What is Voltage Drop?
Voltage drop is the decrease of electrical potential along the path of a current flowing in an electrical circuit. It is caused by the internal resistance and reactance of the cables over a distance.
By international standards like IEC 60364-5-52 and BS 7671, the voltage drop typically shouldn't exceed 3% for lighting and 5% for other uses.
Where:
- I = Current in Amperes (A)
- L = Length of cable in meters (m)
- mV/A/m = Millivolt drop per ampere per meter (dependent on cable size and material)
Frequently Asked Questions
By international standards such as IEC 60364-5-52 and BS 7671, voltage drop should typically not exceed 3% for lighting circuits and 5% for other uses (from the origin of the installation). NEC recommends a maximum 3% voltage drop for branch circuits.
The formula for a 3-phase circuit is: Voltage Drop (V) = (√3 × I × L × (R cos Φ + X sin Φ)) / 1000, where I is the current, L is the length of the cable, R is the resistance per km, X is the reactance per km, and Φ is the phase angle.
Voltage drop is the reduction in electrical potential (voltage) as current flows through the resistance of a conductor (cable or wire). It occurs because every conductor has some resistance, and per Ohm's Law (V = I × R), current flowing through resistance causes a voltage loss.
Per IEC 60364, NEC (USA), and most international standards, the maximum allowable voltage drop is 3% for branch circuits and 5% total (from supply to final load). Exceeding these limits causes equipment malfunction, overheating, and energy waste.
Common causes of excessive voltage drop include: undersized cable (too thin for the current load); excessively long cable run; high ambient temperature reducing conductor capacity; loose or corroded connections adding resistance; and load currents higher than originally designed.
To reduce voltage drop: use a larger conductor cross-section (thicker cable); shorten the cable run where possible; increase the supply voltage; use a higher-conductivity material (copper vs aluminium); or split the load across multiple circuits.
The voltage drop formula is: VD = (2 × L × I × R) / 1000, where VD is voltage drop in volts, L is the one-way cable length in metres, I is the load current in amperes, and R is the conductor resistance in Ω/km. The factor of 2 accounts for both the live and return conductor.