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When the voltage at the end of a circuit is lower than the voltage at the beginning of it we call this a voltage drop.

When an electric current flows through a conductor it faces a resistance and the voltage drops along the conductor.

The longer the conductor the higher the voltage drop. The voltage drop appears at the start of large induction motors. The high starting current causes a voltage drop that can affect other loads on the same circuit. That’s why motor-starting methods are being used.

Also, we face a voltage drop when transferring electrical energy through long cables or transmission lines. For this reason, electrical energy is transferred with high voltages to overcome the voltage drop.

Why check the Voltage Drop?

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

Checking the voltage drop after setting up a new device is really important, as it tells you whether the voltage applied at the load end meets the voltage ratings of the load or not.

Voltage drop is destructive for induction motor-based appliances and equipment because induction motors are fixed-power electrical devices i.e. it will draw a higher current to operate in case of voltage drop.

The lower the voltage than its nominal, the higher the current the motor will draw. The point is, that the voltage drop can burn out the motor winding if the overload protection device fails to protect it.

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
  
• Unstable Source

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

Conditions and size of Wires and cables

Wires are the main and the most essential part of the power supply to any type of load. When the wires are in poor condition the voltage drops. The poor condition refers to the age and the physical condition of the wire.

A wire or a cable with many joints means a higher resistance and, as a result, a higher voltage drop value.

The life of a typical wire is from 50 to 70 years given that not too many current fluctuations (surges) occur while conducting.

Cable sizing is a key factor when talking about voltage drop, when the cable is overloaded, it starts to drop voltage. Every cable has a size printed over its outer jacket.

When choosing a cable for a load, we calculate the voltage drop, if it has an unaccepted value of voltage drop we choose a larger size cable to overcome the voltage drop.

I’ve written a detailed article about How does cable sizing affect voltage drop, read it for more information.

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 the case of AC supply.

These differences in voltages and impendence often lead to voltage drop 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 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 a 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.

Voltage drop is proportional to the length of the cable. You can find this clearly in cables voltage drop tables, The voltage drop is represented by mv/km/amp

If you need cable Voltage Drop Tables, you can use my FREE Android app “Stable Cables” Try it now.

Overload

Overloading the wires leads 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 higher the current the higher the voltage drop. Keep current of all circuits and load within the cable ampacity.

Interference

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, introduce an electrical surge in the circuit that leads to an instantaneous voltage drop in the devices connected to the same wire for power.

Can a loose connection cause a Voltage drop?

Yes, a Loose connection increases the resistance of the circuit and causes a voltage drop.

The more loose connections in a circuit, the higher the voltage drop will be. It’s like adding resistances to the cable or the current path.

Loose also heats up the circuit and can melt down the components, meltdown components, spark, or even a fire eruption.

The Joints are normally offering a small resistance that causes 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.

When you face a voltage drop in a circuit, check the connections and re-tighten all joints. We apply joints and bolts and re-tighten them during preventive maintenance at my work.

How Does Low Voltage Affect Different Loads?

Voltage drop effect on home appliances and equipment like refrigerators, TV, or lights is different, we will be observing different effects on these appliances:

I have written a detailed article about Voltage drop problems and solutions, read it for more information.

• Effects on Refrigerators

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

The refrigerator motor will not start if the voltage drops to a 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 out. 

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.

Use an automatic type voltage stabilizer to correct the voltage in case of any drop.

• 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 them well protected against voltage drop.

While old types of lamps, like incandescent or fluorescent, are affected by the voltage drop.  incandescent lamps dim with 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 fluctuations, the lights of higher voltage ratings might get damaged but again there is little chance of it happening.

• Cables

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 overheat 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 burn out in case of low voltage which leads to a high flow of current.

While many modern electronic devices have a wide range of voltage, they still work while the voltage is low.

• Electric Motors

To turn on the electric motor, it needs a high starting current for starting torque. At low voltage, that starting torque cannot be achieved and the motor will draw much more current to start.

In case of voltage drop, the induction motor won’t start and will draw a higher current, overheated, and then burn out if the un-voltage or overcurrent protection devices fail to trip the circuit breaker.

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. It even caused other loads on the same circuit to stop due to the voltage drop.

How can we overcome 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 the current.

This way of solving voltage drops 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. 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 an inductor or capacitor bank. That usually eliminates the voltage drop.

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

Find the voltage drop reason and solve it as required. Measure the source voltage, check the loads, make sure the cable sizing is correct, and check the equipment voltage requirement.

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.

Voltage Drop Calculation in DC Circuits

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

Voltage Drop Calculation in AC Circuits

3 Phase V_d = √3(I*L)*{(R CosΦ)+(X SinΦ)}

Where

V_d: voltage drop

I: Load current

L: Cable length

Φ: The phase angle between voltage and current.

R: Cable resistance

X: Cable reactance

The values of the cable parameters can be found in cable tables or manufacturer catalogs.

In an AC circuit, the resistance is comprised of Reactance. The reactance is denoted by X. Further 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.

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 a 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 Using cable tables

The easy way to calculate the voltage drop is using cable tables. The 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.

Example:

Let’s 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

V_d = Current * V_d (v/amp/km from tables) * Length of the cable

For the same example above, if the current is 39 amp and the length of the cable is 2 KM, then we can calculate voltage drop V_d = 39*0.03*1*2 = 2.34 V

Never worry about keeping formulas and cable data, I’ve created a FREE Android app for you, StableCable, It has cable voltage drop tables and can help you calculate voltage drop.

Why does Voltage decrease on the increase in Load?

The more the load the more current it draws. From the voltage drop equation, the Voltage drop is proportional to the current.

The better way to understand this problem is to see it while keeping Ohm’s Law in mind which 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.

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.

 

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