Voltage Drop Calculation – The Basics of Calculating Voltage Drop

Posted on

Voltage drop means the reduction in voltage or voltage loss. How do you know voltage drop calculation? Due to the presence of the impedance or passive elements, there will be some loss in voltage as the current moves through the circuit. That is, the energy supplied from the voltage source will get reduced as the current flows through the circuit. Too much voltage drop may result in damage and improper function of the electrical and electronics apparatus. Basically, the voltage drop calculation is done by Ohm’s law.

In 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.

Voltage Drop Calculation – The Basics of Calculating Voltage Drop

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 Line

Here, 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 Circuits

In 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)

Circular Mils and Voltage Drop Calculation

Circular 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 Conclusion

After going through the above portion of voltage drop calculation we can now establish a basics of calculating voltage drop. I hope you enjoy when reading this article, thank you.

Leave a Reply

Your email address will not be published. Required fields are marked *