Voltage drop calculator
See how many volts are lost along a cable for a given length, size, material and current, and whether it meets the usual limits.
How it is calculated
Voltage drop depends on length, current, power factor and conductor resistance:
ΔV = 2 · L · I · PF / (γ · A) single-phase
ΔV = √3 · L · I · PF / (γ · A) three-phase
γ is the conductivity in m/(Ω·mm²): copper 56 at 20 °C, 48 at 70 °C and 44 at 90 °C; aluminium 35, 30 and 28.
The NEC recommends no more than 3 % on a branch circuit and 5 % overall. IEC 60364 (and BS 7671) set 3 % for lighting and 5 % for other uses on installations supplied from the public low-voltage network.
Frequently asked questions
What happens if the voltage drop is too high?
Motors start poorly and run hot, lighting loses output, and energy is wasted as heat in the cable.
How can I reduce voltage drop?
Use a larger wire, shorten the run, split the load across circuits, or use a higher voltage (240 V instead of 120 V, or three-phase).
Is cable reactance included?
No. Up to about 1/0 AWG (50 mm²) its effect is small; for larger conductors use a full impedance calculation.