Size the earthing conductor, main protective bonding and supplementary bonding conductors to BS 7671:2018+A2:2022 Chapter 54.
Table 54.7 selection or the Reg 543.1.3 adiabatic method for the earthing conductor; Table 54.8 (PME) or the half-earthing rule for main bonding.
Copper conductor sizing only: for design guidance only, always verify with a qualified engineer.
About this earthing & bonding conductor sizing calculator
This free earthing and bonding conductor sizing calculator sizes the three protective conductors an installation needs, the earthing conductor, main protective bonding and supplementary bonding, to BS 7671:2018+A2:2022 Chapter 54. It is aimed at UK electricians, electrical designers and inspectors selecting conductor cross-sectional areas at design or verification stage, handling both TN-C-S/PME and TN-S/TT supplies. Everything runs in your browser. Nothing is uploaded.
How the conductor sizes are calculated
The earthing conductor is sized by Table 54.7 selection (S ≤ 16 → S; 16 < S ≤ 35 → 16 mm²; S > 35 → S/2) or the Reg 543.1.3 adiabatic equation S = √(I²t) ÷ k, with k read from Tables 54.2/54.3 for the chosen material, insulation and installation condition; under PME it is additionally floored at the Reg 544.1.1 main-bonding size (Reg 542.3.1). Main protective bonding is Reg 544.1.1 (≥ half the earthing conductor, ≥ 6 and ≤ 25 mm² copper) or Table 54.8 keyed to the supply neutral for PME. Supplementary bonding follows Reg 544.2 (full or half CPC conductance, with 2.5 / 4 mm² minimums). Every raw requirement rounds up to the next standard conductor size.
Frequently asked questions
What changes between PME and TN-S/TT supplies?
On a TN-C-S / PME supply the main bonding is sized from the copper-equivalent supply neutral via Table 54.8, and the earthing conductor is floored to that size (Reg 542.3.1). On TN-S or TT the main bonding uses the Reg 544.1.1 half-earthing rule instead. Confirm the neutral size with the DNO.
When is the adiabatic method valid?
The Reg 543.1.3 adiabatic equation assumes all fault energy heats the conductor, which holds for disconnection times of 5 seconds or less. Above 5 s the calculator flags the result as indicative only. Use a non-adiabatic method for slow faults.
Does it size aluminium conductors?
The k values include aluminium for the adiabatic step, but the standard-size ladder and bonding outputs are presented as copper-conductor sizing. Always verify the final selection against BS 7671, the DNO's conditions and the project specification.