Bonding Conductor Calculator — BS 7671

Main protective bonding connects extraneous-conductive-parts (metallic services entering the building) to the main earthing terminal (MET). BS 7671 Regulation 411.3.1.2 requires bonding of: metallic water pipes, metallic gas pipes, other metallic services (oil, structural steelwork), and any metallic service pipes entering the building. The bonding conductor must be connected as near as practicable to the point of entry — within 600mm of the meter for gas and water.

With PME (TN-C-S) supplies, if the PEN conductor is lost, extraneous-conductive-parts could rise to dangerous touch voltages. BS 7671 Regulation 544.1.1 requires minimum 10mm² main bonding for PME supplies regardless of fuse size. Additional requirements include: main bonding to gas and water within 600mm of meter, all metallic services bonded, and consideration of external influences. Some DNOs require even larger bonding conductors. Check our Earthing System Calculator and Earth Fault Loop Impedance Calculator for related earthing design.

Supplementary bonding is required where the disconnection time conditions of Regulation 411.3.2 cannot be met — typically in locations with increased risk such as bathrooms (Section 701). It connects simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts, and may be omitted where Regulation 415.2.2 is satisfied (resistance R between the parts ≤ 50V ÷ Ia, where Ia is the disconnection current of the protective device). The minimum supplementary-bonding conductor size is 2.5mm² copper if mechanically protected, or 4mm² if not, per Regulation 544.2. Supplementary bonding in bathrooms connects all metal pipes, radiators, baths, and accessible structural metalwork.

Main protective bonding is sized by conductor cross-sectional area, NOT the supply fuse rating. For PME (TN-C-S) supplies, Table 54.8 keys the bonding size to the copper-equivalent CSA of the supply neutral (PEN) conductor: neutral ≤35mm² → 10mm² bonding; >35–50mm² → 16mm²; >50–95mm² → 25mm²; >95–150mm² → 35mm²; >150mm² → 50mm². The PME minimum is always 10mm² (Reg 544.1.1). For non-PME supplies (TN-S, TT), Table 54.8 does NOT apply — instead the main bonding must be at least half the CSA of the earthing conductor (sized from the line conductor via Table 54.7), with a floor of 6mm² and a ceiling of 25mm². The conductor must be Green/Yellow identified, continuous (no joints unless accessible and properly made), and connected with clamps to BS 951.

No — you must never use gas or water pipes as earth conductors. Bonding conductors are connected TO these pipes to ensure they remain at earth potential, but the pipes themselves are not part of the earthing arrangement. The connection must be made with a BS 951 bonding clamp labelled 'Safety Electrical Connection — Do Not Remove'. The bonding conductor should be Green/Yellow and as short as practicable.