Select the circuit
For three-phase AC, use line-to-line voltage and line current. The tool applies the corresponding circuit multiplier.
ELECTRICAL & DUTY / TOOL 26
Cable Voltage Drop Calculator
Compare the resistance-related voltage drop of different conductor sizes and cable lengths for a stated load current.
Before you start: required inputs
- Identify DC, single-phase AC or balanced three-phase AC, together with supply voltage and current.
- Obtain the one-way cable route length, area of each conductor, conductor material, operating temperature and AC power factor.
Calculation diagram
A PRACTICAL WALKTHROUGH
How to use this calculator
Describe the conductor
Use one-way length and the cross-sectional area of one conductor. Set the expected conductor temperature.
Compare the resistive drop
Read volts and percent of supply. For AC, assess the omitted reactance and starting conditions separately.
400 V three-phase copper supply
THE INPUTS
40 A, 100 m one-way length, 16 mm² copper, 70 °C conductor temperature and 0.85 power factor.
THE RESULT
Estimated resistive drop is about 7.593 V, or 1.898% of the 400 V supply.
What does the result mean?
01
Higher conductor temperature raises the assumed resistance.
02
Larger conductor area lowers the resistive drop at the same current and length.
03
AC reactance, harmonics, starting current, cable ampacity and protection coordination are outside the model.
Watch for this
Doubling the cable route length manually for a two-wire circuit; the formula already includes the return path.
Formula & model conditions +
R_conductor = ρ20[1 + α(T − 20)] L / A
DC: ΔV = 2 I R; single-phase AC: ΔV ≈ 2 I R cos φ
Balanced three-phase AC: ΔV ≈ √3 I R cos φ
- Idealized conductor resistance from 20 °C bulk resistivity, corrected linearly for temperature.
- For AC, the approximate resistive term uses cos φ and excludes reactance, harmonics, motor starting and unbalanced current.
- Cable ampacity, protection coordination and allowable voltage-drop limits are not assessed.
Common questions
Does this verify that my cable size is suitable?
It estimates one voltage-drop component. Ampacity, installation method, short-circuit withstand, protection and applicable drop limits need separate assessment.
Why can the real AC drop be larger?
The real cable has reactance and may have unbalanced current or other effects omitted by this resistance-only approximation.