Cable Size Calculation: Find the Cross-Section for Voltage Drop

What this cable sizing method determines

This method solves for conductor cross-section that meets a chosen resistive voltage-drop limit. It does not determine the complete installation rating. A cable satisfying voltage drop may still be unsuitable for its load, installation method or protective device.

Cable size formula

Amin = k × I × ρ(T) × L / [V × p / 100]. Use k = 2 for DC or single-phase at unity power factor and k = √3 for balanced three-phase at unity power factor. I is amperes, L is one-way meters, V is supply volts and p is the allowed percentage drop. The result is mm² when ρ is Ω·mm²/m.

Worked example: 230 V, 20 A and 30 m

For copper at 20°C, DC mode and a user-selected 3% limit: allowable drop = 230 × 3 / 100 = 6.9 V. Amin = 2 × 20 × 0.017241 × 30 / 6.9 = 2.9984 mm².

The next size in Toolnoza’s listed metric series is 4 mm². At 4 mm² the calculated drop is 5.1723 V, or 2.2488%. A listed size is not a statement that this cable is approved for the installation.

Temperature, material and limit selection

Higher conductor temperature raises the resistivity in this model and can increase the required section. Aluminum and copper use different resistivity values. Enter the voltage-drop limit applicable to your project; the prefilled value is an example, not a universal code requirement.

Using the Cable Size Calculator

Choose material and circuit mode. Enter current, voltage, one-way length, conductor temperature and allowed drop. Compare the calculated minimum with the next listed metric size and its predicted drop. Then check actual available cable products and their data sheets.

Checks that remain after the calculation

Verify current-carrying capacity with installation, insulation temperature, ambient conditions and grouping accounted for. Check protection coordination, short-circuit withstand, terminal compatibility and local requirements. Long AC runs and non-unity power factors need a calculation including reactance. Conduit Fill Calculator uses outside cable diameter, not the metallic cross-section.