A simple voltage drop calculator to assist with cable sizing and specification.

Even though copper wires are fantastic conductors, they still have a small amount of resistance. Ohm's Law states that voltage equals current multiplied by resistance (V = I*R). Therefore, a small amount of voltage is lost in the wires as current flows through your electrical system. This is known as voltage drop.

SANS 10142-1 states that when all conductors of an a.c. installation are carrying their design load, the difference in voltage (the voltage drop) between the point of supply and any point of outlet or terminals of fixed appliances shall not exceed 5 % of the standard or declared voltage.

In the case of a 230/400 V system, the voltage drop for single-phase circuits shall not exceed 11,5 V (5 % of 230 V) and the voltage drop for three-phase circuits shall not exceed 20 V (5 % of 400 V). In the case of single-phase circuits derived from three-phase supplies with balanced loading, the phase-to-neutral voltage drop is calculated from three-phase values divided by the square root of 3.

When all conductors of a d.c. installation are carrying their maximum estimated load, the difference in voltage (the voltage drop) between any point of supply and any point of consumption shall not exceed 5% of the circuit nominal voltage or shall comply with the requirements of the manufacturer of the equipment connected to the circuit.