Free 12V Cable Size Calculator
Use this tool to estimate the minimum DC cable cross-section for a 12-volt system based on current, cable length, and allowed voltage drop.
How this 12 volt wire size calculator works
In low-voltage systems, cable sizing is critical. At 12V, even a small voltage drop can cause dim lights, slower motors, or electronics that shut down unexpectedly. This calculator estimates cable size by limiting voltage drop to a value you choose (commonly 3% for sensitive devices and 10% for non-critical loads).
The result includes a recommended cable cross-section in mm² and an approximate AWG equivalent. It also estimates the actual voltage drop and power loss for the selected size.
Why cable sizing matters in 12V systems
- Performance: Loads receive closer to their intended voltage.
- Efficiency: Lower cable resistance means less wasted power as heat.
- Reliability: Prevents under-voltage faults in inverters, fridges, pumps, and radios.
- Safety: Undersized cables can overheat, especially in enclosed spaces.
Formula used
The calculator uses the DC voltage-drop relation for a two-wire circuit:
- ρ = conductor resistivity in Ω·mm²/m (Copper ≈ 0.0175, Aluminum ≈ 0.0282)
- Length = one-way cable run in meters
- Current = load current in amps
- Allowed Voltage Drop = 12V × (drop % / 100)
Step-by-step: using the calculator
1) Enter current draw
Use the expected continuous current, not just startup surge. If your device draws 18A normally and 30A briefly at startup, base cable sizing on the continuous load plus margin.
2) Enter one-way length
Measure from power source to load in one direction. The calculator automatically doubles it for return path resistance.
3) Choose voltage drop target
For electronics, 2-3% is common. For less sensitive loads (for example, heating elements), higher values may be acceptable.
4) Pick conductor material
Copper has lower resistance than aluminum, so it usually allows a smaller cross-section for the same performance.
Typical voltage drop targets in 12V applications
| Application | Suggested Max Drop | Reason |
|---|---|---|
| LED lighting, electronics, radios | 2-3% | Stable voltage helps avoid flicker and resets |
| Fridges, pumps, DC appliances | 3-5% | Good balance between cost and performance |
| Non-sensitive resistive loads | 5-10% | Can tolerate larger drop, but less efficient |
Practical sizing tips
- Add design margin: If results are close to a cable boundary, choose the next size up.
- Check ampacity: Voltage drop is not the only limit. Verify current-carrying capacity for your insulation type and installation method.
- Account for heat: High ambient temperature and cable bundling reduce ampacity.
- Fuse correctly: Protect conductors with appropriately rated fuses or breakers near the source.
- Keep runs short: Shorter cables reduce voltage drop and copper cost.
AWG and mm² quick guide (approximate)
| mm² | Approx AWG |
|---|---|
| 1.5 | 15 AWG |
| 2.5 | 13 AWG |
| 4 | 11 AWG |
| 6 | 9 AWG |
| 10 | 7 AWG |
| 16 | 5 AWG |
| 25 | 3 AWG |
| 35 | 2 AWG |
| 50 | 1/0 AWG |
Important note
This calculator provides an engineering estimate for voltage drop sizing. Final cable selection should also consider local electrical codes, insulation temperature rating, installation environment, strand class, connector quality, and maximum fault current.