Earth Conductor Sizing

Parameters

Every field on the form, what it controls, the range accepted, and where each one stops applying.

Calculation methodBoth, Table 5.1 or adiabatic

Two completely different routes to an answer. The Table 5.1 lookup reads a minimum earth size straight off the active conductor size and ignores fault current entirely. The adiabatic equation sizes the conductor from the energy it has to survive, so it needs a fault current and a disconnection time. Both is the default: it runs the two side by side, shows each result, marks which one governs, and returns the larger. You no longer have to pick a method before you know which one is more demanding. The single-method settings are still there for when you want one answer on its own.

Active conductor sizesquare millimetres, 1 to 300

The cross-sectional area of one active conductor in the circuit being earthed. Standard sizes only: 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240 and 300. Under the Table 5.1 method this is the lookup key and it fully determines the answer. Under the adiabatic method it is carried through to the results for reference and is used for the minimum-size check, but it does not change the calculated size.

Conductor materialcopper or aluminium

Under the Table 5.1 method, aluminium takes the next standard size above the copper answer. Under the adiabatic method it sets the reference material constant k, 115 for copper and 76 for aluminium at the reference rise of 75 to 160 degrees Celsius, and it also sets the temperature constant the calculator uses to rescale k when you change the temperatures (234.5 for copper, 228 for aluminium). Because k sits on the bottom of the equation, aluminium always lands on a larger conductor for the same fault. Most Australian installations use copper.

Fault currentamps, 1 to 100000, adiabatic method only

The prospective earth fault current at the point of the fault, not the protective device rating. Get it from a loop impedance test at the board, from the switchboard fault-level schedule, or from a fault study. Entering a low figure produces a small conductor, so an optimistic number here is the single easiest way to undersize an earth.

Disconnection timeseconds, 0.001 to 10, adiabatic method only

The time the protective device actually takes to clear that fault current, read off the device time-current curve. It is not the AS/NZS 3000 maximum disconnection time. A device in its instantaneous region may clear in 0.01 seconds, which produces a far smaller conductor than the 0.4 second limit would. Using the limit rather than the real clearing time is conservative but expensive. This field is disabled and replaced by a derived value whenever you select a protective device below.

Parallel earth conductors1 to 4, advanced, both methods

How many earth conductors run in parallel between the same two points. The calculator assumes they share the fault current evenly, so each one is sized for its own share: the required area is divided by the count before rounding up to a standard size. The headline result becomes the size of one conductor, and the total earth area is reported alongside it. Leave it at 1 for the normal single-conductor case. Above 1 the calculator warns you that the conductors must match in material, size, length and route, because unequal sharing puts more current through one of them than the arithmetic assumes.

Installation conditionfive conditions, advanced, adiabatic method only

How the earth conductor is actually run, which is what fixes the temperatures the adiabatic equation is allowed to work between. Insulated and laid up in a cable or bunched with other conductors is the default and the most restrictive: it starts at the conductor operating temperature and stops at the insulation limit. Insulated but not incorporated in a cable and not bunched starts cold at 30 degrees Celsius, so it tolerates far more energy for the same size. Bare and in contact with a cable sheath is limited by the sheath rather than by any insulation. Bare and clear of combustible material has the highest limit of all, and bare in a fire risk location has the lowest of the bare cases. Changing this field moves the temperatures, which moves k, which moves the size.

InsulationPVC, XLPE or EPR, advanced, adiabatic method only

The insulation on the earth conductor. PVC runs at 75 degrees Celsius and is limited to 160, XLPE and EPR run at 90 and are limited to 250, so a cross-linked conductor gets a larger k and a smaller minimum size. The field is disabled for the bare installation conditions, where the limit comes from the surroundings and the insulation is irrelevant.

Set the temperatures manuallycheckbox, advanced, adiabatic method only

Off by default, in which case the initial and final temperatures come from the installation condition and insulation you selected and are shown in the results panel. Turn it on when you have specific figures from a cable data sheet or a design brief, and the two fields below override the condition defaults.

Initial temperaturedegrees Celsius, 0 to 200, advanced, adiabatic method only

The conductor temperature at the instant the fault starts, normally the operating temperature of the insulation. It genuinely feeds k: the calculator rescales the material constant by the ratio of the temperature terms, so a conductor already sitting at 90 degrees Celsius has less thermal headroom and lands on a larger size than one starting at 30.

Final temperaturedegrees Celsius, 50 to 700, advanced, adiabatic method only

The highest temperature the conductor and its surroundings may reach at the end of the fault, typically 160 degrees Celsius for a PVC-insulated conductor and up to 500 for a bare conductor clear of anything combustible. It feeds k the same way the initial temperature does, in the opposite direction: a higher permitted final temperature gives a larger k and a smaller conductor. It must be greater than the initial temperature.

Protective deviceMCB type B, C or D, MCCB, gG fuse, or direct entry, advanced

Select a device and the calculator derives the disconnection time instead of taking it from the field above. For circuit breakers it compares the fault current against the magnetic threshold, which is 5 times rating for a type B, 10 for a type C or an MCCB, and 20 for a type D. Above the threshold the device is in its instantaneous region and clears in 0.01 seconds, 0.02 for an MCCB. Below it the device is in its thermal region, which a rating alone cannot model, so the calculator falls back to the maximum disconnection time for the circuit and says so in the working. A gG fuse uses an indicative inverse curve anchored at 5 seconds for a fault of 5 times rating. Leave it on direct entry to keep using your own number.

Device ratingamps, 6 to 400, advanced, shown once a device is selected

The nominal rating of the upstream protective device. It sets the magnetic threshold or the fuse curve, and it also selects the maximum disconnection time the result is checked against: 0.4 seconds at 63 amps and below, 5 seconds above that. A device that cannot reach its instantaneous region at the fault current you entered will fail that check.

Assumptions and limits

The adiabatic equation is simple. The judgement about which inputs to feed it is not. Here is exactly what this implementation does and does not do.

What the calculator assumes

• The Table 5.1 values used here are indicative. They are a working set of active-to-earth pairings, not a transcription of AS/NZS 3000:2018 Table 5.1. They are broadly conservative but they will not match the standard row for row, and the reduced-earth columns of the real table are not implemented.
• k is calculated from the temperatures, off a calibrated pair of reference constants. The calculator anchors k at 115 for copper and 76 for aluminium for the reference rise of 75 to 160 degrees Celsius, then rescales it for any other pair by the square root of the ratio of the logarithmic temperature terms. Because that anchor is calibrated to the legacy constants rather than to the published prefactors, the derived k values sit roughly 3 percent above the IEC 60364-5-54 figures. Treat every k this calculator reports as indicative pending verification against AS/NZS 3008.1.1 Table 52.
• The installation condition temperatures are indicative. The five conditions follow the structure of the IEC tables, insulated and bunched, insulated and separate, bare against a sheath, bare and clear, and bare in a fire risk area, but the temperature pairs behind them have not been transcribed from AS/NZS 3000 or AS/NZS 3008.1.1. They are the right shape and the right direction, not verified numbers.
• The protective device models are not manufacturer curves. The magnetic multiples are the upper edge of the published trip bands, so the derived time errs long. Below the magnetic threshold the calculator cannot model the thermal region from a rating alone, so it substitutes the maximum disconnection time for the circuit, which is conservative but often much longer than the real clearing time. The gG fuse curve is an indicative fourth-power fit anchored at 5 seconds for a fault of 5 times rating. Read the real time off the device curve when the answer matters.
• The maximum disconnection time is picked from the device rating. 0.4 seconds at 63 amps and below, 5 seconds above. AS/NZS 3000 Table 1.1 sets this by the role of the circuit, not by rating alone, so check the limit that actually applies to your circuit.
• Parallel conductors are assumed to share the fault evenly. The calculator divides the required area by the number of conductors. That only holds if they are the same material and size and follow the same length and route. It does not check terminations, and it does not model one conductor carrying more than its share.
• The aluminium rule under Table 5.1 is a simplification. The calculator takes the copper answer and steps to the next standard size up. That is not how the standard expresses the aluminium requirement.
• The fault is adiabatic. No heat leaves the conductor during the fault. This holds for short faults, up to roughly 5 seconds, and errs on the conservative side. Past 5 seconds the calculator warns you that the assumption has broken down.
• The result is always rounded up. A calculated 5.50 square millimetres becomes 6 square millimetres, and the calculator warns you that it rounded.
• The first size check always passes. It compares the size the calculator just produced against itself, so it reports a pass by construction. The lines that can genuinely fail are the 2.5 square millimetre minimum, which trips when the active is larger than 1 square millimetre and the method returns something smaller than 2.5; the both-methods line, which trips when you run the adiabatic method on its own and Table 5.1 would have demanded more; and the disconnection time line, which trips when the device you selected cannot clear the fault inside its permitted time.

What this should not be used for

• Confirming that the protective device will actually clear in the time used. The device model here is a simplified magnetic threshold, not a manufacturer time-current curve, and it says nothing about whether the real earth fault loop impedance will let that current flow.
• Earth continuity, earth fault loop impedance, or automatic disconnection verification. Use the cable sizing calculator for the loop length check and the earthing system calculator for the electrode side.
• Main earthing conductors, MEN bonding conductors, equipotential bonding, and lightning protection down-conductors, which carry their own separate minimum sizes.
• Mechanical strength, corrosion allowance, or buried and unprotected conductor minima, none of which the adiabatic equation addresses.
• High-voltage earthing, which is covered by AS 2067 rather than AS/NZS 3000.

Nothing here has been validated by a chartered professional engineer. Verify the conductor size against the current edition of AS/NZS 3000 and AS/NZS 3008.1.1, confirm the k value that suits the actual insulation and temperature limits, and have the responsible person for the installation sign off the final design.

Worked examples

Four examples covering both methods, both conductor materials, and the device-derived disconnection time, with the arithmetic written out so you can check the tool by hand.

Earth Conductor Sizing Guide for AS/NZS 3000:2018

The protective earth conductor is a critical safety element in every electrical installation. Its purpose is to provide a low-impedance path for earth fault current so that protective devices (circuit breakers or RCDs) can disconnect the supply quickly enough to prevent electric shock and fire. AS/NZS 3000:2018 Section 5 sets out the requirements for earth conductor sizing in Australian and New Zealand installations. This calculator determines the minimum earth conductor size using either the simplified Table 5.1 lookup method or the more precise adiabatic equation, depending on the information available for your circuit.

Key concepts

• Fault current path. During an earth fault, current flows through the earth conductor back to the source (transformer neutral). The conductor must carry this fault current without damage for the full duration of the protective device clearance time. If the earth conductor is undersized, it can overheat, melt, or cause a fire before the breaker trips.
• Adiabatic equation. The formula S = sqrt(I squared x t) / k calculates the minimum cross-sectional area (S in mm squared) needed to survive a fault. I is the prospective earth fault current in amps, t is the disconnection time in seconds, and k is a material constant that depends on conductor material and the initial and final temperatures, taken from AS/NZS 3008.1.1 Table 52. AS/NZS 3000 Clause 5.3.3.1.3 cites K = 136 for copper with PVC insulation not laid up with other conductors and K = 170 for bare copper, while a copper conductor starting at 75 degrees Celsius and limited to 160 degrees Celsius gives K = 111. This equation assumes no heat dissipation during the fault, which is conservative for short fault durations.
• Table 5.1 method. When the prospective fault current is not precisely known, AS/NZS 3000 Table 5.1 provides minimum earth conductor sizes based on the cross-sectional area of the associated active conductor. This method is conservative and suitable for the majority of general installations, especially domestic and light commercial work.
• Conductor material matters. Copper has a significantly higher k factor than aluminium (AS/NZS 3008.1.1 Table 52 gives 111 for copper against 73.6 for aluminium over a 75 to 160 degrees Celsius rise). This means a copper earth conductor can safely carry more fault energy per unit area and can be a smaller cross-section than an aluminium conductor for the same fault level. Most Australian installations use copper earth conductors for this reason.

Common scenarios

• Sizing the earth conductor for a submain. When running a submain from a main switchboard to a sub-distribution board, the electrician needs to size the earth conductor for the full prospective fault current at the main board. For a 95 mm squared active conductor on a submain with a known fault level of 8 kA and a 0.4-second disconnection time, the adiabatic equation gives the precise minimum size. This is a common scenario in commercial and industrial fit-outs where fault levels are documented on the switchboard schedule.
• Domestic final subcircuit using Table 5.1. For a standard domestic power circuit wired in 2.5 mm squared TPS cable, the electrician uses Table 5.1 to determine that the minimum earth conductor is 2.5 mm squared (which is already integral to the TPS cable). No fault current measurement is needed because the table method accounts for typical domestic fault levels.
• Verifying earth conductor adequacy during a periodic inspection. When an electrician inspects an existing installation, they measure the prospective earth fault current at the switchboard using a loop impedance tester, then check whether the installed earth conductor meets the adiabatic equation requirements for the measured fault current and the installed protective device clearance time. This is especially important in older installations where conductors may have been sized to superseded standards.

Two calculation methods, run together

• Table 5.1 lookup: Simplified method based on active conductor size. Suitable for general installations where fault current is not precisely known.
• Adiabatic equation: Precise method based on prospective earth fault current and protective device disconnection time. Required for critical circuits.
• Both: The default. The calculator runs the two methods side by side, reports each result, marks which one governs, and returns the larger of the two. Neither method is a substitute for the other: the table can ask for more than the fault energy needs, and a long clearing time can push the adiabatic answer well past the table.

Additional principles

• Protective earthing: The earth conductor forms the protective earthing system that connects all exposed conductive parts to the main earthing terminal.
• Temperature rise: The adiabatic equation limits the conductor temperature rise during the fault. A PVC-insulated conductor bunched in a cable runs from about 75 degrees Celsius to about 160, a cross-linked conductor from 90 to 250, and a bare conductor clear of combustible material can be allowed as far as 500. The installation condition on the form selects the pair, and k follows it.
• Standard sizes: 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300 mm squared.
• Conductor material: Copper requires smaller conductors than aluminium for the same fault scenario due to its higher thermal capacity and conductivity.