Voltage Drop Calculator

How it works

1. 1Enter the load current, one-way cable length, conductor cross-section, and system voltage.
2. 2Select the conductor material (copper or aluminum) and the system type (single- or three-phase).
3. 3The calculator applies Vd = k·I·ρ·L/A and shows the drop in volts, as a percentage, and the voltage at the load.

Use cases

• Checking whether a cable run stays within the 3% or 5% voltage-drop limit in wiring codes.
• Comparing copper and aluminum conductors to find a cost-effective size.
• Evaluating long runs to outbuildings, EV chargers, or sub-panels.

Frequently asked questions

What formula does the calculator use?

Vd = k·I·ρ·L/A, where k = 2 for single-phase or √3 ≈ 1.732 for three-phase, ρ is conductor resistivity (copper ≈ 1.68×10⁻⁸, aluminum ≈ 2.82×10⁻⁸ Ω·m), L is one-way length, and A is the cross-section.

What percentage voltage drop is acceptable?

Most codes recommend keeping the total drop to about 3% for lighting and 5% for power circuits. On 230 V, 3% is about 6.9 V. This is a physics estimate — follow your local wiring code.

Worked example: 20 A, 30 m, 4 mm² copper, 230 V single-phase?

Vd = 2 × 20 × 1.68×10⁻⁸ × 30 ÷ (4×10⁻⁶) ≈ 5.04 V, about 2.2% of 230 V — within the 3% limit.