Line Voltage Drop: What, Why, How And Detailed Facts

This article describes the line voltage drop and its characteristics. The line voltage is the potential difference between two phases or lines in a polyphase system. High resistance is the primary reason behind line voltage drop.

Voltage drop becomes a crucial factor in case of long cables or transmission lines. Excess line voltage drop may damage electrical appliances, damage them and shorten their lifespan. To minimize the line voltage drop, one efficient way to increase the size or diameter of the conductor which lowers the overall line resistance. 

What is voltage drop in transmission line?

The impedance in the transmission line is the primary reason for voltage drop across it. The impedance generates from the transmission line parameters such as resistance, inductance, capacitance and shunt conductance. 

The four transmission line parameters sum up to provide impedance to the current flow and thus voltage drop occurs in the entire length of the transmission line. At zero load, the voltage drop at both the ends are equal. In load, if the voltage drop rises, the voltage at the receiving end of the line decreases and vice versa. 

The Lethaba Power Station
Transmission line; Image credit: Flickr

What causes line voltage drop?

Line voltage drop is the result of multiple factors present in the transmission line. Excessive load, redundant connections, increased conductor resistance etc are responsible for line voltage drop.

The two main reasons of line voltage drop are- 

  1. Voltage drop in line due to inductive reactance- It is almost 10 times higher than the general line resistance voltage drop.
  2. Voltage drop caused by high line resistance- It is nominal as compared to the inductive reactance voltage drop.

Read more on…Voltage Drop For Single Phase: How to Calculate And Detailed Facts

Line voltage drop formula?

There are two different formulas for calculating voltage drop in single phase and three phase. In the case of a single phase system, there is only one power line. In the case of a three phase system, there are three power lines.

The line voltage drop for single phase is –

The line voltage drop for three phase is –

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Where, Z = Impedance of the line 

I = Load current

L = Length in ft ( divided by 1,000 as standard impedance values are given for every 1,000 ft)


Line voltage drop chart

Maximum of 3% voltage drop is permissible through the wire made of any material. Here is the chart of 3% voltage drop in single phase connection for 110 volt- 

Image credits: Pinterest

Line voltage dropping resistor

Although every resistor drops potential when current passes through it, a dropping resistor is a specific equipment used to reduce voltage. It is connected in series with the load to bring down load voltage.

The sole purpose of using a line voltage dropping resistor is providing the circuit with extra resistance. The voltage drop can be calculated just by using the general ohm’s law.

Overhead line voltage drop 

An overhead line is an electrical cable that transmits electrical energy over large areas or in electrical locomotives. Generally overhead lines have higher voltage drop than underground cables. 

In overhead lines, the inductance is much higher than the inductance of the insulated underground cables. As voltage drop increases with inductance,higher voltage drop occurs in same length overhead lines. Also, the longer distance between conductors causes voltage drop in overhead lines. 

Overhead line voltage drop calculation

Overhead line voltage drop can be obtained in either exact or in approximate method. In the latter, Voltage drop

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where I= line current, R= resistance X= reactance and theta is phase angle.

In exact method, one more quantity Es or the source voltage gets added. So the exact line voltage drop

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Cosθ and sinθ are also known as the power factor and reactive factor of the load respectively. 

Read more on…Transformer Voltage Drop: What, Why, How To Find And Detailed Facts

Capacitor line voltage drop

The conductors present in the transmission line form a capacitor acting as parallel plates and air work as the dielectric medium. The capacitance depends on the line length and it amplifies the current in the lines. 

Capacitance in the transmission line depends upon the shape, size and the separation between the conductors. As capacitance is inversely proportional with the voltage, lesser capacitance will yield greater voltage drop through the transmission line. Similarly, high capacitance value will result in low voltage drop. 

Supply line voltage drop

Supply lines are the combination of long electrical wires and the structures to support them for the transmission of electrical power. 

Many factors like load, too many conductors, high resistance etc induce the voltage drop in the supply line. For a branch circuit or a feeder individually, the recommended voltage drop in the conductors is a maximum of 3%. The combined voltage drop of the two must not exceed the level of 5%.

Line reactor voltage drop

A line reactor is an electrical component ( basically an inductor) that can be used to protect the semiconductor devices like variable frequency drives and other devices from transients, surges and power spikes. 

The percentage mentioned in the line reactor isn’t the measure of voltage drop across it. As the reactance is inductive and the voltage is in phase with current, the voltage drop is tangential to the line current. So if we have a 5% line reactor, the voltage drop across it might be somewhere around 2-3% of the total voltage.

Linear regulator voltage drop

A linear voltage regulator is a device that maintains a certain voltage. The input voltage in a linear regulator is always more than the output voltage. This difference in voltage makes the linear regulator work. 

Linear or step down regulators control a set voltage and supply the load with electrical energy. The regulated voltage sometimes appears different due to voltage drop occured in the interconnected lines. The voltage drop depends upon the resistance or the net impedance between the load and the linear regulator.

Line to neutral voltage drop calculation

For a single phase system, the line to neutral voltage is the lower voltage( generally 120 Volt). This is the voltage between the neutral and one of the lines. The line to neutral voltage drop is single phase value by 2. 

For a three phase electrical system, we can find the line to neutral voltage using the same process. It is the lower voltage ( generally 277-347 Volt). This is the voltage between the neutral and one of the three phase lines. The line to neutral voltage drop is the three phase value by √3. 

Linear power supply voltage drop

When lines use power supply regulators, they regulate a set voltage to supply the load with electrical energy. In several cases, the regulated voltage faces fluctuations due to voltage drop across the lines. 

The effect of a high current on the voltage drop is higher than the low current. If the electricity is divided according to the area and the load to be supplied, there will be a decrease in the voltage between the controlled voltage and the area where the power is required. This decrease in power depends on the resistance that exists between the controller and the load.

Line loss vs voltage drop

Line loss in a transmission line refers to the power loss due to various losses such as ohmic loss, copper loss, dielectric loss etc. Voltage drop in a transmission line is the loss in potential caused by all impedance factors.

Here is a comparison table of the reasons of line loss and line voltage drop-

Line lossVoltage drop
The I2R loss is the most significant cause of line loss. One of the main contributing factors of voltage drop is line resistance. 
The other losses responsible are- Dielectric and conductance lossCorona loss in overhead high voltage linesRadiation loss in high frequency linesInduction loss due to magnetic coupling between wires. Voltage drop caused by the inductive reactance is also crucial as it is very high. 

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