The protective conductor (earth wire) must survive the fault current that flows during an earth fault long enough for the protective device to trip. If the earth wire burns out before the breaker opens, the installation loses its fault path — potentially leaving exposed metalwork live. Getting the sizing right is not optional, and the calculation is more nuanced than simply matching the live conductor size.
The purpose of the protective conductor
When a line conductor comes into contact with exposed conductive parts — a damaged cable touching a metal chassis, for example — the protective conductor provides a low-resistance path back to the source. This causes a large fault current to flow, which trips the overcurrent protective device and disconnects the supply before the voltage on the metalwork can cause electrocution.
The protective conductor must do two things simultaneously: carry the fault current without overheating to the point of failure, and present low enough impedance that the fault current is sufficient to trip the protective device within the required disconnection time.
The adiabatic equation (BS 7671 and AS/NZS)
The minimum cross-sectional area of a protective conductor is determined by the adiabatic equation from BS 7671 Regulation 543.1.3:
S = √(I² × t) ÷ k
Where:
- S = minimum conductor cross-sectional area (mm²)
- I = fault current (RMS) in amperes
- t = fault duration in seconds (the disconnection time of the protective device)
- k = a factor from BS 7671 Table 54.3 (or 54.2 for conductors within a cable) that accounts for the conductor material, initial temperature, and maximum allowable temperature
For a copper conductor with PVC insulation (initial temperature 70 °C, maximum 160 °C), k = 115. For bare copper exposed to touch, k = 228. For aluminium conductors within a cable, k = 76.
Worked example (BS 7671)
A circuit is protected by a 32 A type B MCB. The measured earth fault loop impedance is 0.8 Ω. What is the minimum protective conductor size?
- Fault current: I = 230 ÷ 0.8 = 287.5 A
- A 32 A type B MCB disconnects at 5× rated current (160 A). At 287.5 A (9× rated), it will disconnect in well under 0.1 s — use t = 0.1 s as a conservative estimate from the MCB time-current characteristic
- S = √(287.5² × 0.1) ÷ 115 = √(82,656 × 0.1) ÷ 115 = √8,266 ÷ 115 = 90.9 ÷ 115 = 0.79 mm²
The calculated minimum is 0.79 mm². The standard 2.5 mm² twin-and-earth cable includes a 1.5 mm² earth conductor — this easily exceeds the requirement. But if the fault current were lower or the disconnection time longer, you might need a larger earth.
Simplified table method (BS 7671 Table 54.7)
Where the protective device is not a fuse or MCB with a known time-current characteristic, or where you want to avoid the adiabatic calculation, BS 7671 Table 54.7 gives minimum protective conductor sizes based on the cross-sectional area of the associated line conductor:
- Line conductor up to 16 mm²: PE = same as line conductor
- Line conductor 25–35 mm²: PE = 16 mm²
- Line conductor above 35 mm²: PE = half the line conductor area (minimum 16 mm²)
This table is conservative — it will always give a conductor that is at least as large as the adiabatic equation requires in typical domestic and light commercial installations. Use it where simplicity is preferred over optimisation.
NEC equipment grounding conductors (EGC)
Under the NEC, the equivalent of the protective conductor is the equipment grounding conductor (EGC). NEC Table 250.122 specifies minimum EGC sizes based on the rating of the overcurrent protective device:
- 15 A OCPD: 14 AWG copper (2.08 mm²)
- 20 A OCPD: 12 AWG copper (3.31 mm²)
- 30 A OCPD: 10 AWG copper (5.26 mm²)
- 40–60 A OCPD: 10 AWG copper
- 100 A OCPD: 8 AWG copper (8.37 mm²)
- 200 A OCPD: 6 AWG copper (13.3 mm²)
- 300 A OCPD: 4 AWG copper (21.1 mm²)
NEC 250.122 also requires the EGC to be increased in proportion to any increase in the ungrounded (phase) conductors above the minimum for the circuit — so if you upsize the live conductors for voltage drop reasons, the EGC must also be upsized accordingly.
Main protective bonding conductors
The main protective bonding conductors connect gas, water, oil, and other metallic service pipes to the main earthing terminal. These are not sized by the adiabatic equation — they are sized by Table 54.8 of BS 7671 as a function of the supply neutral conductor:
- Supply neutral up to 35 mm²: main bonding conductor = 10 mm² (minimum)
- Supply neutral 35–50 mm²: main bonding conductor = 16 mm²
- Supply neutral 70–95 mm²: main bonding conductor = 25 mm²
Under no circumstances should main bonding conductors be less than 6 mm² copper, and for most domestic installations the standard is 10 mm² green-and-yellow.
Calculate the minimum size
Use the Voltix earthing and grounding conductor calculator to apply the adiabatic equation or look up the simplified Table 54.7 values for any circuit. For the fault current input, use the measured or calculated Zs from your loop impedance test — the earth fault loop impedance guide explains how to find that value.