Voltage Drop Calculation for Electrical Power Cable

What is Voltage Drop for Cable?

When current flows in a cable conductor a voltage drop is developed in between the ends of the conductor which is the product of flowing current and the impedance of the cable.

Why Calculation Voltage Drop?

It is common obligation for designer to calculate the voltage drop as requirements is the main factor in determining the minimum conductor sizes (cross-sectional area) that can be used for a particular electrical circuit.

How Calculate Voltage Drop?

Calculating the voltage drop of a circuit the designer has two options to choose one of them depending on the project requirements. These pretty two options are as below:

Simple Design Approach

Accurate Design Approach

The simple design approach is a rough estimated one which sometimes leads to use larger conductor cross-sectional areas than are necessary for the project. If you consider as simple approach, then basically following information is needed:
  •     Type of cable;
  •     Conductor cross-sectional area;
  •     Method of installation (for a.c. circuits only);
  •     Circuit route length;
  •    Type of circuit (d.c., single-phase a.c. or three-phase          a.c.);
  •     Load on the circuit.

Information is not needed for simple approach:

  •   Type and nominal current rating of the associated over      current device;
  •   Ambient temperature;
  •   Whether the circuit is run singly or grouped with other  circuits;
  •    Power factor of the load.

On the other hand accurate design approach takes more consideration of conductor operating temperature. Above mentioned four “not needed information” for simple approach should consider for more accurate approach, and specially two factors namely:

  •    The ambient temperature
  •    Whether the circuit is to be run singly or grouped with      other circuits.

Calculate Voltage Drop Smartly:

We who work with cable system sometimes have to calculate voltage drop in short and very smartly. Considering this point of view following two formulas may be helpful for voltage drop calculation smartly.

Voltage Drop Calculation Formula for Single Phase Circuit:

Vd=2Il (R cos ø + X sin ø)  V

Voltage Drop Calculation Formula for Three Phase Circuit:

Vd=√ 3 I l (R cos ø + X sin ø)  V

Vd = Voltage drop in Volt;
I = Load current in Amper;
R =A C Resistance in Ω/km;
X =Reactance in Ω/km;
cos ø = Power factor;
l  = Length in km;
X = ωL10-3
 ω=2 π f
L = from table mh/km

The limit of voltage drop depends on national code and standard which vary country to country; and hope we got some basic idea on Voltage Drop for Electrical Power Cable.

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