REFERENCE 24-30 · ELECTRICAL POWER
12V DC Wire Ampacity & Voltage-Drop Chart
In low-voltage DC systems, voltage drop — not fire-safety ampacity — is usually what actually governs wire size. This chart covers both. Applies to vans, truck campers, trailers, fifth wheels, motorhomes, and skoolies alike; the physics don’t change with the chassis.
Two Rules, Not One
1. Ampacity (safety): the wire must carry the circuit’s current without overheating. This is what the fuse/breaker protects — size the fuse to the wire, not the load. A 10 AWG wire rated for 30A gets a fuse of 30A or less, regardless of what’s actually plugged in.
2. Voltage drop (performance): even a wire that’s technically “big enough” for the amperage can drop so much voltage over distance that chargers misbehave, lights dim, and inverters fault. Per ABYC E-11, size critical circuits (battery charging, inverter feeds, main panel runs) for 3% max voltage drop, and non-critical circuits (interior lighting, accessories, fans) for up to 10%. Always calculate on the full round-trip (loop) length — both the positive and negative legs.
Ampacity by Wire Gauge (Free Air, Copper)
| AWG | Ampacity (approx.) | Typical Use |
|---|---|---|
| 14 AWG | 15A | Lighting, small accessories |
| 12 AWG | 20A | Water pump, fans, USB/12V outlets |
| 10 AWG | 30A | Furnace, larger accessory circuits |
| 8 AWG | 40–55A | Converter feeds, small inverter runs |
| 6 AWG | 55–75A | Mid-size inverter, DC-DC charger runs |
| 4 AWG | 70–95A | 1000–1500W inverter feeds |
| 2 AWG | 95–130A | 2000W+ inverter feeds |
| 1/0 – 4/0 AWG | 150–260A | Battery bank interconnects, 3000W+ inverters |
Ampacity figures are free-air baseline values consistent with NEC Table 310.16 practice, adapted for RV low-voltage wiring. Derate roughly 30% for bundled conductors or conduit runs, and roughly 15% for high ambient temperatures (≥50°C / 122°F) — both common conditions inside a wall chase or engine bay.
Minimum Wire Gauge by Distance & Load (3% Drop — Critical Circuits)
Round-trip (loop) length in feet. Use this table for battery charging, inverter feeds, and main panel runs.
| Load | 10 ft | 20 ft | 30 ft | 40 ft |
|---|---|---|---|---|
| 10A | 14 AWG | 12 AWG | 10 AWG | 10 AWG |
| 20A | 12 AWG | 10 AWG | 8 AWG | 8 AWG |
| 50A | 8 AWG | 6 AWG | 4 AWG | 2 AWG |
| 100A | 4 AWG | 2 AWG | 1/0 AWG | 2/0 AWG |
| 150A | 2 AWG | 1/0 AWG | 3/0 AWG | 4/0 AWG |
These are conservative starting points for planning, not a substitute for a proper voltage-drop calculation on your specific run — ambient temp, bundling, and connector quality all matter. For non-critical circuits (interior lighting, small accessories) up to 10% drop is acceptable per ABYC E-11, which allows one size smaller in many cases.
The Fuse Protects the Wire, Not the Device
A common and dangerous mistake: sizing the fuse to the appliance’s rated draw instead of the wire’s ampacity. If you run 10 AWG wire (30A rated) to a device that only draws 8A, the fuse still needs to protect the wire from a fault condition — so it should be sized at or below 30A, not oversized to “cover” a bigger future load. If you want to run a bigger load later, upsize the wire first, then the fuse.
Related Reference Pages
Battery Charging Voltage Chart
Electrical & Power Systems
Full Field Guide
⚠ Safety note: This page is for informational and planning purposes only and is not a substitute for the advice of a qualified professional. Electrical work carries fire and shock risk. Values are conservative planning figures per ABYC E-11 practice — always verify against your specific component and wire manufacturer’s current specifications, and have work inspected by a certified technician. United Mobile RV LLC is not responsible for outcomes of DIY electrical work performed using this reference. Rev. 1 — August 2026.
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