How is the protective earthing conductor size determined?

AS/NZS 3000:2018 Table 5.1 gives the minimum copper earth size for each active conductor size. A separate single-core earthing conductor must be at least 2.5 mm²; an earth core built into a multi-core cable or flexible cord can be as small as 1 mm² (Clause 5.3.3.4).

Can the earthing conductor be smaller than the active?

Yes, and by a long way. From 4 mm² actives upward Table 5.1 already allows a smaller earth (a 4 mm² active takes a 2.5 mm² earth), and on larger cables the earth is roughly a quarter to two-fifths of the active: 50 mm² takes 16 mm², 95 mm² takes 25 mm², 240 mm² takes 95 mm². Table 5.1 is only the minimum. Where fault current is high and the protection is slow, also check the earth against the adiabatic equation in Clause 5.3.3.1.3, which can call for a bigger conductor.

Does the earthing conductor need to be the same material as the active?

Table 5.1 always gives you a copper earth size: its two columns tell you whether the active you are pairing it with is copper or aluminium, and an aluminium active needs a smaller copper earth than the same size in copper. Aluminium earthing conductors are allowed under Clause 5.3.2.1.2, but with conditions: sizes up to 10 mm² must be solid, main earthing conductors must be at least 16 mm², and they must not run underground or in damp situations unless specifically designed for it.

What size earth cable do I need for a 25 mm² active under AS/NZS 3000?

AS/NZS 3000 Table 5.1 requires a 6 mm² copper earth for a 25 mm² copper active. Table 5.1 is only the minimum: on submains with high fault current and slow protection you should also check the earth against the adiabatic equation in Clause 5.3.3.1.3, which can call for a bigger conductor. This calculator gives you the Table 5.1 minimum only.

When does the Appendix B adiabatic check govern over Table 5.1?

When the prospective short-circuit current is high and the protective device clearing time is slow (typically thermal-only MCCB protection clearing in 1–5 seconds). For fast-clearing MCBs (≤100 ms) the Table 5.1 minimum almost always governs.

Can the earth conductor be smaller than the neutral in a TN-C-S installation?

AS/NZS 3000 Table 5.1 sizes the earth (PE) conductor independently of the neutral (N) conductor. The neutral is sized for load current carrying capacity; the earth is sized for fault clearing. In a TN-C-S system the combined PEN conductor must satisfy both: typically the larger of the two governs.

ElecAS

Earthing Cable Size Calculator

Determine minimum protective earthing conductor sizes using AS/NZS 3000 Table 5.1 criteria.

Key facts

Minimum earth conductor sizes come from AS/NZS 3000 Table 5.1, stepping with the active conductor size.
A copper earth is typically half the active conductor area from 16 mm² upwards.
The earth must also pass the adiabatic short-circuit check I²t ≤ k²S², which can force a larger size than Table 5.1.

Who this page is for

Designers checking protective earthing conductor sizing against active conductor and device conditions.

Relevant standards

AS/NZS 3000 Table 5.1

What this tool helps with

Look up protective earthing conductor sizes from active conductor data.
Use the result during final circuit design and compliance review.
Connect earthing outcomes with cable sizing and demand calculations.

How to size an earth conductor under AS/NZS 3000:2018

Enter the active conductor cross-sectional area: Enter the active conductor size in mm². The calculator applies Table 5.1 to give the minimum regulatory earth size.
Pick the active and earth conductor material: Choose copper or aluminium for the active and the earth. The k-factor used in the Appendix B adiabatic check depends on the material combination.
Enter the prospective short-circuit current and clearing time: Enter the prospective short-circuit current at the protective device in kA and the device clearing time in seconds. These drive the adiabatic check.
Pick the insulation type: V-90 PVC (k=115 for Cu, 76 for Al), X-90 XLPE (k=143 for Cu, 94 for Al) or high-temperature insulation. The k-factor enters the adiabatic formula.
Review the governing result: The calculator displays both the Table 5.1 minimum and the Appendix B adiabatic result, and selects the larger as governing. Export the branded PDF citing both clauses.

Earth cable sizing under AS/NZS 3000:2018 Table 5.1

What AS/NZS 3000 Table 5.1 specifies

AS/NZS 3000:2018 Table 5.1 specifies the minimum earth conductor cross-sectional area as a function of the active conductor cross-sectional area for circuits where the earthing conductor is run in the same enclosure or cable as the actives. For active conductors up to 16 mm² the earth conductor must equal the active size; from 16 mm² to 35 mm² the earth must be at least 16 mm²; above 35 mm² the earth must be at least half the active size.

Table 5.1 is the regulatory minimum for residual current device and overcurrent device fault loop performance. It does not account for adiabatic-equivalent sizing under high-prospective-fault-current conditions: for that, AS/NZS 3000 Appendix B Clause B6 provides the adiabatic calculation method.

When Appendix B adiabatic sizing supersedes Table 5.1

If the prospective short-circuit current at the protective device is high and the device clearing time is slow (e.g., upstream MCCB with thermal-only protection clearing in 5 seconds), the Table 5.1 minimum earth size may be insufficient to dissipate the I²t energy without melting the insulation. The Appendix B Clause B6 adiabatic formula gives Smin = √(I²t) / k where I is the prospective fault current, t is the device clearing time and k is the insulation k-factor (115 for PVC / copper, 143 for XLPE / copper).

The ElecAS earth cable sizing calculator applies both Table 5.1 and the Appendix B adiabatic calculation and reports the governing result. For most LV installations with fast-clearing MCBs the Table 5.1 minimum dominates; for slow-clearing MCCB-protected submains the adiabatic result frequently governs.

Aluminium and parallel earth conductors

Where the active conductor is aluminium, AS/NZS 3000 Table 5.1 applies as for copper but the earth-conductor material is typically still copper for thermal reasons. The k-factor for aluminium / PVC is lower (76) and the adiabatic-sized earth in aluminium would be substantially larger than the copper equivalent.

Parallel installations require either (a) a single earth conductor sized for the total fault current per Table 5.1 referenced to the largest single active size, or (b) one earth conductor per parallel run, each sized per Table 5.1 referenced to the individual active size. The ElecAS calculator supports both approaches and flags the AS/NZS 3000 Clause 5.3.3.1.1 requirement.

Reviewed by

Wisam Tozah: Associate Electrical Engineer. B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus). See how these calculations are verified. LinkedIn.

Frequently asked questions

How is the protective earthing conductor size determined?: AS/NZS 3000:2018 Table 5.1 gives the minimum copper earth size for each active conductor size. A separate single-core earthing conductor must be at least 2.5 mm²; an earth core built into a multi-core cable or flexible cord can be as small as 1 mm² (Clause 5.3.3.4).
Can the earthing conductor be smaller than the active?: Yes, and by a long way. From 4 mm² actives upward Table 5.1 already allows a smaller earth (a 4 mm² active takes a 2.5 mm² earth), and on larger cables the earth is roughly a quarter to two-fifths of the active: 50 mm² takes 16 mm², 95 mm² takes 25 mm², 240 mm² takes 95 mm². Table 5.1 is only the minimum. Where fault current is high and the protection is slow, also check the earth against the adiabatic equation in Clause 5.3.3.1.3, which can call for a bigger conductor.
Does the earthing conductor need to be the same material as the active?: Table 5.1 always gives you a copper earth size: its two columns tell you whether the active you are pairing it with is copper or aluminium, and an aluminium active needs a smaller copper earth than the same size in copper. Aluminium earthing conductors are allowed under Clause 5.3.2.1.2, but with conditions: sizes up to 10 mm² must be solid, main earthing conductors must be at least 16 mm², and they must not run underground or in damp situations unless specifically designed for it.
What size earth cable do I need for a 25 mm² active under AS/NZS 3000?: AS/NZS 3000 Table 5.1 requires a 6 mm² copper earth for a 25 mm² copper active. Table 5.1 is only the minimum: on submains with high fault current and slow protection you should also check the earth against the adiabatic equation in Clause 5.3.3.1.3, which can call for a bigger conductor. This calculator gives you the Table 5.1 minimum only.
When does the Appendix B adiabatic check govern over Table 5.1?: When the prospective short-circuit current is high and the protective device clearing time is slow (typically thermal-only MCCB protection clearing in 1–5 seconds). For fast-clearing MCBs (≤100 ms) the Table 5.1 minimum almost always governs.
Can the earth conductor be smaller than the neutral in a TN-C-S installation?: AS/NZS 3000 Table 5.1 sizes the earth (PE) conductor independently of the neutral (N) conductor. The neutral is sized for load current carrying capacity; the earth is sized for fault clearing. In a TN-C-S system the combined PEN conductor must satisfy both: typically the larger of the two governs.