Comprehensive Jemma Tech Guide to Voltage Drop Calculation in Electrical Power Systems in 2025

Introduction

Voltage Drop

Understanding voltage drop is essential for anyone working with electrical systems, from engineers and electricians to system designers. This article provides a complete, beginner-friendly explanation of voltage drop in power systems, its implications on different electrical components, how to calculate it manually, and practical ways to reduce it.

What is Voltage Drop?

Voltage drop occurs when electrical current travels through a conductor such as a wire or cable. Every conductor has inherent resistance and inductance, which together create impedance. As current flows, this impedance leads to a reduction in voltage from the power source to the load.

For instance, if the supply voltage is 120V AC and only 112V AC reaches the load, the voltage drop is 8 volts. This voltage loss is due to the resistance and reactance of the connecting cable.

How Voltage Drop Affects Electrical Systems

1. Impact on Electric Motors

Voltage drop significantly affects electric motor performance. The torque of a motor is directly proportional to the square of its supply voltage. So, a drop in voltage causes a substantial decrease in starting and running torque, which can lead to inefficiencies or even failure in starting the motor.

2. Impact on Lighting Systems

In lighting applications, voltage drops reduce light output, leading to dimmer lights and lower efficiency. Over time, this may affect both visibility and the lifespan of lighting equipment.

3. Impact on Sensitive Electronics

Electronic devices are particularly sensitive to voltage variations. Even small drops can lead to malfunctioning or permanent damage, especially in precision equipment or microprocessor-based systems.

Acceptable Voltage Drop Limits

The International Electrotechnical Commission (IEC) provides standard guidelines for permissible voltage drops:

Application Type	Public Supply (%)	Private Supply (%)
Lighting Circuits	3%	6%
General Use Circuits	5%	8%

Typically, a voltage drop of less than 5% is considered acceptable for most applications.

Exceptions to IEC Standards

Certain conditions allow for greater voltage drops:

• During the starting phase of electric motors
• For equipment with high inrush currents like transformers

Temporary voltage transients due to switching operations or abnormal system behavior are also excluded from these restrictions, provided they don’t exceed the equipment’s voltage tolerance specifications.

How to Calculate Voltage Drop Manually

To manually calculate voltage drop, follow these steps:

1. Get Voltage Drop per Unit Length from Cable Catalog

Cable manufacturers provide data such as voltage drop (in millivolts per ampere per meter). For example, for a 3×70 mm² copper cable with PVC insulation, the voltage drop might be 0.524 mV/A/m.

2. Convert and Calculate Total Voltage Drop

Convert millivolts to volts:

0.524 mV = 0.524 × 10⁻³ V

Now calculate:

Voltage Drop = Drop per Unit × Current × Length

Example:

• Cable: 3×70 mm² copper
• Length: 120 meters
• Load: 75 HP motor (approx. 112.5A using power factor 0.8)
• Voltage Drop = 0.524 × 10⁻³ × 112.5 × 120 = 7.074 volts

3. Find the Percentage Voltage Drop

For a 3-phase system:

% Voltage Drop = (Voltage Drop / Line Voltage) × 100

Assuming line voltage is 380V:

% Voltage Drop = (7.074 / 380) × 100 ≈ 1.86%

This is within the 5% limit, making the design acceptable.

Tips to Reduce Voltage Drop

1. Increase Cable Cross-sectional Area

Larger cable diameters reduce resistance, lowering voltage drop. This is the most common and effective method.

2. Use Parallel Conductors

Installing multiple smaller cables in parallel reduces the total impedance, achieving a similar effect as increasing conductor size.

3. Reduce Cable Length

Shorter distances mean lower resistance and reactance. Design your layout to minimize the length between the power source and the load.

4. Improve Power Factor

Adding capacitor banks for power factor correction reduces the reactive power component (inductive reactance), leading to a lower overall impedance and therefore less voltage drop.

Real-Life Example

Scenario:

• Supply Voltage: 400V (3-phase)
• Load: 75 HP motor
• Cable: 3×70 mm² copper + 1×35 mm² neutral, 120 meters long
• Voltage Drop: 7.074V
• % Voltage Drop: 1.86%

The system is operating within the 5% permissible range, making it compliant with IEC standards.

Conclusion

Voltage drop may seem like a minor issue, but it plays a critical role in the efficiency, safety, and longevity of your electrical systems. Knowing how to calculate and mitigate voltage drop manually gives you the confidence to design more reliable installations.

Always ensure voltage drop stays within regulatory limits, and consider professional-grade tools or simulation software like ETAP for complex networks. However, the manual calculation approach is invaluable for quick assessments and educational purposes.

Key Takeaways:

• Voltage drop results from conductor impedance.
• It affects motor torque, lighting brightness, and electronic stability.
• Use cable catalogs, load currents, and distances for manual calculation.
• Keep voltage drop under 5% (public) or 8% (private).
• Reduce it by increasing conductor size, using parallel cables, reducing length, and improving power factor.