Wire Gauge & Voltage Drop Calculator
Choose the right cable section (mm² or AWG) for a DC or AC run from the current, the one-way length and the maximum acceptable voltage drop — with resistance, power loss, ampacity and a suggested fuse.
Loading tool…
Two limits: voltage drop and ampacity
A cable must satisfy two independent conditions. First, it must carry the current without overheating (ampacity). Second, it must not drop so much voltage that the device at the end misbehaves (voltage drop). A cable can pass one test and fail the other, which is why both are shown here.
- Ampacity: how much current the conductor can carry safely
- Voltage drop: how much voltage is lost along the run
- Undersized cable = heat, melting insulation, fire risk
- Over-long runs need a bigger section even at low current
The voltage drop formula
For a two-wire DC circuit the drop is Vd = 2 × L × I × ρ ÷ A, where L is the one-way length in metres, I the current in amps, ρ the conductor resistivity (copper ≈ 0.0172 Ω·mm²/m at 20 °C) and A the section in mm². The factor 2 accounts for the supply and the return conductor. When the return goes through the chassis, the vehicle body has a very low resistance and only the supply length counts (factor 1).
- Copper ρ ≈ 0.0172 Ω·mm²/m at 20 °C
- Aluminium ρ ≈ 0.0283 Ω·mm²/m — about 60% more
- Resistance rises with temperature (≈ +0.4%/°C)
- Drop % = Vd ÷ system voltage × 100
How much drop is acceptable
3% is the usual target for sensitive electronics, lighting and audio. Simple loads such as a fan or a pump often tolerate 5%, and non-critical loads can accept up to 10%. On a 12 V system, 3% is only 0.36 V — that is why long 12 V runs quickly need large cable.
- 3% — electronics, lighting, audio
- 5% — pumps, fans, general loads
- 10% — non-critical, short-term loads
- 12 V systems are far more sensitive than 230 V because the drop is a bigger share of the total