Voltage drop estimates
What does voltage drop mean?
Voltage drop is the difference between the supply voltage and the voltage at a load due to impedance in the circuit conductors.
Simplified formula
For single-phase two-wire circuits: ΔU ≈ 2 × I × ρ × L / A. For balanced three-phase circuits: ΔU ≈ √3 × I × ρ × L / A. I is current in amperes, ρ is resistivity in Ω·mm²/m, L is one-way cable length in metres, and A is conductor area in mm².
Worked example
A single-phase circuit with 20 A, 50 m one-way copper cable, and 6 mm² cross-section, using copper resistivity 0.01724 Ω·mm²/m at 20°C: ΔU ≈ 2 × 20 × 0.01724 × 50 / 6 = 5.75 V, or about 2.50% of 230 V.
Important limitations
This is a resistive estimate at 20°C, not a final cable-sizing or regulatory-compliance check. Real voltage drop depends on operating temperature, reactance, power factor, cable construction and circuit configuration. The load-end voltage approximation may not be physically meaningful if the estimated drop exceeds the supply voltage.