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Earthing & Main Protective Bonding Sizer

Determine statutory minimum cross-sectional areas for circuit protective conductors (CPCs), earthing conductors, and main protective bonding conductors. Compares tabulated selection (Table 54.7) against the thermodynamic adiabatic calculation (Regulation 543.1.3), and sizes main bonding for PME (Table 54.8) and TN-S / TT (Regulation 544.1.1) supplies.

Statutory Verification: Fully checked against BS 7671:2018+A4:2026. Applies the 25 mm² copper cap for TN-S/TT bonding (Reg 544.1.1) and the 50 mm² PME cap (Table 54.8).

1. Supply Earthing Arrangement

Earthing System Type

PME bonding is governed by DNO neutral conductor copper equivalent (Table 54.8).

Supply Neutral Conductor Copper Equivalent (mm²)

Main Earthing Conductor CSA (mm²)

TN-S/TT bonding must be ≥ 50% of earthing conductor, min 6 mm², max 25 mm² (Reg 544.1.1).

2. Circuit Protective Conductor (CPC) Parameters

Line / Phase Conductor CSA (mm²)

Conductor Material & Construction (k-factor)

Installed / Proposed CPC CSA (mm²)

e.g. 1.5 mm² in standard 2.5 mm² Twin & Earth cable (6242Y).

3. Fault Current & Disconnection Time (Adiabatic)

Prospective Fault Current I (Amperes)

Design earth fault current (or prospective fault current at circuit origin).

Disconnection Time t (seconds)

Note: Disconnection times > 5 s invalidate adiabatic thermal assumptions.

BS 7671 Sizing

Minimum Required CPC (Governing Size)CHECKING

2.5 mm²

Main Protective Bonding Conductor

10 mm² Copper

BS 7671 Protective Conductor & Bonding Formulation

BS 7671:2018+A4:2026 Table 54.7, Table 54.8, Reg 543.1.3 & Reg 544.1.1

AM2 PORTFOLIO & DESIGN AUDIT FORMATTRANSPARENT FIRST-PRINCIPLES MATHS

Table 54.7 Tabulated Minimum:	2.5 mm²
Thermal Withstand :	k = 115
Fault Energy :	758.9 A·s½
Calculated Adiabatic (Reg 543.1.3):	6.60 mm²
Installed CPC :	1.5 mm² (PASS)
Main Bonding Statutory Reference:	Table 54.8 (PME)