Selecting the correct cable size for commercial and industrial electrical installations helps ensure safety, performance, and compliance with Australian standards. By using an undersized cable, it can lead to overheating, equipment damage, or energy inefficiency. For large-scale electrical systems, careful cable sizing supports the long-term reliability and efficiency of your operations.
In this guide, it outlines how to select the appropriate size of cables, covering current calculations, environmental factors, and industry standards, particularly AS/NZS 3008.1.1:2017.
What Affects Cable Sizing
Knowing how to size cables properly starts with understanding the key electrical and environmental variables that affect cable performance.
Current-Carrying Capacity
Cables must handle the expected electrical load without overheating. If the current is too high, the cable insulation may overheat and degrade, leading to damage or fire risk.
Voltage Drop
Some voltage is lost as current flows through a cable. Too much drop can cause equipment to underperform or fail. In Australia, the allowable drop is up to 5%.
Short-Circuit Performance
Cables must withstand the intense heat and force during a short circuit until the safety switch or breaker cuts power. The cable should not melt or break under this stress.
Installation Environment
Heat, moisture, grouping with other cables, and how the cable is installed (in air, underground, etc.) all affect its performance. Harsh conditions often require a larger or specially insulated cable.
Step-by-Step Guide to Cable Sizing
Accurate cable sizing involves a series of calculations and checks. The following steps help guide you through the process, using AS/NZS 3008.1.1:2017.
Step 1: Determine Load Current
Start by calculating the expected current. For three-phase loads, use the formula:
Where:
- I = current in amperes
- P = power in watts
- V = voltage
- PF = power factor, usually around 0.85 for motors
Example: A 50 kW motor operating at 400V with a power factor of 0.85:
This becomes your starting point for selecting the appropriate cable size.
Step 2: Evaluate Installation Conditions
Different installation methods affect how heat is dissipated. For example:
- Cables in the air generally cool better than cables buried underground.
- Higher ambient temperatures reduce a cable’s current-carrying capacity.
- Multiple cables grouped together generate additional heat.
Use the tables in AS/NZS 3008.1.1 to apply the correct derating factors for these conditions. This adjustment gives you a more accurate idea of the current requirement.
Step 3: Select the Cable Size Based on Ampacity
After applying correction factors, refer to the ampacity tables in the standard to find a cable that supports the adjusted current. Make sure the selected cable size supports the load under your specific environmental and installation conditions.
- For single-core cables unenclosed in air: see Table 4
- For single-core cables in conduit in air: see Table 7
- For multicore cables unenclosed in air: see Table 10
- For multicore cables in conduit in air: see Table 13
- For direct-buried single-core cables: see Table 17
- For direct-buried multicore cables: see Table 21
Step 4: Calculate Voltage Drop
Voltage drop occurs when current flows through a cable and loses energy due to resistance and reactance. If the voltage drop is too high, equipment may underperform, overheat, or fail prematurely.
That’s why it’s important to calculate voltage drop early in the design process and ensure it remains within the limits set by Australian standards. You can calculate voltage drop using one of two formulas, depending on the system type:
For Single-Phase AC Systems:
For Three-Phase AC Systems:
Where
- Vd = voltage drop (volts)
- I = current (amps)
- R = resistance of the cable (ohms per metre)
- X = reactance of the cable (ohms per metre)
- cos⏀ = power factor of the load
- L = one-way cable length (metres)
To apply these formulas, determine the current load, the cable’s physical and electrical properties, and the actual installation length. The values of RRR and XXX are available in AS/NZS 3008.1.1:2017, depending on cable type, conductor material, and installation method.
Use:
- Table 35 to Table 50 for copper conductors
- Table 51 to Table 52 for aluminium conductors
These tables provide the R and X values (Ω/km), which you convert to Ω/m by dividing by 1000.
Use the equipment’s actual power factor when available (e.g., 0.95 for lighting, 0.8 for motors), or apply a typical assumed value if none is specified. With CableHero, these values are built-in, ensuring precise voltage drop calculations without the need to manually refer to the tables.
Step 5: Confirm Short-Circuit Capability
Finally, check the cable’s ability to withstand short-circuit conditions. This ensures the conductor won’t be damaged by the heat generated during a fault before the protective device operates.
Short-circuit withstand is calculated using the formula from AS/NZS 3008.1.1:2017, Clause 9.3:
Where
- S = minimum cross-sectional area of the conductor (mm2)
- I𝓀 = prospective fault current (A)
- t = disconnection time of the protective device (seconds)
- 𝓀 = material constanct (e.g., 115 for copper/PVC; refer to Table 52 of the standard)
The value of k depends on the conductor material, insulation type, and permissible temperature rise — all provided in Table 52 of AS/NZS 3008.1.1.
If the selected cable’s actual size is greater than or equal to the calculated minimum SSS, it’s suitable for short-circuit conditions.
Practical Considerations in Cable Selection
When planning a commercial or industrial electrical installation, several practical factors influence how to size cables. By considering these early in the design process, you can make better decisions and avoid costly corrections.
Material Choice
Copper and aluminium are the most common conductor materials. Copper offers better conductivity and flexibility, while aluminium is lighter and more cost-effective. Choose based on your load requirements, installation conditions, and budget.
Environmental Factors
Temperature, humidity, exposure to sunlight, and the presence of chemicals or water can affect cable performance. For example, cables in hot or damp environments may require larger sizes or specialised insulation to maintain safety and durability.
Mechanical Protection
Consider whether the cable will face physical stress, such as from foot traffic, machinery, or abrasion. In these cases, select cables with added protection like armoured sheathing or install them in conduits for extra resilience.
Future Expansion
Oversizing cables slightly can accommodate future increases in load without requiring major upgrades. If you expect the system to grow, plan cable sizes accordingly during the initial design. To simplify the process, you can also use a cable calculator to estimate the correct size based on load, length, and conditions.
Take the Guesswork Out of Cable Sizing
Choosing the correct cable size supports safe and efficient operation in commercial and industrial environments. By understanding how to size cables, it helps you select a conductor that can handle the expected load, minimise voltage drop, and withstand fault conditions. Finally, proper cable sizing ensures that your system performs as expected and complies with AS/NZS 3008.1.1:2017.
To simplify the process, you can use CableHero’s cable sizing calculator—a convenient tool that helps you estimate the right cable size based on your installation’s specific conditions. For more information on our innovative tools, visit our website today!
Frequently Asked Questions (FAQs)
Here are answers to common questions about using a cable sizing calculator for commercial and industrial electrical installations:
How to determine cable size visually?
To visually determine a cable size, check printed markings on the cable sheath, which usually indicate the conductor size. If markings are missing, use a wire gauge tool for measurement. However, a reliable cable size calculator is still recommended for better accuracy and efficiency.
Does cable length always affect voltage drop?
Yes. The longer the cable, the more resistance it introduces, which leads to a higher voltage drop. This is why length must be included when using a cable sizing calculator.
