Quick answer
Voltage drop = 2 × length × current × resistivity ÷ cable area (DC and single-phase). Keep it under 3% for 12 V and solar systems, and under 3-5% for mains circuits.
How to use the Voltage Drop Calculator
- Choose DC, single-phase AC or three-phase AC.
- Enter the supply voltage and the current the load draws.
- Enter the one-way cable length from the power source to the load.
- Select the wire size in mm² or AWG and the conductor material.
- Press Calculate. If the drop is too high, use the recommended size.
Example
A 12 V fridge in a camper van draws 5 A through 6 m of 1.5 mm² copper cable:
Vdrop = 2 × 6 m × 5 A × 0.0172 Ω·mm²/m ÷ 1.5 mm² ≈ 0.69 V at 20 °C, which is 5.7%. The fridge sees only about 11.3 V and may cut out. Switching to 4 mm² cable brings the drop down to about 2%.
Low-voltage DC systems are much more sensitive to voltage drop than 230 V circuits, because the same voltage loss is a bigger share of the supply.
How it works
Every cable has resistance, and current flowing through it causes a voltage loss. The calculator uses:
- DC and single-phase AC:
Vdrop = 2 × L × I × ρ ÷ A(the current flows out and back) - Three-phase AC:
Vdrop = √3 × L × I × ρ ÷ A
where L is the one-way length in meters, I is the current, A is the cross-section in mm², and ρ is the resistivity: 0.0172 Ω·mm²/m for copper and 0.0282 Ω·mm²/m for aluminum at 20 °C. Resistance rises with temperature, so the calculator adjusts ρ for the conductor temperature you choose.
Common limits: many wiring rules recommend no more than 3% for lighting and 5% for other circuits, measured from the supply origin. For 12 V and 24 V systems, 3% or less is a good target. This calculation ignores cable reactance, which is accurate for small and medium cables at power factors close to 1.
Cable sizing tips for 12 V, solar and camper setups
- Low voltage needs thick cable: 5 A over 6 m loses about 5.7% with 1.5 mm² at 12 V, but only about 0.3% at 230 V.
- Measure the real route: cables rarely run in a straight line. Add 10-20% to your estimated length.
- Solar panel to charge controller: wiring panels in series raises the voltage and lowers the current, which allows thinner cable for the same loss.
- Inverters draw big currents: a 1,000 W inverter on 12 V can pull over 90 A. Keep battery-to-inverter cables short and thick, often 25-35 mm².
- Fuse for the cable: the fuse protects the wire, so choose it to suit the cable's current rating, not just the device.
Frequently asked questions
What is an acceptable voltage drop?
A common guideline is 3% for lighting circuits and 5% for other circuits, but the exact limit depends on your country's wiring rules. For battery and solar systems, aim for 2-3%.
Why does voltage drop matter?
Devices may run poorly or shut down if the voltage is too low, and the lost voltage turns into heat in the cable, wasting energy. In solar systems, voltage drop directly reduces the power that reaches the batteries.
Should I enter the one-way or round-trip length?
Enter the one-way distance from the source to the load. The calculator already accounts for the return conductor.
Does this calculator check if the cable can carry the current?
No. Voltage drop and current-carrying capacity (ampacity) are separate checks. A cable must meet both, and its ampacity depends on installation method and local regulations.
How do I reduce voltage drop?
Use a thicker cable, shorten the cable run, or use a higher system voltage. Doubling the voltage for the same power halves the current and cuts the percentage drop to a quarter.