---
title: "Earthing Cable Size Calculator: AS/NZS 3000 Table 5.1 Earth Conductor Sizing"
url: "https://elecas.com.au/calculator/earth-cable-size"
description: "Free AS/NZS 3000 Table 5.1 earthing cable size calculator: the minimum protective earth conductor for any copper or aluminium active size, rule shown."
updated: "2026-09-14"
standards:
  - "AS/NZS 3000 Table 5.1"
author: "Wisam Tozah (B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus))"
site: "ElecAS"
license: "https://elecas.com.au/terms"
usage: "Cite and link. Do not reproduce or train on this content."
---

# Earthing Cable Size Calculator: AS/NZS 3000 Table 5.1 Earth Conductor Sizing

Source: https://elecas.com.au/calculator/earth-cable-size

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.
- The copper earth is between a quarter and 40 percent of the active area from 16 mm² up (6 mm² for a 25 mm² active, 25 mm² for 70 mm², 120 mm² for 300 mm²), and it stops at 120 mm² for actives of 300 mm² to 630 mm².
- The earth must also pass the adiabatic short-circuit check of Clause 5.3.3.1.3, S = √(I²t) / k, which can force a larger size than Table 5.1.
- Enter the earth-fault current and the device clearing time and the calculator runs that check for you. k comes from AS/NZS 3008.1.1 Table 5.1: 143 for PVC, 176 for XLPE and 166 for rubber when the earth is a separate conductor starting at 30 °C (the AS/NZS 61439.1 Table B.1 figures), or 111.2, 142.9 and 134.0 when it is a core of the same cable starting at its operating temperature. The larger of the two answers governs.
- Inside a switchboard the same formula sizes the protective conductor the external earths land on (AS/NZS 61439.1 Clause 8.4.3.2.3), and a PEN conductor is never smaller than 10 mm² copper or 16 mm² aluminium.

## 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 does

- 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

1. **Pick the active conductor material** — Choose copper or aluminium for the active. Table 5.1 has a column for each, and the aluminium column starts at 16 mm². The earthing conductor is copper.
2. **Enter the active conductor cross-sectional area** — Pick the largest active conductor supplying the part of the installation the earth protects. The calculator reads Table 5.1 for the minimum copper earth.
3. **Enter the earth-fault current and clearing time** — Optional. Enter the r.m.s. earth-fault current through the protective device in kA and its disconnection time in seconds (0.1 s to 5 s). These drive the Clause 5.3.3.1.3 adiabatic check; leave the current at 0 to size by the table alone.
4. **Say how the earth runs and what insulates it** — A separate earthing conductor starts a fault at 30 °C (k = 143 PVC, 176 XLPE, 166 rubber); a core of the same cable starts at its operating temperature (k = 111.2, 142.9, 134.0). Both sets are AS/NZS 3008.1.1 Table 5.1.
5. **Review the governing result** — The calculator displays both the Table 5.1 minimum and the Equation 5.1 result, and selects the larger as governing. Export the branded PDF citing both.

## 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 copper earthing conductor as a function of the largest active conductor supplying the part of the installation being protected (Clause 5.3.3.1.2). The copper earth equals the active up to 2.5 mm², holds at 2.5 mm² for 4 mm² and 6 mm² actives, then falls behind: 4 mm² for a 10 mm² active, 6 mm² for 16 mm² and 25 mm², 10 mm² for 35 mm², 16 mm² for 50 mm², 25 mm² for 70 mm² and 95 mm², and so on up to 120 mm², which serves every active from 300 mm² to 630 mm². An aluminium active reads its own column, which is one to two sizes smaller because aluminium carries less current for the same area.

Table 5.1 is a deemed-to-comply minimum. It does not account for the prospective fault current or the clearing time of the device: for that, Clause 5.3.3.1.3 gives the adiabatic calculation, Equation 5.1, as the alternative to the table.

### When the Clause 5.3.3.1.3 adiabatic check supersedes Table 5.1

If the prospective short-circuit current at the protective device is high and the device clearing time is slow (an upstream MCCB on its thermal element clearing in a few seconds, for example), the Table 5.1 minimum may not absorb the I²t energy without the insulation exceeding its short-circuit temperature limit. Equation 5.1 of Clause 5.3.3.1.3 gives S = √(I²t) / k, where I is the r.m.s. fault current through the device, t its disconnection time (the equation is written for 0.1 s to 5 s) and k a constant for the conductor material and its initial and final temperatures, which AS/NZS 3000 refers to AS/NZS 3008.1.1.

k is where most hand calculations go wrong. AS/NZS 3008.1.1 Table 5.1 tabulates it by the temperature the conductor starts the fault at. A separate earthing conductor carries no load current and starts at 30 °C: k = 143 for PVC, 176 for XLPE, 166 for rubber. An earth core of the same multicore cable lies against loaded actives and starts at their operating temperature, 75 °C for PVC and 90 °C for XLPE, which gives 111.2 and 142.9 respectively, the figures 3008 uses in its own worked example A.9. Using the 30 °C values on a cable core over-rates it by about 20 percent.

The ElecAS calculator applies both Table 5.1 and Equation 5.1 with the k for the run you pick, and reports the governing result. For most LV final subcircuits on fast-clearing MCBs the Table 5.1 minimum dominates; for slow-clearing MCCB-protected submains at a high fault level the adiabatic result frequently governs.

### Aluminium actives, parallel runs and the main earthing conductor

Where the active conductor is aluminium, Table 5.1 has its own column, starting at 16 mm²: the copper earth for a 70 mm² aluminium active is 10 mm² where a 70 mm² copper active needs 25 mm². The earthing conductor itself is always copper in Table 5.1. Where an aluminium earthing conductor is used its size is found by calculation, with the aluminium k (95 for PVC from 30 °C, 116 for XLPE), and it comes out roughly one and a half times the copper figure.

Parallel actives are treated as one conductor of their summed cross-sectional area (Clause 5.3.3.1.2), so a submain of two 240 mm² copper cables per phase reads Table 5.1 at 480 mm² and takes a 120 mm² earth. This calculator takes one active size; the Cable Selection calculator reads Table 5.1 that way for a parallel run.

The main earthing conductor to the electrode is also read from Table 5.1, against the consumer mains, but is then held between 4 mm² and 120 mm² (Clause 5.3.3.2). The MEN connection is sized to the current-carrying capacity of the main neutral, or from Table 5.1 on a switchboard rated 800 A or more (Clause 5.3.5.2).

### 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.

### 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 constant in the adiabatic equation, set by the conductor material and by the temperature the conductor starts and finishes a fault at. AS/NZS 3008.1.1 Table 5.1 tabulates it: for copper with PVC, 143 from 30 °C (a separate earthing conductor) or 111.2 from 75 °C (a core of the same cable); for copper with XLPE, 176 or 142.9.
- **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 (and also 6 mm² for a 25 mm² aluminium 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. Enter the fault current and clearing time and this calculator runs that check beside the table.

### When does the Clause 5.3.3.1.3 adiabatic check govern over Table 5.1?

When the prospective short-circuit current is high and the protective device clearing time is slow (typically an MCCB on its thermal element clearing in 1 s to 5 s). For fast-clearing MCBs (0.1 s or less) the Table 5.1 minimum almost always governs.

### How do I run the short-circuit check on the earth conductor?

Enter the r.m.s. earth-fault current through the protective device (in kA) and its clearing time (in seconds, 0.1 s to 5 s), say whether the earth is a separate conductor or a core of the same cable, and pick its insulation. The calculator works S = √(I²t) / k, rounds up to the next standard copper size and compares it with the Table 5.1 figure; whichever is larger is the size to install. k is from AS/NZS 3008.1.1 Table 5.1: for a separate conductor from 30 °C it is 143 for PVC (160 °C final), 176 for XLPE (250 °C) and 166 for rubber (220 °C), the same as AS/NZS 61439.1 Table B.1; for a core of the same cable it is 111.2 for PVC (from 75 °C), 142.9 for XLPE (from 90 °C) and 134.0 for rubber (from 85 °C). A 10 kA fault cleared in 1 s on a separate PVC earth needs 69.9 mm², so a 70 mm² earth, where Table 5.1 would have allowed 25 mm² on a 70 mm² active; the same fault on a PVC core of the cable needs 89.9 mm², so 95 mm², which is 3008 example A.9.

### Why is the k factor different for an earth core inside the cable?

The adiabatic equation assumes all the fault energy goes into heating the conductor from its initial temperature to the insulation limit. A separate earthing conductor carries no load, so it starts at ambient, 30 °C, and has the full 130 °C of PVC headroom to absorb the fault. A core of a multicore cable sits against actives running at 75 °C, so it starts there and has only 85 °C of headroom, which is why its k drops from 143 to 111.2 and the conductor comes out about 30 percent larger for the same fault. AS/NZS 3008.1.1 Table 5.1 gives both rows.

### 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.

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## About this page

Written and reviewed by Wisam Tozah, Associate Electrical Engineer (B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus)), Sydney, Australia. How the calculations are tested and reviewed: https://elecas.com.au/verification. Profile: https://elecas.com.au/creator.

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