Earthing and Protective Conductor Calculator
Size earthing and circuit protective conductors the way BS 7671 Chapter 54 requires: the adiabatic equation S = √(I²t) ÷ k sets the minimum cross-section that survives the fault, the k factor comes from the conductor material and how it is installed, Table 54.1 sets minimum sizes for buried earthing conductors, and any existing conductor can be checked for thermal withstand against the same fault.
The fault current and clearing time come from the protection design — pair this sheet with the Circuit Breaker Sizing Calculator and the Cable Sizing and Voltage Drop Calculator.
Earthing schedule CALC-09 · REV 2026-07
| Fault current x clearing time | – |
|---|---|
| k factor used | – |
| Adiabatic minimum, S = I√t ÷ k | – |
| Governing minimum applied | – |
| Protective conductor (CPC) | – |
| Earthing conductor (to electrode) | – |
| Table 54.7 selection method | – |
| Thermal withstand of checked size | – |
Guidance tool only, for thermoplastic (PVC 70°C) conditions; XLPE cables use higher k values and manufacturer data. The full design must verify disconnection times from the actual device curves and Zs, bonding conductor sizes (Reg 544), and electrode resistance — work for a registered engineer.
How this calculator works
- The adiabatic equation. During a fault the conductor has no time to shed heat, so all the energy I²t goes into raising its temperature. Reg 543.1.3 gives the minimum section S = √(I²t) ÷ k, valid for clearing times up to 5 s.
- The k factor. k encodes the material and the temperature it may start from and rise to: for PVC (70 °C) conditions, copper is 115 when the CPC runs inside or bunched with the cable (Table 54.3), 143 as a separate insulated conductor (54.2) and 159 bare (54.6); aluminium 76/95/105; steel 52 separate, 51 as wire armour (54.4). XLPE cables run hotter and use different values.
- Floors. Conductors not forming part of a cable must be at least 2.5 mm² (protected) or 4 mm² (unprotected) for mechanical reasons (Reg 543.1.1); buried earthing conductors must meet Table 54.1 — 16 mm² corrosion-protected, 25 mm² copper or 50 mm² steel unprotected. The calculator applies whichever governs and says which it was.
- Selection method vs calculation. If the line size is given, the Table 54.7 quick method is shown alongside — equal to the line up to 16 mm², 16 mm² for lines to 35, half the line above — illustrating how the adiabatic calculation usually justifies a smaller (cheaper) CPC.
- Withstand check. For an existing conductor, the tool checks I√t ≤ kS, reports the utilisation and the maximum clearing time the size can survive at that current — the direct answer to ‘is my earth wire big enough?’
Assumptions and limits
- Thermoplastic (PVC 70 °C) initial and final temperatures assumed; XLPE and mineral-insulated cables need their own k values.
- The clearing time must come from the actual device time–current curve at the actual fault current; main equipotential bonding (Reg 544) and electrode resistance design are outside this sheet.
Need a full earthing and lightning-protection design or test report? See our consultancy services and design document library, or learn the method in a training program.