Approximate copper conductor voltage drop calculation for single-phase circuits.
Voltage drop is a key part of cable selection and electrical installation design. As current flows through a conductor, the conductor resistance causes a reduction in voltage between the source and the load. If the drop is too high, motors may start poorly, lighting may become dim, electronic equipment may underperform, and energy losses can increase.
This voltage drop calculator gives a quick estimate for a single-phase copper circuit using cable length, current, and conductor area. It is useful for early design checks, troubleshooting, and verifying whether a cable run is likely to stay within common voltage drop limits.
It is particularly useful on longer cable runs, where a fast estimate can show whether a larger conductor may be needed before final installation decisions are made.
Vdrop = (2 x L x I x rho) / A
The factor of 2 accounts for the round-trip path in a single-phase circuit.
Vdrop = voltage drop in volts
L = one-way cable length in meters
I = current in amperes
rho = conductor resistivity in ohm mm2 per meter
A = conductor cross-sectional area in mm2
Length is entered in meters, current in amperes, and conductor area in square millimeters. This page uses an approximate copper resistivity value of 0.0175 ohm mm2 per meter at about 20 C and returns the voltage drop in volts.
Suppose a single-phase circuit has a 30 m one-way cable run, carries 20 A, and uses a 4 mm2 copper conductor.
Vdrop = (2 x 30 x 20 x 0.0175) / 4 = 5.25 V
This gives a quick indication of whether the cable size is acceptable for the connected load and supply.
For a 50 m run carrying 15 A on a 2.5 mm2 copper cable:
Vdrop = (2 x 50 x 15 x 0.0175) / 2.5 = 10.50 V
This higher drop may indicate the need for a larger conductor depending on the circuit voltage and design limit.
Common design guidance often limits voltage drop to around 3 percent for lighting circuits and 5 percent for power circuits, although the exact limit depends on the applicable standard. Final cable selection should also consider installation method, ambient temperature, grouping, conductor reactance, and ampacity. This calculator is a fast planning estimate, not a substitute for the final code-based design check.