Earth Conductor Sizing

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 5.2 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 type and insulation (143 for copper with PVC, 176 for copper with XLPE). This equation assumes no heat dissipation during the fault, which is conservative for short fault durations.
• Table 5.2 method. When the prospective fault current is not precisely known, AS/NZS 3000 Table 5.2 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 (143 vs 95 for PVC insulated conductors). 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 5.2. For a standard domestic power circuit wired in 2.5 mm squared TPS cable, the electrician uses Table 5.2 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

• Table 5.2 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.

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 conductor temperature rise from approximately 75 degrees Celsius to approximately 160 degrees Celsius for PVC insulation during the fault.
• 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.