Wire Voltage Drop Calculator
Voltage drop is the loss of voltage as current flows through the resistance of a wire run. Too much drop starves the load, dims lights, and wastes energy as heat in the cable. The drop depends on the conductor material, the run length, the cross-sectional area, and the current. Enter the conductor resistivity, the one-way run length, the cross-sectional area, the current, and the source voltage; this calculator returns the round-trip wire resistance, the voltage drop in volts, the drop as a percentage, and the power lost in the wire.
Voltage drop formula
Area (m^2) = area (mm^2) / 1,000,000
Resistance = resistivity * (2 * length) / area
Voltage drop = current * resistance
Voltage drop % = voltage drop / source voltage * 100
Power lost = current^2 * resistance
Resistivity is in ohm metres, length in metres, area in square millimetres (converted to square metres), and current in amperes. The factor of two accounts for the supply and return conductors of the run.
Voltage drop context
- Copper resistivity is about 1.68 x 10^-8 ohm metres; aluminium about 2.65 x 10^-8 ohm metres at 20 degrees Celsius.
- The US National Electrical Code recommends roughly 3 percent maximum drop on a branch circuit and 5 percent total.
- Doubling the conductor area halves the resistance and the voltage drop for the same current.
- Resistivity rises with temperature, so a hot conductor drops slightly more voltage than a cold one.
- This DC model omits AC reactance and skin effect, which add to drop at higher frequencies.
Voltage drop: frequently asked questions
How is voltage drop along a wire calculated?
Voltage drop equals the current times the round-trip wire resistance. The wire resistance is resistivity times the conductor length divided by its cross-sectional area, doubled to account for both the supply and return conductors over a one-way run length.
What resistivity should I use?
Annealed copper at 20 degrees Celsius has a resistivity of about 1.68 x 10^-8 ohm metres, and aluminium about 2.65 x 10^-8 ohm metres. Resistivity is a user-editable input so you can match your conductor material and temperature.
Why is the length doubled?
Current must travel out to the load and back along a return conductor, so the total conductor path is twice the one-way run length. This calculator doubles the entered one-way length automatically for a standard two-wire DC circuit.
What is an acceptable voltage drop?
Guidance commonly cited from the US National Electrical Code is a recommended maximum of about 3 percent on a branch circuit and 5 percent total including the feeder. This calculator reports the drop as a percentage of the source voltage so you can compare against your own limit.
Does this include AC effects?
No. This calculator covers DC resistive voltage drop. Alternating-current circuits also have reactance from inductance and capacitance, and the skin effect raises resistance at high frequency, so AC drop can be larger for the same conductor.
Official sources
- NIST: SI electrical units and conversion factors (SP 811).
- U.S. Department of Energy: Federal Energy Management Program (electrical efficiency).
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.