The voltage drop in distribution system is calculated by following Ohm,s Law. Let us consider a simple do radial distribution system as shown in Figure.

The system have concentrated load I_a, I_b, I_c, I_d and I_e at load point A,B,C,D and E respectively.

The feeder is fed at point O. Let the voltages at different nodes are V_a, V_b, V_c, V_d and V_e and the feeder is fed at the voltage V_o . Hence the voltage drop is given by:

VD_Total =VD_OA + VD_AB +VD_BC + VD_CD + VD_DE

Current flowing in sections:

• The current flowing in the section ‘OA’ is: I_oa = I_a + I_b + I_c + I_d + I_e

• The current flowing in the section ‘AB’ is: I_ab = I_b + I_c + I_d + I_e

• The current flowing in the section ‘BC’ is: I_bc = I_c + I_d + I_e

• The current flowing in the section ‘CD’ is: I_cd = I_d + I_e

• The current flowing in the section ‘DE’ is: I_de = I_e

The total voltage drop therefore, is given by:

VD_Total = I_oaR_oa + I_abR_ab + I_bcR_bc + I_cdR_cd + I_deR_de

*Similarly, we can determine the voltage drop for AC distribution system. In many cases the load in the system is not concentrated, it may be either uniform loading or a combination of uniform and concentrated loading.

If the load is uniform then the voltage drop is calculated for a very small length of the feeder such as dx and then integrate it over the whole length.