Ohms Law & Circuit
Voltage Drop Calculator
Estimate how much voltage a cable run loses, using a simplified resistive model for DC, single-phase and three-phase circuits.
Voltage Drop
A simplified resistive estimate for a single run.
The Model This Calculator Uses
Every conductor has resistance, and current flowing through that resistance produces a voltage drop along the run. The resistance of a single conductor comes from its resistivity, length and cross-sectional area.
A complete circuit needs a return path, so a DC or single-phase run has roughly twice that resistance in the loop. Three-phase circuits are conventionally expressed with a factor of √3 for the line-to-line drop.
Resistivity at 20 °C is taken as 1.724 × 10⁻⁸ Ω·m for annealed copper and 2.826 × 10⁻⁸ Ω·m for aluminium. When you supply a conductor temperature, the calculator corrects the resistivity with the linear model ρ(T) = ρ₂₀ × [1 + α(T − 20)].
How to Use the Voltage Drop Calculator
Start by choosing the circuit type, because it sets the multiplier applied to the one-way resistance. Select the conductor material next, then enter the one-way run length rather than the total length of conductor used; the calculator adds the return path itself.
Enter the load current and the nominal supply voltage. The supply voltage is not used to find the drop in volts, only to express it as a percentage and to work out the voltage remaining at the load.
Size the conductor either by cross-sectional area in square millimetres, square centimetres, square metres or kcmil, or by choosing an AWG size from the list. Switching between the two modes swaps the visible field. A conductor temperature is optional and defaults to the 20 °C reference figure.
Worked Example: A Long Copper Run
Consider a 230 V single-phase circuit feeding a 16 A load through 30 metres of 2.5 mm² copper cable.
Converting the area to 2.5 × 10⁻⁶ m² gives a one-way conductor resistance of about 0.207 Ω, a loop resistance of 0.414 Ω, and a drop of roughly 6.6 V. As a percentage of 230 V that is about 2.9%, leaving 223 V at the load and dissipating around 106 W in the cable itself.
Increase the run to 60 metres and the drop doubles to about 13.2 V, or 5.8%, which is beyond what most installation practice accepts for a branch circuit. Moving up to 4 mm² cable brings the 60 metre run back to roughly 3.6%, which shows the usual remedy: increase the conductor area rather than accept the loss.
Reading Your Voltage Drop Results
The voltage drop in volts is the difference between the voltage at the source and the voltage at the load under the current you specified. The percentage expresses that as a share of the nominal supply voltage, which is the form most guidance is written in. A figure near 3% for a branch circuit and 5% overall is a widely used target, though the actual requirement depends on your local rules.
The end voltage is what the equipment sees. Motors, in particular, draw more current at reduced voltage and can run hot or start poorly when the drop is excessive. The circuit resistance figure shows both the loop value used in the calculation and the resistance of one conductor on its own.
Power lost in the conductors is the energy converted to heat in the cable rather than delivered to the load, calculated as I² multiplied by the resistance in the current path. On long runs with heavy loads this can amount to a meaningful ongoing cost.
Where Voltage Drop Matters Most
Long runs are where drop matters most: a workshop at the end of a garden, a pump in an outbuilding, lighting along a driveway, or low-voltage LED strip where even a fraction of a volt causes visible dimming at the far end. In 12 V and 24 V systems the percentage drop grows very quickly, because the same absolute loss is a much larger share of the supply.
The calculator is also useful in reverse, as a way of comparing conductor sizes before pricing a job. Running the same load and length against several areas shows how quickly the drop falls as the area increases, and where the sensible stopping point is.
For DC solar and battery wiring, where currents are high and voltages low, checking drop early usually leads to larger conductors than intuition suggests.
Limits of the Voltage Drop Estimate
This is a simplified resistive estimate. It does not replace code-based conductor sizing, ampacity and derating checks, or full AC impedance calculations. Installation design should be carried out or verified by a qualified electrician.
- Reactance is ignored. On larger conductors and longer AC runs, inductive reactance adds to the impedance and the true drop is higher than a purely resistive figure suggests.
- Power factor is not modelled. The three-phase result assumes a resistive load; inductive loads shift the relationship between current and drop.
- Ampacity is a separate question. A conductor can satisfy a voltage drop target and still be too small to carry the current safely once grouping, insulation type and ambient temperature are considered.
- Nominal AWG areas are used for the size table, and stranded conductors differ slightly from the nominal figure.
Questions
Voltage Drop FAQs
Should I enter the one-way length or the total conductor length?
Enter the one-way distance from the source to the load. The calculator applies the factor of 2 for DC and single-phase circuits, or √3 for three-phase, to account for the current path.
What voltage drop is acceptable?
Many installations aim for about 3% on a branch circuit and about 5% overall, but the binding figure comes from the electrical code that applies where you are working. Treat the percentage here as a planning estimate.
Why is aluminium worse than copper?
Aluminium has a resistivity of about 2.83 × 10⁻⁸ Ω·m against copper's 1.72 × 10⁻⁸ Ω·m, so the same size conductor drops roughly 64% more voltage. Aluminium runs are usually sized up to compensate.
Does temperature really change the result?
Yes. Copper resistance rises by about 0.39% per degree, so a conductor running at 70 °C has roughly 20% more resistance than the 20 °C reference value. Enter the expected operating temperature to include that.
Check the Whole Circuit on the Ohms Law Wheel
Open the full Ohms Law Wheel calculator to solve voltage, current, resistance and power from any two known values.