Voltage Drop Calculator

Why Voltage Drop Matters

When current flows through a cable, the cable's resistance causes a voltage drop between source and load. Excessive voltage drop causes motors to run hot, lighting to dim, and sensitive electronics to malfunction. Most electrical codes specify a maximum of 3–5% voltage drop in final circuits.

The Formula

Voltage Drop (V) = 2 × ρ × L × I / A

Where:

• ρ = resistivity (copper: 0.0175 Ω·mm²/m; aluminium: 0.028)
• L = one-way cable length (m)
• I = current (A)
• A = cable cross-section (mm²)
• Factor 2 accounts for both conductors (live and neutral/earth return)

Percentage drop = (V_drop / V_supply) × 100%

How to Use This Calculator

Enter cable length, current, conductor cross-section, material (copper/aluminium), and supply voltage. Results show voltage drop in volts and percentage.

Practical Examples

Example 1: 2.5mm² copper cable, 20A, 30m run, 230V supply. V_drop = 2 × 0.0175 × 30 × 20 / 2.5 = 8.4V = 3.65% — borderline acceptable.

Example 2: Same run with 4mm² cable: V_drop = 5.25V = 2.28% — well within limits.

Solutions for Excessive Voltage Drop

• Use larger cable cross-section.
• Run a separate circuit closer to the load.
• Upgrade to a higher supply voltage (240V instead of 120V halves the current).
• Use aluminium conductors for long MV cable runs (lower cost, but larger diameter).