How to Calculate Voltage Drop in a Series Circuit?

To calculate voltage drop in a series circuit multiply the circuit current by the resistance of each section using Ohm's Law (V = I × R) then add every individual voltage drop together. Because current remains constant throughout a series circuit, each resistor, cable or electrical load receives a share of the total supply voltage based on its resistance.

Whether you're designing a low voltage system troubleshooting an electrical circuit or studying circuit analysis understanding how voltage drop works helps ensure equipment operates safely and efficiently.

In this guide you'll learn the formula for voltage drop in a series circuit, follow a step by step calculation and see a practical example you can apply to your own project. If you want a broader step by step guide covering different circuit types and calculation methods, see our guide on how to calculate voltage drop .

Key Takeaways

Current Stays the Same Voltage Splits

In a series circuit the same electric current flows through every component. However the supply voltage is divided across each resistor cable or load according to its resistance.

The Same Core Formula Applies

Calculating voltage drop in a series circuit uses the standard Voltage Drop = Current × Resistance (VD = I × R) equation. Calculate the voltage drop for each segment separately then add them together.

Total Voltage Drop Should Stay Within Recommended Limits

A commonly used design guideline is to target no more than 3% voltage drop on a branch circuit and no more than 5% for the combined feeder and branch circuit path although the applicable requirements can vary by installation and electrical standard.

What Is Voltage Drop in a Series Circuit?

A series circuit connects electrical components one after another creating only one path for current to flow. Since there is only one path the same circuit current passes through every resistor conductor connector and electrical load.

As current flows through each component electrical resistance causes part of the supply voltage to be consumed. This reduction in electrical potential is known as voltage drop. According to Ohm's Law (V = I × R) the voltage drop across any component depends on:

Because the current remains constant throughout the circuit components with higher resistance experience a larger voltage drop than components with lower resistance.

Another important principle governing series circuits is Kirchhoff's Voltage Law (KVL) which states that the sum of all voltage drops around a closed electrical loop always equals the total supply voltage. In other words:

Supply Voltage = Voltage Drop₁ + Voltage Drop₂ + Voltage Drop₃ + ...

This relationship makes voltage calculations in series circuits predictable and straightforward.

• The current flowing through it
• Its electrical resistance

Why Does Voltage Drop Matters?

Voltage drop isn't simply a mathematical concept it directly affects how electrical equipment performs. Excessive voltage drop can lead to:

For electricians and engineers calculating voltage drop is an important part of cable sizing electrical design and troubleshooting.

• Dim LED lights
• Motors losing torque
• Electronics operating below their rated voltage
• Increased power losses
• Reduced system efficiency

Why Series Circuits Behave Differently from Parallel Circuits?

One of the biggest differences between series and parallel circuits is how voltage and current behave. In a series circuit:

In a parallel circuit:

Understanding this distinction helps avoid one of the most common mistakes when calculating voltage drop.

• Current stays constant.
• Voltage divides across components.
• Total resistance increases as more components are added.
• Voltage remains the same across every branch.
• Current divides between branches.
• Total resistance decreases as more branches are added.

Where Series Circuits Are Used?

Although most household wiring uses parallel circuits series circuits still appear in many practical applications including:

In these systems accurately calculating voltage drop helps maintain reliable performance and prevents unnecessary energy losses.

• LED light strings
• Battery powered devices
• Flashlights
• Automotive sensor circuits
• Long cable runs containing multiple connectors

How Do You Calculate Voltage Drop in a Series Circuit?

The process is straightforward because current remains the same throughout the entire circuit. Instead of calculating different current values for each component you calculate the voltage drop across every resistance individually and then add the results together.

If you need a quick reference for voltage drop formulas and cable resistance values see our voltage drop formula and calculation table .

Voltage Drop Formula for a Series Circuit

Voltage Drop (per component) VD = I × R

Where:

For an entire series circuit:

Total Voltage Drop VD(total) = VD₁ + VD₂ + VD₃ + ...

Or equivalently:

Supply Voltage = V₁ + V₂ + V₃ + ...

This relationship follows Kirchhoff's Voltage Law which states that the algebraic sum of all voltages around a closed electrical loop equals zero or practically speaking that all individual voltage drops equal the supply voltage.

• VD = Voltage drop
• I = Circuit current
• R = Resistance of the component or cable

Understanding Each Variable

Before performing the calculation it's important to understand what each value represents.

Current

Current is measured in amperes (A). In a series circuit you only need to determine the current once because it remains identical through every resistor and cable section. Current can be measured using a clamp meter or calculated from the total resistance and supply voltage.

Resistance

Resistance is measured in ohms (Ω). Each component contributes its own resistance to the circuit including:

Higher resistance always produces a larger voltage drop when current remains constant.

• Copper cable
• Aluminum conductor
• Electrical connectors
• Switches
• Resistors
• Electrical loads

Voltage Drop

Voltage drop is measured in volts (V). It represents the amount of supply voltage consumed by each individual component as electrical current flows through it.

Professional electricians often verify calculated voltage drops using a digital multimeter especially during commissioning or troubleshooting to confirm that the actual measured values closely match the calculated results.

How to Calculate Voltage Drop in a Series Circuit Step by Step?

Follow these simple steps whenever you need to calculate voltage drop in a series circuit.

Step 1 – Determine the Circuit Current

Since current is identical throughout a series circuit you only need to determine it once. You can obtain the current by:

Once you know the current you'll use the same value for every component in the circuit.

• Measuring it with an ammeter or clamp meter
• Reading the equipment's rated current
• Calculating it using Ohm's Law if the supply voltage and total resistance are known

Step 2 – Identify the Resistance of Each Component

Next list the resistance of every component in the series path. This may include:

Remember that every conductor has some resistance even if it's very small.

• Cable resistance
• Electrical connectors
• Resistors
• Switches
• Lamps
• Motors
• Electronic loads

Step 3 – Calculate the Voltage Drop Across Each Section

Apply the voltage drop formula individually to every component.

Voltage Drop = Current × Resistance

Because current remains constant only the resistance changes from one component to another.

Step 4 – Add All Voltage Drops Together

Once you've calculated each individual voltage drop add them together. The result is the total voltage drop for the entire series circuit. In a simple closed series circuit the sum of the voltage drops across all components equals the source voltage consistent with Kirchhoff's Voltage Law.

Step 5 – Calculate the Percentage Voltage Drop

To evaluate whether the circuit is operating within recommended limits calculate the voltage drop percentage.

Voltage Drop (%) = (Total Voltage Drop ÷ Supply Voltage) × 100

For most electrical installations:

Higher values may indicate that larger conductors or a shorter cable run are needed.

• 3% is the recommended maximum for a branch circuit.
• 5% is generally accepted for the combined feeder and branch circuit.

Worked Example

Let's calculate the voltage drop for a simple DC series circuit.

Given

• Supply Voltage = 12 V
• Circuit Current = 2 A
• Copper Cable 1 = 10 meters total resistance 0.5 Ω
• Connector Resistance = 0.2 Ω
• Copper Cable 2 = 8 meters total resistance 0.4 Ω

Calculate Each Voltage Drop

Cable 1

VD = 2 × 0.5 = 1.0 V

Connector

VD = 2 × 0.2 = 0.4 V

Cable 2

VD = 2 × 0.4 = 0.8 V

Total Voltage Drop

Total VD = 1.0 + 0.4 + 0.8 = 2.2 V

Percentage Voltage Drop

(2.2 ÷ 12) × 100 = 18.3%

This exceeds the commonly recommended 3 to 5% guideline meaning the circuit would likely experience noticeable performance issues. Increasing the conductor size shortening the cable run or reducing the load current would help lower the voltage drop.

Factors That Affect Voltage Drop in a Series Circuit

Several variables determine how much voltage is lost as electricity travels through a series circuit.

Circuit Current

Current has a direct relationship with voltage drop. As current increases voltage drop increases proportionally. For example doubling the current doubles the voltage drop if resistance remains unchanged.

Total Resistance

Higher resistance always creates greater voltage loss. Resistance can come from:

Reducing unnecessary resistance helps improve circuit efficiency.

• Resistors
• Long cable runs
• Electrical connectors
• Switches
• Terminal blocks
• Electrical loads

Cable Length

Longer conductors have greater electrical resistance. This is why long cable runs generally experience larger voltage drops than shorter ones. Whenever possible keeping cable lengths shorter reduces energy loss.

Wire Size

Larger conductors have lower resistance. Using a thicker copper cable is one of the most effective ways to reduce voltage drop especially in higher current applications.

Conductor Material

Different materials have different electrical resistivities. For example:

Because of this aluminum conductors typically experience slightly higher voltage drop than equivalent copper conductors.

• Copper has lower resistance and better conductivity.
• Aluminum has higher resistance for the same conductor size.

Temperature

As conductor temperature increases electrical resistance also increases. This means hot conductors produce slightly greater voltage drop than cool conductors. Temperature becomes especially important in industrial installations, high current systems and long cable runs.

Common Mistakes When Calculating Voltage Drop

Even experienced technicians occasionally make errors when calculating voltage drop in a series circuit. Understanding these common mistakes can help you avoid inaccurate results.

Forgetting to Include Every Component

Many people calculate voltage drop for only the main cable while forgetting smaller resistances such as:

Although each resistance may seem small together they can noticeably increase the total voltage drop.

• Connectors
• Terminal blocks
• Switches
• Inline fuses

Assuming Voltage Is Constant Everywhere

Current remains constant in a series circuit but voltage does not. Each component receives only part of the total supply voltage depending on its resistance. This is one of the most common misconceptions among beginners.

Using the Wrong Resistance Values

Always use the actual resistance of the conductor or component being installed. Resistance varies depending on:

Using incorrect resistance values leads to inaccurate calculations.

• Wire gauge
• Material
• Length
• Temperature

Ignoring Voltage Drop Percentage

Knowing the voltage drop in volts is useful but comparing it with the supply voltage provides a much better understanding of circuit performance. Always calculate the percentage voltage drop before deciding whether the circuit meets recommended electrical standards.

Skipping Real World Verification

Manual calculations are highly accurate when the correct values are used but field conditions can introduce small variations. Professional electricians often confirm calculated results using a digital multimeter measuring the voltage across individual components while the circuit is under load.

Comparing measured values with calculated ones helps verify proper installation and identify hidden problems such as loose connections or damaged conductors.

Series Circuit vs Parallel Circuit

Although both series and parallel circuits follow Ohm's Law they distribute current and voltage differently.

Understanding these differences helps you choose the correct calculation method and avoid applying the wrong formula.

• Tip: If your circuit includes multiple cable runs, connectors or resistors our Voltage Drop Calculator can automatically calculate the total voltage drop, percentage voltage loss and remaining voltage saving time and reducing the risk of manual calculation errors.

Conclusion

Calculating voltage drop in a series circuit is straightforward once you know the circuit current and the resistance of each component. Simply calculate the voltage drop for each section using VD = I × R then add the results to find the total voltage drop.

For faster and more accurate results especially in circuits with multiple components or long cable runs try our Voltage Drop Calculator to instantly calculate voltage drop, percentage loss and remaining voltage.

FAQs

How do you calculate voltage drop in a series circuit?

Calculate the voltage drop across each component using VD = I × R where I is the circuit current and R is the component's resistance. Then add all individual voltage drops together to find the total voltage drop across the series circuit.

What is the voltage drop of a series circuit?

A series circuit doesn't have one fixed voltage drop. Instead the total supply voltage is divided among all components according to their resistance. The sum of all individual voltage drops always equals the supply voltage.

Is voltage drop 3% or 5%?

Both values are commonly used in electrical design.

Keeping voltage drop within these limits helps electrical equipment operate efficiently.

• 3% is generally recommended as the maximum voltage drop for an individual branch circuit.
• 5% is typically accepted as the maximum combined voltage drop for the feeder and branch circuit together.

Do you measure voltage drop in series or parallel?

Voltage drop is always measured across the component being tested by connecting the voltmeter in parallel with that component even if the component itself is part of a series circuit.

Why is voltage divided in a series circuit?

Voltage divides because every resistor, conductor or electrical load consumes part of the electrical energy supplied by the source. Since the same current flows through every component each resistance produces its own voltage drop according to Ohm's Law (V = I × R).

Does every resistor in a series circuit have the same voltage drop?

No. Resistors only have the same voltage drop if they have identical resistance values. When resistances differ the resistor with the greater resistance experiences the larger voltage drop because the circuit current is constant.

Can voltage drop exceed the supply voltage?

No. According to Kirchhoff's Voltage Law the total of all voltage drops in a closed series circuit always equals the supply voltage. If your calculations exceed the supply voltage there is an error in the calculation or measurement.