Voltage drop calculations are essential for designing safe, efficient overhead electrical lines following IEC standards. IEC 60364-5-52 specifies maximum voltage drops between the installation origin and load points for proper equipment operation.

Voltage Drop Calculation Formulae

1. Single-Phase Circuit (AC or DC)

For a single-phase circuit, the voltage drop (V) can be calculated using:

• I: Current in amperes (A)
• R: Resistance of the conductor in ohms per kilometer (Ω/km)
• L: Length of the conductor in kilometers (km)

2. Three-Phase Circuit

For a three-phase circuit, the voltage drop (V) is:

• I: Current in amperes (A)
• R: Resistance of the conductor in ohms per kilometer (Ω/km)
• L: Length of the conductor in kilometers (km)

These formulae are derived from Ohm’s Law, considering the resistive properties of the conductor. The factor of 2 in the single-phase formula accounts for the round trip of the current, while the factor of √3 in the three-phase formula adjusts for the phase difference.

Common Values and Parameters

Conductor Materials and Resistances

These values are essential for calculating the resistance per unit length of the conductor.

Standard Voltage Levels

IEC 60038 specifies these standard voltages for electrical systems.

Practical Examples

Example 1: Single-Phase Residential Circuit

Given:

• Current (I) = 20 A
• Conductor Resistance (R) = 0.000000017 Ω·mm²/m
• Length (L) = 100 m
• Conductor Cross-Sectional Area (A) = 2.5 mm²

Calculation:

Analysis:

The calculated voltage drop is minimal, indicating efficient power delivery for the residential application.

Example 2: Three-Phase Industrial Circuit

Given:

• Current (I) = 50 A
• Conductor Resistance (R) = 0.000000017 Ω·mm²/m
• Length (L) = 200 m
• Conductor Cross-Sectional Area (A) = 10 mm²

Calculation:

Analysis:

The voltage drop is within acceptable limits for industrial applications, ensuring the proper functioning of equipment.

Best Practices for Voltage Drop Management

• Conductor Selection: Choose materials with low resistivity, such as copper, to minimize voltage drop.
• Appropriate Sizing: Select conductor sizes that can handle the expected current without excessive voltage drop.
• Shorter Distances: Minimize the length of the conductor to reduce resistance and voltage drop.
• Regular Maintenance: Inspect and maintain overhead lines to ensure optimal performance and safety.

Understanding Voltage Drop in Overhead Lines

Voltage drop occurs when electrical energy is lost as current flows through a conductor due to its inherent resistance and reactance. In overhead lines, this phenomenon is influenced by:

• Conductor material: Copper and aluminum are the most common. Copper has lower resistivity, leading to smaller voltage drops. Aluminum is lighter and cheaper but results in higher drops for the same current.
• Line length: Longer lines naturally experience higher voltage drop. Designing for minimum distance between the power source and load reduces losses.
• Current magnitude: Higher currents increase voltage drop proportionally. Understanding peak and continuous current is essential.
• Conductor cross-section: Larger cross-sectional areas reduce resistance, thus reducing voltage drop.

IEC standards recommend maximum allowable voltage drops, typically 3–5% for low voltage systems and up to 7% for industrial installations, to ensure equipment operates efficiently.

Typical Parameters for Voltage Drop Calculators

When using a Voltage Drop Calculator for Overhead Lines, engineers must input several key parameters. Commonly used values include:

These values allow a calculator to provide realistic and practical voltage drop predictions.

Practical Application Examples

Example 1: Residential Feeder Line

A residential neighborhood is fed from a transformer 500 m away. The line uses copper conductors of 16 mm², supplying multiple houses with a total load of 120 A.

Using standard voltage drop guidelines:

• Voltage drop is calculated per phase.
• The system is designed to remain below 3% drop, ensuring household appliances operate normally.
• The calculator helps engineers confirm conductor sizing or recommend using a larger cross-section if the drop exceeds limits.

Outcome: Proper sizing ensures lights and motors maintain performance without exceeding voltage drop standards.

Example 2: Industrial Plant Supply

An industrial plant requires a three-phase 11 kV supply over 2 km using aluminum conductors of 70 mm². Peak load is 250 A per phase, with a power factor of 0.9.

The voltage drop calculator allows:

• Determining whether conductors meet IEC standards for maximum 5% drop.
• Adjusting conductor size or adding parallel lines if voltage drop exceeds limits.
• Evaluating impact of load factor and operating hours on energy efficiency.

Outcome: Voltage drop remains within safe limits, preventing excessive energy losses and avoiding equipment malfunctions.

Key Considerations for Overhead Line Design

1. Material Choice: Copper is preferred for high-performance lines; aluminum is suitable for budget-conscious installations.
2. Line Routing: Minimizing line length reduces drop and energy losses.
3. Load Balancing: Proper phase load distribution reduces excessive drops in one phase.
4. Power Factor Correction: Using capacitors can reduce reactive losses and improve voltage stability.
5. Environmental Factors: Temperature, wind, and weather conditions can slightly affect resistance and voltage drop over time.
6. Periodic Verification: Using calculators periodically ensures that expansions or added loads do not exceed voltage drop limits.

Benefits of Using a Voltage Drop Calculator

• Accurate sizing of conductors and materials.
• Compliance with IEC standards, ensuring safety and reliability.
• Cost optimization, avoiding over-sizing or under-sizing conductors.
• Energy efficiency, reducing losses over long distances.
• Simplified planning for both residential and industrial overhead lines.