Enter the conductor size, the one-way cable length, the load current and the supply voltage, and get the voltage drop in volts and percent, the power lost in the cable, the longest run that stays inside your target, and the smallest standard size that meets it. For DC, single-phase and three-phase, in copper or aluminium.
Worked example
These are example values. Change any entry above and press Calculate to run your own numbers.
Conductor
AWG 12 (3.309 mm²), copper
Resistance of one conductor over the run
0.1932 Ω at 75 °C
Voltage drop
7.726 V
Voltage drop, percent
6.438 %
Voltage at the load
112.27 V
Power lost in the cable
154.5 W I²R heating of the conductors
Against your 3% target
Over the target
Longest run at this size for the target
46.6 ft at this current, voltage and temperature
Smallest standard size within the target
AWG 8 (2.55 % drop)by voltage drop only
This is resistive voltage drop only. It ignores conductor reactance, skin and proximity effect and the load power factor, so it is accurate for DC and for small conductors on AC and optimistic for large AC conductors at long length.
Size a conductor by ampacity, protection and the applicable electrical code first. Voltage drop is a second check that can call for a larger conductor than ampacity alone.
How it works
Voltage drop is the voltage lost in the cable between the source and the load. It rises with current and cable length and falls with conductor size, and a long run on a small conductor can leave a motor or a control device short of voltage even when the conductor is large enough for the current.
The formulas
Resistance of one conductor: R = ρ × L ÷ A (ρ resistivity, L one-way length, A cross-section)
DC and single-phase: voltage drop = 2 × I × R
Three-phase: voltage drop = √3 × I × R
Percent drop = voltage drop ÷ supply voltage × 100
Power lost in the cable = 2 × I² × R (DC, single-phase) or 3 × I² × R (three-phase)
Frequently asked questions
How do you calculate voltage drop?
Work out the resistance of one conductor over the run, R = ρ × L ÷ A, then multiply by the current and by 2 for DC or single-phase (the current flows out and back) or by √3 for three-phase. For AWG 12 copper, 100 ft one way, carrying 20 A at 120 V and 20 °C, R is 0.159 Ω, the drop is 6.35 V and that is 5.3% of 120 V.
What is an acceptable voltage drop?
There is no single figure. Design guidance often aims for about 3% on a branch circuit and about 5% from the source to the farthest load, and electrical codes such as the NEC mention those numbers as recommendations in informational notes rather than as mandatory limits. The equipment itself, its datasheet tolerance and your specification decide. Motors starting, and sensitive electronics, may need a tighter target.
Why is the length one-way?
The calculator applies the factor of 2 (two conductors) or √3 (three-phase) itself, so you enter the distance from the source to the load, not the round-trip cable length. Entering the round-trip length would double-count it.
Why does temperature matter?
Copper resistance rises by about 0.39% for every °C. A conductor running at 75 °C has about 21% more resistance than the same conductor at 20 °C, so its drop is about 21% higher. Use 75 °C for a loaded conductor, or 20 °C for a cold-conductor figure.
How does aluminium compare with copper?
Aluminium conductor has about 61% of copper's conductivity, so at the same size it drops about 1.64 times as much voltage. Aluminium therefore needs a larger cross-section to match copper on voltage drop, and it has its own termination and installation requirements.
What does this calculator not account for?
It is resistive drop only. It ignores conductor reactance, skin and proximity effect and the load power factor, so it is accurate for DC and for small conductors on AC, and optimistic for large AC conductors on long runs. It also does not size a conductor for ampacity or protection. Use the applicable code and the cable maker's data for those, then use voltage drop as a second check.
