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Minimum cable copper earth sizes from AS/NZS 3000 -with calculated aluminium earth sizes.

The minimum earth sizes for cables in Australia and New Zealand are specified in AS/NZS 3000:2018, section 5.3.3.

According to the standard, the minimum earth size can determined by:

  1. Calculation, or
  2. Selection from Table 5.1 in the standard.

The Cable Size Calculator supports both methods.

Calculation of earth cable sizes

When the earth size is calculated, the cable fault rating and cable loop impedance must be considered.

Selection of earth sizes from Table 5.1

When the earth size is selected, the minimum earth size is determined by the active size.

The minimum earth sizes from Table 5.1 are shown below.

Copper earth sizes are specified for both copper and aluminium active conductors

The standard does not specify aluminium earth sizes.

The aluminium earth sizes in Table 5.1 below have been calculated by jCalc.net, based on the required copper cable impedance and fault rating.

The detailed calculations and background information are covered in Calculation of equivalent aluminium earth conductor sizes for Table 5.1. below.

Table 5.1: Minimum earth conductor sizes from AS/NZS 3008:2018, including calculated aluminium earth sizes.
Active size
mm2 		Copper earth size Aluminium earth sizec
(calculated)
Copper active Aluminium active Aluminium active
1 1 (2.5)a - -
1.5 1.5 (2.5)a - -
2.5 2.5 - -
4 2.5 - -
6 2.5 - -
10 4 - -
16 6 4 16
25 6 6 16
35 10 6 16
50 16 10 25
70 25 10 25
95 25 16 35
120 35 25 50
150 50 25 50
185 70 35 70
240 95 50 95
300 120 70 120
400 ≥120b ≥95b ≥185b
500 ≥120b ≥95b ≥185b
630 ≥120b ≥120b ≥240b
>630 ≥25% of active sizeb ≥25% of active sizeb ≥40% of active sizeb
  1. The smaller size is only applicable for multi-core cables and flexible cords. Excludes lift travelling cables.
  2. A larger earth conductor may be required to comply with:
    1. Fault current rating.
    2. Earth fault loop impedance.
    3. Mechanical strength or other external influences.
    4. Deterioration.
  3. Calculated by jCalc.net based on copper earth impedance and copper fault rating. See section Calculation of equivalent aluminium earth conductor sizes for Table 5.1..

Calculation of equivalent aluminium earth conductor sizes for Table 5.1.

Applicable clauses in AS/NZS 3008:2018

Section 5.3.2.1.2, "Aluminium Conductors," of AS/NZS 3000:2018 states that aluminium earth conductors may be used. It is subject to specific provisions. It does not specify sizes.

Section 5.3.2.1.3, "Other materials," of AS/NZS 3000:2018 states that materials other than copper and aluminium may be used as an earthing conductor if the conductor resistance is not greater than that of the required copper conductor.

Section 5.3.3.1.2, "Selection," of AS/NZS 3000:2018 states that the impedance of an earthing medium (wiring enclosure, cable component, etc.) may not be greater than the impedance of the required copper earthing conductor.

Section 5.3.3.1.1, "General", of AS/NZS 3000:2018 states that the earth conductor must have:

In conclusion, the aluminium earth conductor must have a lower impedance and a higher fault rating than the required copper conductor in Table 5.1.

Calculation

I used the copper earth sizes from Table 5.1 to calculate the equivalent aluminium earth sizes for Table 5.1.

The detailed calculations are shown in Table 2 below.

In Table 2, I selected the smallest aluminium earth cable that has:

The reactances for the calculation are based on Single-core, XLPE cables, in trefoil formation, i.e. Table 30, column no. 4.

The resistances for the calculation is based on the maximum operating temperature for XLPE cables (90°C) i.e. Table 34, column no. 6 (copper) and column no. 12 (aluminium).

Table 2: Aluminium earth size calculations
Aluminium active
Size
mm2		 Copper eartha Aluminium earthb
Size
mm2		 Impedance
Ω/km		 Fault rating
I2t x 106		 Size
mm2		 Impedance
Ω/km		 Fault rating
I2t x 106		 Ratio to active
16 4 5.8815 0.33 16 2.4523 2.29 100%
25 6 3.9319 0.74 16 2.4523 2.29 64%
35 6 3.9319 0.74 16 2.4523 2.29 46%
50 10 2.3328 2.04 25 1.5434 5.58 50%
70 10 2.3328 2.04 25 1.5434 5.58 36%
95 16 1.4738 5.23 35 1.1143 10.94 37%
120 25 0.9326 12.78 50 0.8272 22.33 42%
150 25 0.9326 12.78 50 0.8272 22.33 33%
185 35 0.6752 25.05 70 0.575 43.76 38%
240 50 0.5026 51.12 95 0.4201 80.6 40%
300 70 0.3535 100.2 120 0.3358 128.6 40%
400 95 0.2618 184.55 185 0.2279 305.64 46%
500 95 0.2618 184.55 185 0.2279 305.64 37%
630 120 0.2143 294.47 240 0.1815 514.38 38%
>630 ≥25% - - ≥40% - - 40%
  1. Copper earth sizes from AS/NZS 3000: 2018, Table 5.1.
  2. Aluminium earth sizes are calculated by jCalc.net based on copper earth impedance and copper fault rating.