Current-Carrying Capacity of Cables: How to Calculate for Safety and Compliance

The current-carrying capacity of a cable is calculated using AS/NZS 3008 and is a critical component in guaranteeing the effectiveness and safety of an electrical installation. It describes the highest continuous current a cable can handle under given circumstances without going beyond its temperature rating. The idea of current-carrying capacity will be explored in this part along with a practical instance to help understand it.

Current-Carrying Capacity: Key Factors

The current-carrying capacity of a cable depends on several factors:
Conductor Material
Copper and aluminum are the most common materials. Copper has a higher conductivity than aluminum, allowing for smaller cross-sectional areas for the same current-carrying capacity.
Insulation Type
The type of insulation determines the maximum temperature the cable can withstand. Common insulation materials include PVC, XLPE, and rubber.
Installation Method
How and where the cable is installed affects its ability to dissipate heat. Cables installed in free air can dissipate heat more effectively than those buried underground or enclosed in conduits.
Ambient Temperature
Higher ambient temperatures reduce the cable’s ability to carry current because the temperature difference between the conductor and the surrounding environment is reduced.
Grouping of Cables
When multiple cables are installed close to each other, they generate additional heat, requiring a reduction in their current-carrying capacity.
Derating Factors
These factors account for variations in installation conditions, such as ambient temperature, grouping, and soil thermal resistivity.

Determining Current-Carrying Capacity

Scenario
A 3-phase, 400V electrical system is designed to supply power to a 50-kW motor.
• The motor’s full-load current is approximately 87 A.
• The installation method is in a conduit in free air, and the ambient temperature is 50°C.
• The cable insulation is XLPE (cross-linked polyethylene).
A. Now we will determine the Required Current-Carrying Capacity
First, calculate the full-load current of the motor:
I = P/√3×V×PF
Where:
• P is the motor power (50 kW),
• V is the line voltage (400V),
• PF is the power factor (assumed to be 0.85).
I=50000 /√3×400 V×0.85 ≈ 85 A
B. Select the Base Current-Carrying Capacity as per cable requirements
Refer to Table 14 in AS/NZS 3008.1.1, which provides the current-carrying capacities for various cable types and installation methods. For a 3-core, XLPE-insulated copper cable installed in a conduit spaced in free air, the table provides the following information:
• Base current-carrying capacity at 40°C ambient temperature (Iz): 87 A for a 16 mm² cable.
Current-Carrying Capacity
C. Then we will apply Correction Factors
Since the ambient temperature is 50°C, a correction factor must be applied. From Table 27 in AS/NZS 3008, the derating factor for an ambient temperature of 50°C is 0.88 for a cable conductor suitable up to 90°C in air.
Calculate the adjusted current-carrying capacity:
Iz′= Iz × Correction Factor
Where:
• Iz​ is the base current-carrying capacity at 40°C (87 A),
• The correction factor for 50°C is 0.88.
Iz′ = 87 A×0.88 =76.56 A
This is lower than the required current limit.
Current-Carrying Capacity
D. Evaluate the Cable Size required by selecting the next size available
The adjusted current-carrying capacity (76.56 A) is less than the required full-load current (85 A). Therefore, a larger cable size is needed.
Refer back to Table 14 to select the next larger cable size. A 25 mm² XLPE-insulated copper cable has a base current-carrying capacity of 116 A at 40°C. Applying the same correction factor for 50°C:
Iz′=116 A×0.88= 102.05 A
This capacity (102.05 A) is greater than the required load current (85 A), so a 25 mm² cable is appropriate for this installation.
Current-Carrying Capacity
E. Verification  

Determining the correct current-carrying capacity is a crucial step in cable selection, ensuring that the cable can handle the electrical load without overheating. By following the guidelines provided in AS/NZS 3008.1.1 and applying the necessary correction factors, you can select the appropriate cable size that ensures safe and efficient operation. In this example, the 25 mm² XLPE cable was selected based on its ability to meet the load current requirements while considering the installation conditions and ambient temperature.

   
Tags: