Van Electrical Wire & Fuse Sizing (Battery-to-Inverter) Explained
The battery-to-inverter cable is the highest-current wire in a van, and it is the one people most often get wrong. Undersize it and it runs hot; fuse it wrong and it will not protect anything. This guide explains the method the wire sizing tool uses, so you understand every number it gives you.
Start with continuous current
Work out the steady current the inverter draws from the battery:
current = inverter watts ÷ system volts ÷ 0.9. The 0.9 is inverter
efficiency. A 2,000 W inverter at 12V pulls
2000 ÷ 12 ÷ 0.9 ≈ 185 A continuously — a lot of current for a short,
thick cable to carry.
Size the fuse next
The fuse protects the wire from a short or overload. Size it just above the
continuous current with the standard 1.25 margin, then round to the next
standard fuse: fuse = ceil(current × 1.25). For our 185 A example,
that is ceil(185 × 1.25) = 232 A → 250 A. The golden rule: the fuse
must never be larger than the wire's ampacity, or the wire could melt before the
fuse blows.
Then size the wire two ways
The gauge must satisfy both of these, and you use the thicker result:
1. Ampacity (can it carry the fuse rating?)
Use the ABYC E-11 105°C ampacity table for cable outside the engine space. The wire's rated ampacity must be at least the fuse rating. For a 250 A fuse, the smallest compliant conductor is 1/0 AWG (rated 285 A). Never mix ampacity standards — the household NEC table and the ABYC marine table give different numbers, and this cable uses ABYC.
2. Voltage drop (will the inverter get enough volts?)
Long runs lose voltage, and at these currents even a few feet matters. Keep drop
under 3% using the round-trip length — the current flows down the positive and
back through the negative, so a 5 ft install is a 10 ft round trip. The circular
mils needed are (10.75 × current × round-trip feet) ÷ (volts × 0.03).
If that calls for a thicker wire than ampacity did, voltage drop wins.
Worked example
A 1,000 W inverter at 12V, 5 ft from the battery: current ≈ 92.6 A, fuse 125 A. Ampacity needs 4 AWG (160 A). Voltage drop over the 10 ft round trip also lands at 4 AWG. Both agree, so the answer is 4 AWG with a 125 A fuse. Double the run to 10 ft and voltage drop, not ampacity, would push you to a thicker gauge.
Common mistakes
- Using one-way length instead of round-trip for voltage drop — it doubles the required copper.
- Fusing larger than the wire can carry, defeating the protection.
- Mixing NEC and ABYC ampacity tables and getting an unsafe gauge.
- Forgetting a fuse within a few inches of the battery positive terminal.
Enter your inverter size and real cable length in the wire calculator and it applies both checks automatically, showing the ampacity gauge, the voltage-drop gauge, and the thicker one it chooses.