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What Is Voltage Drop?

Voltage drop occurs whenever current passes through a resistance.

According to Ohm’s Law:

Where:

• V = Voltage (Volts)
• I = Current (Amps)
• R = Resistance (Ohms)

The higher the resistance or current, the greater the voltage drop.

For example:

• Long cables create more resistance
• Thin wires have higher resistance
• Heavy electrical loads draw more current

All of these increase voltage drop in a circuit.

Voltage Drop Across a Resistor

In electronics, voltage drop across a resistor is calculated directly using Ohm’s Law.

The formula is:

This means the voltage across a resistor equals the current flowing through it multiplied by the resistor value.

Example 1: Voltage Drop Across a Single Resistor

Suppose a circuit contains:

• Supply voltage = 12V
• Resistor = 4Ω
• Current = 2A

Using the formula:

The voltage drop across the resistor is 8V.

If the supply voltage is 12V, the remaining voltage in the circuit is:

This follows Kirchhoff’s Voltage Law, where the total voltage drops equal the source voltage.

Voltage Drop in Series Circuits

In a series circuit:

• Current is the same through all resistors
• Voltage is divided between the resistors

Voltage division depends on resistance ratio.

The voltage drop across each resistor depends on its resistance value.

Formula for Total Resistance

Example 2: Voltage Drop in a Series Circuit

Circuit values:

• Supply voltage = 24V
• R₁ = 4Ω
• R₂ = 8Ω

Step 1: Find Total Resistance

Step 2: Find Circuit Current

Step 3: Calculate Voltage Drop Across Each Resistor

For R₁:

For R₂:

Step 4: Verify Total Voltage

The voltage drops add up to the supply voltage.

Voltage Drop in Electrical Wiring

Voltage drop also occurs in cables and conductors because wires have resistance.

Long cable runs and high-current loads can cause noticeable voltage loss between the power source and equipment.

For single-phase circuits, the common formula is:

Where:

• I = Current (A)
• R = Resistance per unit length
• L = One-way cable length

For single-phase two-wire circuits, the factor 2 represents the total conductor length of the outgoing and return paths.

For three-phase systems, different voltage drop formulas are used because current flows through three conductors with phase relationships.

Example 3: Voltage Drop in a Cable

Assume:

• Current = 20A
• Cable resistance = 0.0015Ω/m
• Cable length = 30m

Using the formula:

If the supply voltage is 230V:

The voltage drop is approximately 0.78%.

Voltage Drop Percentage Formula

Voltage drop is often expressed as a percentage of supply voltage.

The formula is:

Many electrical design guidelines commonly recommend keeping voltage drop within approximately 3% for branch circuits and 5% for total circuits, although exact limits depend on local standards and application requirements.

Common Causes of Excessive Voltage Drop

Excessive voltage drop can be caused by several factors within an electrical installation. Common causes include long cable distances, undersized conductors, high current loads, loose or damaged connections, corroded terminals, and poor-quality wiring.

These conditions increase electrical resistance within the circuit, leading to greater voltage loss between the power source and the connected equipment. Reducing circuit resistance or lowering the load current can help minimize voltage drop and improve system performance.

How to Reduce Voltage Drop

Several methods can be used to reduce voltage drop in an electrical installation. Common approaches include using larger cable sizes, shortening cable runs, reducing load current, using higher supply voltages where appropriate, and improving the quality of electrical connections.

Among these methods, proper conductor sizing is one of the most effective ways to control voltage drop and maintain efficient system performance.