How to Calculate Voltage Drop of BBT and Its Optimum Limit

Voltage Drop Calculation

CONDUCTOR (HERE BBT) carrying current always have inherent resistance, or impedance, to current flow. Voltage drop is defined as the amount of voltage loss that occurs through all or part of a circuit due to impedance.

CAUSE: Voltage drop is caused by resistance in the conductor or connections leading to the electrical load. There are many causes of resistance in the conductor path. There are four fundamental causes of voltage drop:

1. Material         

2. Wire Size       

3. Wire Length           

4. Current Being Carried 

VOLTAGE DROP FORMULA: For three-phase circuits,

Here,        L       = Length of the BBT

                I        = Current Capacity of the BBT

                R       = Resistance (mOhm/m)

                X       = Impedance (mOhm/m)

Optimum Level of Voltage Drop

According to the IEC 60364, the maximum allowable voltage drop at any point of LV installation is given in the below table:

Type of installations	Lighting circuits	Other uses (heating and power)
Low voltage installations supplied directly from a public low voltage distribution system	3%	5%
Low voltage installation supplied from private LV supply	6%	8%

An Excel sheet for Voltage Drop Calculation. Download.