Voltage Drop CalculationPublished on 24/2/2012 & updated on 3/8/2018
Voltage Drop in Direct Current CircuitsIn direct current circuits, the reason for voltage drop is the resistance. For understanding the voltage drop in DC circuit, we can take an example. Assume a circuit which consist of DC source, 2 resistors which are connected in series and a load.
Here; every element of the circuit will have a certain resistance, they receive and lose energy to some value. But the deciding factor of the value of energy is the physical features of the elements. When we measure the voltage across the DC supply and first resistor, we can see that it will be less than the supply voltage. We can calculate, the energy consumed by each resistance by measuring the voltage across individual resistors. While the current flows through the wire starting from the DC supply to the first resistor, some energy that is given by the source gets dissipated owing to the conductor resistance. To verify the voltage drop, Ohm’s law and Kirchhoff’s circuit law are used which are briefed below. Ohm's law is represented by V → Voltage Drop (V) R → Electrical Resistance (Ω) I → Electrical Current (A)
For DC closed circuits, we also use Kirchhoff’s circuit law for voltage drop calculation. It is as follows: Supply Voltage = Sum of the voltage drop across each component of the circuit.
Voltage Drop Calculation of a DC Power LineHere, we are taking an example of 100 ft power line. So; for 2 lines, 2 × 100 ft. Let Electrical resistance be 1.02Ω/1000 ft and current be 10 A.
Voltage Drop in Alternating Current CircuitsIn AC circuits; in addition to Resistance (R), there will be a second opposition for the flow of current – Reactance (X) which comprises of XC and XL. Both X and R will oppose the current flow also the sum of the two is termed as Impedance (Z). XC → Capacitive reactance XL → Inductive reactance
The amount of Z depends on the factors such as magnetic permeability, electrical isolating elements and the frequency of AC. Similar to Ohm’s law in DC circuits, here it is given as E → Voltage Drop (V) Z → Electrical Impedance (Ω) I → Electrical Current (A) IB → Full load current (A) R → Resistance of the cable conductor (Ω/1000ft) L → Length of the cable (one side) (Kft) X → Inductive Reactance (Ω/1000f) Vn → Phase to neutral voltage Un → Phase to phase voltage Φ → Phase angle of load
Circular Mils and Voltage Drop CalculationCircular mil is really a unit of area. It is used for referring the circular cross sectional area of the wire or conductor. The voltage drop using mils is given by L → Wire length (ft) K → Specific Resistivity (Ω-circular mils/foot). P → Phase constant = 2 meant for single phase = 1.732 meant for three phase I → Area of the wire (circular mils)
Voltage Drop Calculation of Copper Conductor from TableThe voltage drop of the copper wire (conductor) can be found out as follows: f is the factor we get from the standard table below.
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