ElecAS
Earthing Cable Size Calculator: AS/NZS 3000 Table 5.1 Earth Conductor Sizing
Determine minimum protective earthing conductor sizes using AS/NZS 3000 Table 5.1 criteria.
Overview
The protective earthing conductor is sized from the active conductor it runs with, and AS/NZS 3000 Table 5.1 sets the minimum. Enter the active conductor cross-sectional area and this calculator returns the minimum earthing conductor size Table 5.1 requires for it, so the earth is sized from the standard rather than from habit or from whatever was on the last job.
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.
Table 5.1 at a glance
Note the shape of the copper column: it tracks the active size up to 2.5 mm², then falls behind it, and above 300 mm² it stops rising at 120 mm² altogether. Beyond 630 mm² the table runs out and the earth is sized by calculation instead.
Read this as a floor rather than an answer. Table 5.1 is a deemed-to-comply minimum based on the active conductor size alone: it knows nothing about the prospective fault current at the point of installation or how long the protective device takes to clear it, which are the two quantities that actually determine whether the earth survives a fault.
Always confirm the tabulated size against the adiabatic check, and against the earth fault loop impedance the circuit needs to achieve its required disconnection time. Both can force a larger conductor than the table asks for.
| Active conductor (mm²) | Earth with Cu active (mm²) | Earth with Al active (mm²) |
|---|---|---|
| 1 | 1 | — |
| 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 |
Why the earth is often larger than Table 5.1 requires
Three things commonly push the protective earthing conductor above the tabulated minimum. The first is the adiabatic check: on an installation with a high prospective fault current, or a device with a slow clearing time, I²t can exceed k²S² for the Table 5.1 size and force the next size up.
The second is earth fault loop impedance. The loop includes the earthing conductor, so on a long circuit a larger earth lowers Zs, raises the fault current and brings the disconnection time back inside the AS/NZS 3000 limit. Where a circuit only just fails its Zs check, increasing the earth is often the cheaper remedy than increasing the active.
The third is simply the installation: a combined active-and-earth cable is supplied with a fixed earth core, so the available size is whatever the manufacturer builds. Where that core is smaller than the checks require, the cable has to change rather than the earth.
Key terms
Protective earthing conductor (PE)
- The conductor connecting exposed conductive parts to the main earthing terminal, so a fault current has a path back to the source and the protective device operates.
Table 5.1
- The AS/NZS 3000 table giving the minimum protective earthing conductor size for a given active conductor cross-sectional area. It is a minimum, not necessarily a sufficient size.
Adiabatic equation (I²t ≤ k²S²)
- The short-circuit thermal withstand check: the fault energy the conductor must survive against what its cross-section can absorb before its insulation is damaged. It assumes no heat escapes during the fault, which is why it is called adiabatic.
k factor
- The material and insulation constant in the adiabatic equation, from AS/NZS 3008.1.1 Table 5.1: for example 111.2 for copper with PVC and 142.9 for copper with XLPE.
Earth fault loop impedance (Zs)
- The total impedance of the fault current path, including the earthing conductor. A larger earth conductor lowers Zs, which raises the fault current and shortens the disconnection time.
MEN system
- Multiple Earthed Neutral, the Australian earthing arrangement in which the neutral is earthed at the supply and again at each installation, and which the Table 5.1 sizing assumes.
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.