Voltage Drop Calculator

How it is calculated

The drop is Ohm's law applied to the cable's resistance: Vdrop = 2 × L × I × ρ / A, where L is the one-way cable length in metres, I the current in amperes, A the conductor cross-section in mm², and ρ the resistivity in Ω·mm²/m: 0.0175 for copper and 0.0282 for aluminium. The factor 2 counts the outgoing and return conductor. Then drop % = Vdrop / supply × 100 and voltage at load = supply − Vdrop. This is a simple resistive model for single-phase or DC circuits: it ignores temperature, cable reactance, power factor and three-phase factors. Area and supply must be above zero.

Common mistakes

• Entering the total length of both conductors. The tool already doubles the one-way length, so typing the round-trip distance doubles the drop twice over.
• Entering the area in the wrong unit. The field is mm² of conductor cross-section, not the cable diameter or AWG number.
• Using the circuit breaker rating as the current. The drop depends on the real load current, so enter the actual or design current.
• Applying the result to three-phase circuits. The formula here is for single-phase or DC, so a three-phase run needs a different factor.
• Treating the percentage limit as universal. Allowed drop depends on the circuit type and the standard you follow, so check your local rules for the limit.