12V Wire Size Calculator
Size DC wire for RV, van, boat, trailer and solar circuits. Enter amps or watts and the one-way length to get the right AWG and fuse size.
Quick examples
Example values are shown. Change any field to calculate your own circuit.
Result
Enter positive numbers to see a result.
on a breaker or fuse
Ampacity Sets the size
needed, rated
Voltage drop Sets the size
of the allowed
- Voltage drop
- of
- Voltage at the load
- Power lost in the wire
More details
- Cross-section
- Nearest metric size
- Load current used
Show the math
Based on NEC Table 310.16 ampacities and Chapter 9 Table 8 resistance. No temperature or bundling derating. Check with a licensed electrician. How this is calculated
On this page
Why 12 V wiring needs thicker wire
At 12 V, a 3% drop is only 0.36 V. The same current that is trivial on a 240 V circuit therefore needs much heavier wire, and short extra lengths matter. Doubling the run length doubles the voltage drop, and doubling the current does the same.
This calculator is set up for low-voltage DC work: RVs and campervans, boats, trailers, off-grid cabins and solar. It defaults to 12 V, a 3% drop target and room-temperature wire, and it accepts watts or amps.
How to use it
- Pick a preset such as fridge, water pump or solar panel to controller, or type your own amps or watts.
- Enter the one-way length between the battery or source and the device. The return conductor is included automatically.
- Choose your drop target. Use 1–2% for charging circuits and sensitive electronics; 3% for most other loads.
- Read the recommended AWG, the voltage at the load, and the suggested fuse size.
Maximum one-way length by wire size at 12 V
This table is generated by the same calculation engine. It shows the longest one-way run, in feet, that stays at or below a 3% drop (0.36 V) with copper wire at 20 °C.
| Wire size | 5 A | 10 A | 20 A | 30 A | 50 A |
|---|---|---|---|---|---|
| 14 AWG | 14.0 ft [4.3 m] | 7.0 ft [2.1 m] | 3.5 ft [1.1 m] | 2.3 ft [0.7 m] | 1.4 ft [0.4 m] |
| 12 AWG | 22.2 ft [6.8 m] | 11.1 ft [3.4 m] | 5.6 ft [1.7 m] | 3.7 ft [1.1 m] | 2.2 ft [0.7 m] |
| 10 AWG | 35.3 ft [10.8 m] | 17.7 ft [5.4 m] | 8.8 ft [2.7 m] | 5.9 ft [1.8 m] | 3.5 ft [1.1 m] |
| 8 AWG | 56.2 ft [17.1 m] | 28.1 ft [8.6 m] | 14.0 ft [4.3 m] | 9.4 ft [2.9 m] | 5.6 ft [1.7 m] |
| 6 AWG | 89.3 ft [27.2 m] | 44.7 ft [13.6 m] | 22.3 ft [6.8 m] | 14.9 ft [4.5 m] | 8.9 ft [2.7 m] |
| 4 AWG | 142.1 ft [43.3 m] | 71.0 ft [21.6 m] | 35.5 ft [10.8 m] | 23.7 ft [7.2 m] | 14.2 ft [4.3 m] |
| 2 AWG | 225.9 ft [68.8 m] | 112.9 ft [34.4 m] | 56.5 ft [17.2 m] | 37.6 ft [11.5 m] | 22.6 ft [6.9 m] |
| 1/0 AWG | 359.2 ft [109.5 m] | 179.6 ft [54.7 m] | 89.8 ft [27.4 m] | 59.9 ft [18.2 m] | 35.9 ft [10.9 m] |
| 2/0 AWG | 452.9 ft [138.0 m] | 226.4 ft [69.0 m] | 113.2 ft [34.5 m] | 75.5 ft [23.0 m] | 45.3 ft [13.8 m] |
| 4/0 AWG | 720.1 ft [219.5 m] | 360.1 ft [109.7 m] | 180.0 ft [54.9 m] | 120.0 ft [36.6 m] | 72.0 ft [21.9 m] |
Fuse and safety notes
- Fuse or breaker the circuit close to the source (the battery or bus bar), sized to protect the wire, not just the load.
- Never choose a wire whose ampacity is below the fuse rating. The calculator warns you when the suggested fuse exceeds the wire's ampacity.
- Marine, automotive and RV standards have their own ampacity tables and colour codes. This tool uses NEC building-wire ampacity as a conservative guide, so check the standard that applies to your vehicle or vessel.
- For high-current runs, check that your connectors, lugs and terminals are rated for the current as well.
Assumptions and limits
- Copper or aluminum, stranded, resistance corrected to the chosen wire temperature. Switch to the hot 75 °C setting for conductors in warm engine bays or tightly bundled runs.
- No derating for bundling or high ambient temperature is applied automatically.
- Not a substitute for a qualified installer. See the disclaimer.
Need 120/240 V circuits, or to check the voltage drop of a wire you already have? See the wire size calculator and the voltage drop calculator.