How to Determine Voltage Drop in Extended Cable Runs and High-Load Applications

Voltage drop is inherent in all electrical systems. In fact, its impact becomes more pronounced in extended cable runs and high-load applications. Excessive voltage drop can lead to inefficient system performance, overheating conductors, malfunctioning connected equipment, and, in severe cases, non-compliance with electrical safety standards.

That said, knowing how to determine voltage drop is important for installations involving long distances or substantial current demand. This article explains the regulatory standards and practical design considerations of this electrical issue.

Fundamental Concepts of Voltage Drop

Voltage drop is primarily governed by the conductor's resistance and reactance, the magnitude of current and the length of the run. In simple terms, it is calculated using this specific formula:

Voltage Drop single phase (V) = Current (I) × Resistance x the (L) Cable Length

In more complex three-phase systems or those involving inductive loads, impedance (including reactance) and power factor must also be considered. That said, minimising voltage drop is important for system efficiency and preventing excessive losses.

In addition, voltage drop must be assessed following AS NZS 3000, which specifies a maximum allowable voltage drop of 5% for final subcircuits to ensure operational safety and performance.

Additionally, AS/NZS 3008 provides the necessary resistance for various conductor types, sizes, and installation conditions, forming the basis for accurate voltage drop calculations.

Cable Selection Considerations

Read more below to understand how cable size, material, and installation conditions affect voltage drop and compliance with AS/NZS standards.

Impact of Cable Size and Material

The resistance of a conductor is inversely proportional to its cross-sectional area as part of how to determine voltage drop. Larger conductors exhibit lower resistance, thereby reducing voltage drop. In extended runs, upsizing the conductor is a commonly adopted mitigation strategy.

The conductor material also plays a role. For instance, copper has a lower resistivity than aluminium, making it more effective at reducing voltage drop over long distances. However, aluminium is often selected for cost or weight considerations, requiring careful compensation through increased sizing.

Environmental and Installation Conditions

The installation environment directly influences conductor performance. As detailed in AS/NZS 3008, derating factors must be applied for ambient temperature, cable grouping, and installation method (e.g., buried, conduit, free air). These factors affect the conductor’s effective resistance and, subsequently, the total voltage drop in the system.

High-Load Scenarios and Mitigation Techniques

With the following scenarios and techniques, learn how to identify high-load conditions and apply targeted strategies to effectively reduce voltage drop.

Identifying High-Load Conditions

High-load applications are typically characterised by continuous current flow or substantial startup inrush currents. Examples include industrial machinery with high-duty cycles, HVAC systems with large compressors and electric vehicle charging stations operating at maximum output.

Such conditions elevate the potential for voltage drop and require more rigorous cable and protective device selection, which contributes to how to determine voltage drop.

Strategies to Minimise Voltage Drop

When faced with high-load and long-distance requirements, several design strategies can be applied:

• Using parallel conductors to divide the load and reduce individual cable resistance
• Minimising cable length wherever possible through layout optimisation
• Selecting larger conductors to compensate for extended runs
• Choosing low-resistance cable types, especially in aluminium installations where additional size is necessary

These measures reduce voltage drop and ensure circuits remain within allowable performance margins.

Use of Online Tools for Compliance and Accuracy

Many tools, such as a voltage drop calculator from CableHero, can support accurate and standards-compliant design. It incorporates calculation methods based on AS NZS 3000 and AS NZS 3008. The tool factors in current, cable type, material, installation method, circuit length, and power factor offer a precise and compliant voltage drop estimate for single-phase and three-phase systems.

The tool automatically applies the correct correction factors and recommends appropriate conductor sizes for high-load or extended cable run applications. This level of automation ensures alignment with regulatory requirements while improving design reliability.

Common Design Challenges and Avoidance Tips

Several recurring issues can compromise voltage drop calculations, such as:

• Incorrect resistance values: Using nominal values instead of corrected figures for installation conditions can skew results
• Omitting environmental factors: Neglecting temperature or grouping adjustments can lead to under-sizing
• Cost-driven under-sizing: Reducing conductor size to save on material costs may cause non-compliance or equipment failure

To avoid these issues, designers should consistently refer to AS/NZS standards, apply all relevant correction factors, and verify calculations using a validated tool.

Achieve Advanced Voltage Drop Calculations with CableHero!

Learning how to accurately determine voltage drop in extended cable runs and high-load applications ensures the safety, reliability, and compliance of electrical installations. That said, applying the correct calculation methods, considering environmental influences, and adhering to AS NZS 3000 and AS/NZS 3008 standards is essential for achieving optimal performance.

For professionals seeking accurate results and greater design confidence, tools like the CableHero voltage drop calculator provide a practical, standards-aligned solution for simple and complex installations.

To learn more about our online calculator, visit the CableHero website today!