Voltage Drop: Causes, Solutions and Calculations

Voltage Drop: Causes, Solutions and Calculations

Voltage drop causes, solutions and calculations are essential topics to understand. In this article I will discuss this important topic. Let’s get started.

What is Voltage Drop?

Voltage drop is the reduction of voltage in a circuit. It can be further explained that if we connect a conductor to a voltage source. The material of the conductor has resistance to the flow of current.

As current-carrying wires and cables have their own resistance or impedance. And this resistance opposes the Current flow.So there will be a difference in the voltage between both ends of the cable. This phenomenon is called voltage drop.

Why To check the Voltage Drop?

Checking the voltage drop after some time or after setting up new devices is really important as it tells you whether the voltage applied at the load end meets the voltage and current ratings of the load or not.

Whether the cables that you have used to supply power to the device are capable enough to bear the load or not. Say an old cable that is rusty and aged well more than its life or has bad physical health will add hundreds of ohms in the resistance itself causing a great voltage drop at the other end of the wire.

Make sure to check the voltage drop and keep its value within the allowable limits, Voltage drop can cause damage to electric devices like motors, home appliances and measurement instruments.

Causes of Voltage Drop

The main reasons for voltage drop are listed and described below as:

  • Condition of the Wires
  • Imbalanced Loads
  • Distance between the loads and the source
  • Overload
  • Interference of other loads
  • Natural causes
  • Unstable Source

Now let’s see how these factors affect the voltage drops in houses and the industrial sector.

Conditions of Wires:

Wires are the main and the most essential part of the power supply to any type of load. Imagine carrying the power supply without wires. Yes, that is impossible. So, when the wires are in poor condition the voltage drops. The poor condition refers to the age and the physical condition of the wire.

The life of a typical wire is from 50 to 70 years given that not too many current fluctuations (surges) occurred while conducting. When the wire is overloaded, it starts to drop voltage. Every wire has a rating printed over its package.

This tells the allowed voltage and sometimes even the power rating is printed on the wire packages. When we overload the wire, voltage drops. More consumers per wire also cause the voltage drop.

Imbalanced Loads:

In the houses or the industrial units, the loads must be balanced. This statement needs a little explanation and that is when we use one single wire to supply power (the live line) and another wire providing the neutral for the different circuits, each circuit has its own equivalent resistance in DC or impedance in case of AC supply.

These differences in voltages and impendence often lead to voltage drops in the circuit. To be more precise, the circuit that draws more current will have a little different voltage requirement than the circuit that draws lesser current and more voltage.

If the equivalent power (sum of both circuits) exceeds the power rating, voltage drops in either or both the circuits.

Distance between loads and the Source:

The longer the wire, the greater will be the voltage drop. Every wire, when supplied power or in other words, when current is passed through the wire, the wire attains a potential (voltage) in itself.

When the voltage in the wire is extended to a farther place, this voltage drops significantly at the load end. Every wire has resistance and due to this resistance, the load does not get the voltage that was sent from the source side. Ohm’s Law describes the resistance-current relation for the voltage and the power.


Overloading the wires lead to a voltage drop. When the circuit or the total load, that the wire is providing the current, draws more current and thus the voltage drops down. That leads to the malfunction of devices.


The electrical appliances that have timer-controlled circuits such as automatic washing machines, ovens, and other appliances and turn ON or OFF automatically depending upon the timers, introduces an electrical surge in the circuit that leads to an instantaneous voltage drop in the devices connected to the same wire for power.

Natural Causes:

The common natural causes of the Voltage drop are heavy trees, thunderstorms, lightning, and fallen trees. The thunderstorm and the lightning pose an instantaneous voltage drop while fallen trees may lead to a long-time voltage drop.

If a tree falls and knocks off or completely cut-off one the transmission lines, the voltage drops for a long while that is until the wire is repaired or reinstalled.

Can a loose connection cause a Voltage drop?

Yes, Loose connection increases the resistance of the circuit and cause a voltage drop. A loose connection often leads to completely opening and closing the circuit every now and then which results in voltage dropping to zero and 100% in the load and causes the heat up of the circuit.

This heating up of the circuit can meltdown the whole circuit, meltdown some components, spark, or even a fire eruption. The Joints are normally offering a small resistance that cause some voltage drop of the order 10-3 which is bearable but a loose joint is a worst-case scenario causing voltage drop as one of its results.

Effects of  Low Voltage on Different Loads

As the effect on refrigerator, TV or lights is different, we will be observing different effects on these appliances:

Effects on Refrigerator:

Every refrigerator has its own voltage and power rating depending upon the model. As the refrigerator has a motor in it, so the effect on the motor will be defining the effect of the whole device.

Refrigerator motor will not start if the voltage drops to very low value. And you may notice that the motor tries to start and then stops at the same time. Repeating this starting trial will cause the motor to overheat and then burn. 

If the refrigerator is connected through a Voltage stabilizer, it will not be harmed by voltage drop even if there are periodic voltage surges. But if not, there can be different cases.

Say our refrigerator works properly at 220V and voltage dropped to 210 volts, that much low voltage is bearable and the internal PCB won’t start heating up. But when the voltage drops too lower than that the PCB starts heating up due to more drawing of current to meet the power needs and will eventually meltdown. It can even cause a short circuit or fire or component failure.

Effects on TV:

The voltage drop has nothing to do with the “display” part of the TV(s) and thus the display is never affected. The circuitry of the power supply might get damaged if there are fluctuations otherwise the TV(s) can also run over a low voltage.

Effects on Lights:

LED lamps work on a wide voltage range, usually +/- 100 V, this makes it well protected against voltage drop. While old types of lamps, like incandescent or fluorescent affect by the voltage drop.  incandescent  lamps dims with the low voltage, while fluorescent will not work. Discharging Lamps simply switch off.

Lights are the protestors against the voltage drop. At low voltage lights become dim but there is a little chance of them burning down.

If the voltage drop is followed by the fluctuations, the lights of higher voltage rating might get damaged but again there is little chance of it happening.


Low voltage has no direct effect on the power cables. But, as low voltage increases the load current, If that current value is more than the cable ampacity, the cable will overheated and the insulation will break down. The life of the cables shortens, they age faster and malfunction unexpectedly.

Electronic Devices:

Some electronic devices are sensitive and may burnout in case of low voltage that leads to a high flow of current. While many modern devices has a wide range of voltage, and it still working while the voltage is low.

Electric Motors:

To turn on the electric motor, it needs a high amount of voltage for starting torque. At low voltage that starting torque cannot be achieved. So, in case of voltage drop a motor won’t start and will draw higher current, overheated and then burn out.

In my workplace, we have a medium voltage motor, 3.3 KV, the motor is located far away of the power source. The long distance makes a voltage drop. The motor didn’t start before solving the voltage drop issue.

Ways to fix Voltage Drop

We can fix voltage drop with one of the following methods:

  • Increase the source voltage.
  • Increase the power cable size.
  • Use a voltage regulator (stabilizer).
  • Reduce unnecessary loads, to reduce current.

Let’s go into some details.

The most common way to solve the problem of low voltage is to use a voltage stabilizer. The Voltage stabilizers boost the low voltage at the expense of current.

This way of solving voltage drop is popular in rural areas. People in such areas think it to be compulsory to use a stabilizer with their AC (Air Conditioners), refrigerators, and TVs. The modern appliances have a built-in capability to stabilize the incident.

In industry, the voltage drop is generally a result of imbalanced loads. Sometimes loads are capacitive and sometimes inductive. The solution to solve this problem is to rectify the capacitive or the inductive factor by using inductor or capacitor banks. That usually eliminates the voltage drop.

And if the reason behind voltage drop is overloading then it is advised to get another independent wire from the source to distribute the load.

Voltage drop Calculations

We have two types of Circuits i.e. DC and AC. In a DC circuit, the voltage drop can be simply calculated by Kirchhoff’s Law. Which states that the sum of the voltages or current in a closed circuit will be zero. This law is further divided into Kirchhoff’s current law and Kirchhoff’s Voltage Law.

Voltage drop Calculation in DC Circuits

The voltage drop across DC power line is simply calculated by Ohm law i.e.

DC voltage drop
DC voltage drop formula

Voltage drop Calculation in AC Circuits

In an AC circuit, the resistance is comprised of Reactance. The reactance is denoted by X. further that reactance contains Capacitive Reactance and Inductive Reactance .therefore the combined resistance is the sum of both Capacitive and inductive Reactance.

The total Impedance is Z= R + jX

The impedance Z depends upon the resistivity of the material, Frequency of the AC circuit, and Electric permeability.

Therefore according to the ohm law for AC circuit.

E = I * Z


E is the voltage drop

Z denotes Impedance of the AC circuit

I denote Current in Electrical Circuit

 Voltage Drop calculation and Circular Mils

We know that besides other parameters voltage drop is also dependent upon the cross-sectional area of the conductor. Therefore, to find the Voltage drop in a conductor we have to use the Mils equation.

Where K is the specific resistivity of the material

P is Phase constant, i.e. for Single Phase we Use 2, and for three-phase, we use 1.732.

L is the length of the conductor or wire

“I” is the current

A is the Circular Miles (Areas of the conductor)

Voltage drop calculation from cables tables

The easy way to calculate the voltage drop is using cables tables, Voltage drop of each cable is given in the tables in v/amp/km, which you can use to calculate the voltage drop according to the load and the distance of the cable.

Lets say a cable drop in the tables is given as 0.03 v/amp/km, this value means that if we have a piece of that cable its length is 1km, and 1 amp passes through it , then this 1 amp will produce 0.03 volt as a drop

Voltage Drop Tables Free Android APP

If you are in the work location and need to calculate voltage drop for an electrical load like three phase motor, but oops! you don’t have the tables! what to do?

Don’t worry, if you have any android smartphone just get cables tables free android app from google play store, and you will find all voltage drop data you need to perform voltage drop calculation.

Why does Voltage decrease on the increase in Load?

The reason for voltage drop due to the increase in load is that more the load more will be the current required in the circuit.

When we increase the load, the resistance increases. Each and every device/ load needs a required amount of current (rated current) to operate.

When the source is generating enough current, the current is not being generated without any expense but at the expense of the voltage. The voltage level is brought down and the current level is brought up to keep the source and its impedances balanced. The better way to understand this problem is to see it while keeping Ohm’s Law in mind that says:

I = V/R,  Or

V= I R

Where “I” is current in Amperes, “V” is the voltage in Volts and, “R” is the resistance in ohms.

These relations clearly tell that the “I” needs to be increased or the “V” needs to be decreased when higher resistance (load) is increased.

Why does Voltage drop on the increase in Current?

Voltage and current are inversely related as the formula of Electrical Power suggests.

P= IR [watts]

Where “P” is the power in watts, “I” is the Current in Amperes and V is the Voltage in Volts.
So, this relation tells us clearly that if we increase the current flow through a circuit, the voltage will be dropped down by the source to meet the power requirements.