#cablelength

When you’re setting up an electrical system, the size of the cable might look like a simple choice at first glance. But it’s actually influenced by several factors that affect how well your installation performs and how safe it is over time. After all, choosing the right cable isn’t just about picking numbers off a datasheet.

To determine the correct cable size, you need to understand how your electrical load, cable length, installation environment, and system type work together. In this article, you get to learn the clear, technical guidance on what impacts cable size, tailored to your residential or commercial projects, while following Australian standards.

Factors That Affect Cable Size

Understanding the main factors influencing cable size helps you choose the right cable for your electrical needs. Some of these factors include the following!

Electrical Load

One of the primary influences on cable size is the electrical load—the current your cable must safely handle. To determine this accurately, you need to perform a proper load calculation. To start, you need to identify the power consumption of your devices or equipment, measured in watts. Then, calculate the current in amps by using one of these formulas, depending on the system:

For Single-Phase AC Systems:

I = P ( V · PF )

For Three-Phase AC Systems:

I = P (√3 · V · PF ) ​

Where:

I = Current in Amps

P = Power  in Watts

V = Voltage in volts

PF = Power factor, usually between 0 and 1

For example, a 2400-watt appliance operating on a 240-volt supply draws about 10 amps. The cable you choose must safely carry this current continuously without overheating.

Selecting a cable that’s too small risks overheating, potentially causing insulation damage, fire hazards, or equipment failure. Therefore, the cable’s rated current-carrying capacity must meet or exceed the load current calculated. In addition to that, focus on the continuous load rather than just peak or startup currents, as cables heat up based on sustained current flow over time.

Cable Length and Voltage Drop

The longer the cable length, the more electrical resistance it encounters, which causes voltage drop—a reduction in voltage as electricity travels along the cable.

Voltage drop reduces the voltage available to your devices, potentially causing them to underperform or sustain damage over time. To maintain system reliability and comply with standards, the voltage drop is typically limited to a maximum of 5% of the supply voltage.

For example, a cable carrying a 10-amp load over 10 metres might be fine with a 1.5mm² cross-section, but if the run extends to 40 metres, you would need a larger cable, such as 2.5mm² or more, to keep voltage drop within acceptable limits.

To calculate voltage drop accurately, you can use formulas or online calculators. You can also consult AS/NZS 3008, which provides detailed tables and guidelines for various cable types and loads.

Installation Conditions and Cable Grouping

Where and how you install your cable also affects the size you should use. Installation conditions influence how much heat a cable can safely release. If cables are installed in tight spaces, bundled closely, or surrounded by insulation, heat builds up and lowers the amount of current the cable can safely carry.

For example, running multiple cables in a conduit increases heat, and installing cables under floors or in ceiling spaces with poor ventilation limits cooling. Underground cables face different heat conditions depending on soil type and moisture. In these situations, you need to apply derating factors.

These multipliers, specified in AS/NZS 3008.1.1:2017, reduce the cable’s current rating to account for reduced cooling and increased thermal resistance. Derating applies based on:

Installation method (e.g. buried, clipped, enclosed)

Grouping/bundling of cables

Ambient or soil temperature

Cable spacing

To maintain safety and performance, you often need to select a larger cable size when these conditions apply. A cable sizing calculator simplifies this process by automatically applying the correct derating factors based on your selected installation method, ensuring compliance and reducing manual errors.

Cable Type and Insulation Material

Not all cables with the same conductor size have identical current ratings. The type of insulation material covering the conductor affects how much heat the cable can withstand.

Different insulation types affect how much heat a cable can handle and how it's used:

PVC: Standard insulation for general use, rated up to 75°C continuous.

XLPE: Used in higher-load and solar applications, rated up to 90°C continuous and 250°C short-circuit.

EPR: Flexible and durable, rated up to 90°C. Common in industrial settings. 

For solar cables, XLPE or similar high-temperature-rated insulation is typically required to meet performance and durability standards.

Choosing a cable with insulation rated for higher temperatures can allow smaller cable sizes or better performance under harsh conditions. In addition to that, certain applications require cables with extra protection against moisture, UV exposure, or mechanical damage. The right insulation extends cable life and reliability.

Voltage Level and Phase Type (Single vs Three-Phase)

The voltage of your system and whether it uses single-phase or three-phase power significantly impact cable size. In Australia, residential systems usually operate at 230 volts single-phase, while commercial and industrial setups often use 400 volts three-phase.

With three-phase power, the electrical load is divided across three conductors, which lowers the current in each cable. For instance, a 10 kW load on a 230V single-phase supply draws around 43 amps, while the same load on a 400V three-phase system draws only about 14.4 amps per conductor. This difference allows you to use smaller cables in many three-phase installations.

However, knowing how to size cables correctly involves more than just comparing currents. You also need to consider factors like cable length, voltage drop, and environmental conditions. When all elements are properly evaluated, you can select a cable that offers safe performance, long-term reliability, and compliance with Australian electrical standards.

Choose the Right Cable Confidently with CableHero

Selecting the right cable size impacts everything from safety to system efficiency. By considering your electrical load, cable length, installation conditions, cable type, voltage configuration, and future expansion, you can select cables that meet your needs. This also helps ensure your installation complies with regulations.