Why voltage drop occurs in electrical circuits
All electrical conductors possess internal resistance. As electric current flows through a wire, resistance causes a portion of the voltage to drop across the cable run, reducing the voltage delivered to the load.
Excessive voltage drop causes lighting flicker, motor overheating and stall, electronic equipment rebooting, and wasted electrical energy as heat in the walls or conduit.
International standards generally mandate that voltage drop should not exceed 3% for sensitive branch circuits or 5% total across the entire electrical distribution system.
Worked example: Single-phase 230V circuit
A 16A load supplied via 25 metres of 2.5 mm² copper cable on a 230V single-phase supply (mV/A/m = 18):
Voltage drop Vd = (18 mV/A/m × 16 A × 25 m) ÷ 1,000 = 7.20 Volts.
Percentage drop = (7.20 V ÷ 230 V) × 100% = 3.13%, delivering 222.8V at the terminal load.
Inputs for the voltage drop calculator
Voltage and Current
The nominal supply voltage and expected design load current in Amperes.
Circuit length
The one-way distance from the distribution board to the furthest electrical point.
CALCULATOR QUESTIONS
Cable voltage drop & wire loss calculator questions
What is the maximum allowed voltage drop?
Most electrical codes recommend a maximum 3% drop on branch circuits (e.g. 6.9V on 230V or 3.6V on 120V) and 5% total from source to load.
How do you fix excessive voltage drop?
Increase the cable conductor cross-sectional area (e.g. moving from 2.5 mm² to 4.0 mm², or from 12 AWG to 10 AWG), or shorten the circuit run.