Quick answerA voltage drop calculator estimates the loss of voltage along a conductor from current, conductor resistance, circuit length, material, and phase. It reports both the voltage lost and the percentage of supply voltage.
Why voltage drop matters
Every conductor has resistance. When current flows, part of the source voltage is lost along the cable and the load receives less than the nominal supply. Excessive drop can reduce motor torque, dim lighting, create unreliable electronics, and waste energy as heat.
Long runs, high current, and small conductors increase the drop. A larger conductor, shorter route, or lower current reduces it. The percentage result is often more useful than volts alone because a three-volt loss is minor on a high-voltage circuit but substantial on a low-voltage system.
Single-phase, DC, and three-phase formulas
A two-wire DC or single-phase circuit includes the outgoing and returning path, so the resistive formula uses twice the one-way length. A balanced three-phase circuit uses the square root of three multiplier. Conductor resistance must correspond to the selected material, size, length unit, and operating assumptions.
This calculator provides a transparent resistive estimate. Real AC circuits can also be affected by reactance, power factor, temperature, harmonics, connections, and parallel conductors. Those factors become more important in large or engineered installations.
How to use this voltage drop calculator
- Select conductor material and AWG size.
- Enter source voltage, current, one-way distance, circuit type, and conductor temperature.
- Review both volts and percentage. Use manufacturer or code data for final design.
Formula and method
This transparent resistive model uses conductor area and temperature-adjusted material resistivity. It excludes AC reactance, terminals, harmonics, cable construction, parallel conductors, and installation derating.
Voltage drop example
Assume a 120-volt single-phase load draws 15 amperes through a conductor whose resistance over the one-way run is 0.20 ohm. The round-trip resistive path is 0.40 ohm, so the estimated drop is 15 × 0.40, or 6 volts.
The load would receive about 114 volts, and the percentage drop is 6 ÷ 120 × 100, or 5 percent. Whether that is acceptable depends on the equipment, circuit design, and applicable electrical rules.
AWG wire example: a 100-foot copper run
Set conductor to copper, size to 10 AWG, supply to 120 V, current to 20 A, one-way distance to 100 feet, circuit to DC / single-phase AC and conductor temperature to 20°C. With the stored conductor area of 5.2612 mm², the model gives about 0.099883 Ω for one conductor.
Applying the return-path factor gives about 3.995 V of drop, 3.329% of the supply, and 116.005 V at the load. Do not enter 200 feet: the single-phase formula already includes the outgoing and returning paths.
| Change from the example | Model result | Reason |
|---|---|---|
| Double the one-way length | About 7.991 V drop | Resistance grows with length |
| Halve the load to 10 A | About 1.998 V drop | Voltage drop is proportional to current |
| Keep inputs, use three-phase | About 3.460 V drop | √3 replaces the factor of 2 |
These are resistive estimates at the specified conductor temperature. An acceptable voltage-drop percentage does not establish ampacity or code compliance. This tool also does not calculate a diode’s forward voltage or an arbitrary component’s voltage drop.
Use the scientific calculator to evaluate a separate known-resistance expression such as V = I × R.
Common mistakes to avoid
- Entering round-trip distance when the calculator already doubles a one-way run.
- Using resistance data for a different conductor material or size.
- Comparing voltage loss without converting it to a percentage of supply voltage.
- Treating a simplified estimate as a substitute for an electrical design review.
Accuracy and practical limits
The result follows the displayed resistive model and the input values. It does not model every AC effect, terminal loss, temperature rise, code requirement, or equipment starting current. Have a qualified professional verify safety-critical conductor sizing.
Method basis: Standard mathematical definitions and the assumptions stated above. See our editorial and testing standards.
Frequently asked questions
Why is distance one-way?
The formula supplies the return-path factor for DC and single-phase circuits, so enter the physical one-way run.
Is this an NEC compliance calculator?
No. It is an engineering estimate and does not replace conductor ampacity, code, or manufacturer checks.
Why does temperature matter?
Metal resistance rises with temperature, which increases voltage drop.