Cable Selection is an essential component of electrical installations, particularly in commercial and industrial settings where electrical system dependability and safety are crucial. A thorough set of guidelines for choosing cables for alternating voltages up to and including 0.6/1 kV under typical Australian installation conditions may be found in the Australian/New Zealand Standard AS/NZS 3008.1.1:2017. We will examine the specifics of cable selection, taking voltage drop, short-circuit temperature rise, and current-carrying capacity into account. In addition, we will discuss the significance of derating factors and offer an actual scenario to show how these ideas might be applied.

Cable Selection Process with example

The cable selection process involves several steps to ensure that the chosen cable meets all the necessary criteria for safe and efficient operation.

Determine the Load Current

The first step is to calculate the load current for the circuit, which will dictate the required current-carrying capacity of the cable. This value can be found using the power and voltage of the load.

I=P / √3 ×V×PF   ​

Where:

• P is the power in kW,
• V is the voltage in volts,
• PF is the power factor.

Select the Cable Based on Current-Carrying Capacity

Using the calculated load current, refer to the appropriate table in AS/NZS 3008 (Tables 4 to 15) to select a cable that can handle the current without exceeding its temperature rating. Consider the installation method and apply any necessary derating factors from Tables 22 to 29.

Example

Suppose you need to select a cable for a 50 kW, 400V three-phase motor with a power factor of 0.85. The load current is calculated as:

I=50000/√3×400×0.85 ≈ 85 A

Assume the cable will be installed in a conduit spaced in free air. Referring to Table 14 in AS/NZS 3008, a 3 core 16 mm² XLPE insulated cable can carry 87 A under these conditions.

If the cable is to be installed in a 40 deg C ambient temperature, apply the appropriate derating factor from Table 22.

Check for Voltage Drop

After selecting a cable based on current-carrying capacity, ensure the voltage drop is within permissible limits. Use the mV/A.m values from the appropriate table and calculate the voltage drop using the following formula:

Voltage Drop= I×L×(mV/A.m)

Where:

• L is the route length in meters.

Compare the calculated voltage drop with the maximum allowable voltage drop (typically 5% of the nominal voltage).

Example (Table 42)

If the route length is 50 meters, and the mV/A.m value for the selected cable is 2.55 mV/A.m, the voltage drop would be:

Voltage Drop=85×50×2.55=10837 mV=10.83 V

This is 2.709% of 400V, which is within the permissible limit.

Verify Short-Circuit Performance

Finally, calculate the short-circuit current and duration, and verify that the selected cable can handle the thermal energy produced during a fault condition. Use the formula provided in the standard and ensure the cable size meets the short-circuit temperature rise requirements.

Importance of Derating Factors

Derating is necessary to ensure that cables operate within safe temperature limits, considering various external factors such as ambient temperature, soil thermal resistivity, and the proximity of other cables. Without proper derating, cables may overheat, leading to insulation failure, reduced lifespan, and increased risk of fire.

Factors Affecting Derating

Ambient Temperature

Higher ambient temperatures reduce the cable’s current-carrying capacity. The standard provides derating factors in Table 27.

Soil Thermal Resistivity

For buried cables, soil with high thermal resistivity can trap heat, necessitating a reduction in current-carrying capacity (see Table 29).

Cable Grouping

Cables installed close together or in the same conduit generate more heat, requiring a reduction in their current-carrying capacity (see Tables 22 to 26).