Voltage Drop Calculator
What arrives at the far end of the cable.
Voltage Drop Calculator
Vdrop = I × (2 × ρ × L / A), with L as the one-way length
Calculated instantly
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What This Calculator Does
The Voltage Drop Calculator works out how many volts are lost in a cable run and what voltage remains at the load. It models the out-and-back loop resistance of a two-wire circuit using conductor length, cross-section, and resistivity.
How It Works
Resistance of a conductor is resistivity times length divided by cross-sectional area. Because the return conductor carries the same current, the loop resistance doubles. Multiplying by current gives the volts lost, and subtracting from the supply gives the voltage at the load. The lost power is current squared times loop resistance.
Example
A 20 A load 30 m from a 230 V supply, wired in 4 mm² copper, has a loop resistance of 2 × 0.0175 × 30 / 4 = 0.2625 Ω. The drop is 5.25 V, or 2.28%, leaving 224.75 V at the load and wasting 105 W as heat in the cable.
Formula
R = 2 × ρ × L / AVdrop = I × RVload = Vsupply − VdropPloss = I² × RReading the Results
Voltage drop
The volts lost between the supply and the load. It is proportional to current, length, and resistivity, and inversely proportional to conductor area.
Voltage at the load
What the equipment actually receives. Motors and compressors are sensitive to low voltage, especially during start-up when current briefly rises.
Sources and References
IEC 60364-5-52 — Wiring systems
Selection and erection of wiring systems, including voltage drop
The international installation standard that sets voltage-drop guidance for fixed wiring.
NFPA 70 — National Electrical Code, 210.19(A) Informational Note
Branch-circuit and total voltage drop recommendations
Suggests keeping branch-circuit drop to 3% and total to 5% for reasonable efficiency.
IEEE Std 141 — Recommended Practice for Electric Power Distribution
Voltage-drop calculation methods
Covers the resistive and reactive components; this calculator handles the resistive part.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
Why does the formula use twice the cable length?
Current flows to the load along one conductor and returns along another, so the total conductor length in the circuit is twice the one-way distance. Both lengths contribute resistance.
What is an acceptable voltage drop?
Common design guidance is up to 3% on a branch circuit and 5% overall, but the figure that matters is the equipment tolerance and your local wiring regulations. Some installations require tighter limits.
Does this account for power factor?
No. It assumes a resistive load at unity power factor. For motors and other inductive loads, the resistive drop is combined with a reactive component, which can change the answer significantly.
Why does my cable get warm?
The power lost in the conductor becomes heat. A 105 W loss in the example is a real heating effect, and bundling cables or insulating them reduces how easily that heat escapes, which is why derating factors exist.