Earth Cable Size Calculator

Minimum copper protective earth to AS/NZS 3000:2018 Table 5.1, with the fault-current check

How to use

Minimum copper protective earthing conductor for a circuit, to AS/NZS 3000:2018 Table 5.1 and the adiabatic check.

Active Conductors

Construction

Conductor

Active size

Parallel runs

Table 5.1 is read with the total active area (Clause 5.3.3.1.2)

Earth Fault

Insulation

The earth starts the fault at the actives' temperature

Prospective earth fault current

Clearing time

Earth Conductor

Enter the active conductors to size the earth.

How it's calculated

Sizing the protective earth

Two rules, and the larger answer wins: the table minimum, and the size that carries the earth fault current until the device clears it.

Table 5.1 minimum

: total active area across parallel runs, rounded up to the next tabulated size

Adiabatic minimum (Clause 5.3.3.1.1)

: prospective earth fault current, A
: clearing time, s
: copper K, AS/NZS 3008.1.1:2025 Table 5.1

Earth to install

A large earth fault current or a slow device can push the earth above the table size; the result says when it does.

Standards referenced

AS/NZS 3000:2018 Clause 5.3.3.1 and Table 5.1 (size of protective earthing conductors)
AS/NZS 3000:2018 Clause 5.3.3.1.2 (parallel conductors)
AS/NZS 3008.1.1:2025 Clause 5.3, Equation 5.3 and Table 5.1 (K)

Worked example: 2 × 95 mm² single-core submain

A submain of 2 parallel runs of 95 mm² copper X-90 single-core cables, with a 20 kA prospective earth fault current cleared in 0.2 s.

1. Total active area
2. Table 5.1 row
3. Table 5.1 earth
4. K for the earth
5. Adiabatic minimum

The fault needs 62.6 mm², less than the table's 95 mm², so Table 5.1 governs: install a 95 mm² copper earth.

Reference

Minimum earth sizes (Table 5.1)

Copper protective earthing conductor, mm², for each active conductor size.

AS/NZS 3000:2018 Table 5.1 as the calculator reads it
Active, mm²	Earth, copper actives	Earth, aluminium actives
1.5	1.5	–
2.5	2.5	–
4	2.5	–
6	2.5	–
10	4	–
16	6	4
25	6	6
35	10	6
50	16	10
70	25	10
95	25	16
120	35	25
150	50	25
185	70	35
240	95	50
300	120	70
400	120	95
500	120	95
630	120	120

Frequently Asked Questions

Earth Conductor Questions Answered

How do I size an earth cable in Australia?

Start with AS/NZS 3000:2018 Table 5.1, which gives the minimum copper protective earthing conductor for each active conductor size, copper or aluminium. Then check the earth carries the prospective earth fault current for the device's clearing time (Clause 5.3.3.1.1), and that the fault loop impedance lets the device trip in time (Clause 5.7). The largest of the three governs.

How are parallel cables handled?

AS/NZS 3000:2018 Clause 5.3.3.1.2 takes parallel active conductors by their total area. A total that is not a tabulated size reads the next larger row, so 2 × 95 mm² (190 mm²) reads the 240 mm² row and needs a 95 mm² copper earth.

Can the earth be smaller than Table 5.1 if the fault current allows it?

No. Table 5.1 is a minimum. The adiabatic check can only raise the earth size, never lower it.

What K is used for the earth's adiabatic check?

Copper K from AS/NZS 3008.1.1:2025 Table 5.1, starting at the insulation's maximum operating temperature, because an earth core laid up with the actives runs as hot as they do. For a V-75 PVC cable that is 111.2; the 136 in AS/NZS 3000:2018 Clause 5.3.3.1.3 Note 1 is for an earth not run with the actives.

Is the earth size the same for aluminium actives?

No. Table 5.1 has a separate column for aluminium actives; the protective earth is still copper. For example 240 mm² aluminium actives need a 50 mm² copper earth, where 240 mm² copper actives need 95 mm².

Reviewed by Suheib Sherif, electrical engineer at CableHero. The calculator is a design aid: the designer remains responsible for checking the installation against the standards.

Guides

Learn the method

How to use this calculator

Pick the protective earth from the active size, then check it against the fault it must carry.

Read the guide

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Check the Fault Loop Too

A big enough earth for the fault current can still be too long for the device to trip in time. Check the loop impedance next.

Open the Fault Loop Calculator