a vehicle. Use marine-grade or automotive-rated stranded wire.
Does wire gauge matter for low-draw circuits like lighting? Yes, but less critically — most LED lighting circuits are safely covered by 16-18 AWG wire even on longer runs, since amp draw is low.