How the Voltage Drop Calculator works

The Voltage Drop Calculator estimates the electrical resistance of a conductor at the selected temperature, then multiplies that resistance by load current. For DC and single-phase circuits it includes the outgoing and return path. For balanced three-phase circuits it uses the square-root-of-three relationship.

Electrical calculations use SI units: volts, amperes, ohms, watts, meters and square millimeters. The NIST SI guide for electric current explains the ampere and related derived electrical units.

Voltage-drop formulas

Conductor resistance: R = ρ × K × L ÷ A. Here ρ is material resistivity in Ω·mm²/m, K is 2 for DC or single-phase and √3 for balanced three-phase, L is one-way length in meters, and A is conductor area in mm². Voltage drop: Vdrop = I × R. Percent drop: Vdrop ÷ source voltage × 100.

Temperature adjustment

The Voltage Drop Calculator adjusts for the fact that conductor resistance increases as metal temperature rises. The calculator starts with approximate resistivity at 20°C—0.017241 Ω·mm²/m for copper and 0.028264 Ω·mm²/m for aluminum—then applies a linear temperature coefficient of 0.00393 per °C for copper or 0.00403 per °C for aluminum.

Worked example

The Voltage Drop Calculator example uses a 12-volt DC circuit carrying 10 amperes through 30 feet of 12 AWG copper has a one-way length of 9.144 meters. At a conductor temperature of 75°C, the estimated complete-circuit resistance is about 0.116 ohm. Voltage drop is approximately 1.16 volts, or 9.66%, leaving about 10.84 volts at the load.

Wire size, length and current

The Voltage Drop Calculator shows that voltage drop rises with current, conductor length and resistivity. It falls as cross-sectional area increases. A larger conductor may reduce both voltage loss and heat, but wire selection must also consider ampacity, insulation rating, ambient temperature, bundling, installation method, terminals and protective devices.

Accuracy and limitations

The Voltage Drop Calculator is a planning model that uses nominal conductor areas and simplified temperature correction. Actual resistance depends on strand construction, connectors, splices, frequency, power factor, harmonics and real operating temperature. The three-phase estimate assumes a balanced load and does not calculate reactive voltage drop. Always verify critical designs using applicable standards and manufacturer data.

Frequently asked questions

Is circuit length one-way or round-trip?

Enter one-way source-to-load length. The calculator automatically doubles it for DC and single-phase circuits.

Why does temperature matter?

Copper and aluminum resistance increases with temperature, so a warm conductor produces more voltage drop than the same conductor at 20°C.

Can I calculate three-phase voltage drop?

Yes. Select three-phase AC. The calculator uses √3 for a balanced circuit, but does not model reactance or power factor.

Does a low voltage circuit need special attention?

Yes. The same voltage loss represents a larger percentage of a 12-volt supply than of a 120-volt supply.

Does this calculator determine wire ampacity?

No. Voltage drop and ampacity are separate checks. Confirm conductor capacity using the rules that apply to the installation.

What percentage is acceptable?

There is no universal limit for every system. Many designs aim for a small percentage, but requirements depend on code, equipment and application.