What is main protective bonding and what services need it?

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.

How does PME affect bonding requirements?

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.

When is supplementary bonding required?

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. The minimum size is 4mm² copper if mechanically protected, or 6mm² if not. Supplementary bonding in bathrooms connects all metal pipes, radiators, baths, and accessible structural metalwork.

What are the main bonding sizes per BS 7671 Table 54.8?

For copper conductors: 35A supply = 6mm², 60A supply = 10mm², 80A supply = 10mm², 100A supply = 16mm², 200A supply = 25mm². These are minimum sizes — they may be increased for PME supplies or where the bonding run is particularly long. The conductor must be Green/Yellow identified, continuous (no joints unless accessible and properly made), and connected with appropriate clamps to BS 951.

Can I use the gas pipe or water pipe as an earth conductor?

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.

TradeCalculator

Bonding Conductor Calculator — BS 7671

Determine main protective bonding and supplementary bonding conductor sizes per BS 7671 Table 54.8. Covers TN-S, TN-C-S (PME), and TT earthing systems.

Supply Fuse Rating

DNO main fuse rating

Earthing System

TN-S, TN-C-S (PME), or TT

Service to Bond

Type of metallic service

Bonding Cable Length

MET to bonding clamp

Cable Price

Your price per metre for bonding cable

How We Calculate This

This calculator determines the required bonding conductor sizes based on BS 7671 18th Edition requirements, considering the supply fuse rating, earthing system, and service type.

BS 7671 Table 54.8 — Main Bonding Sizes

35A supply: 6mm² copper minimum
60A supply: 10mm² copper minimum
80A supply: 10mm² copper minimum
100A supply: 16mm² copper minimum
PME (TN-C-S): Minimum 10mm² regardless of fuse size
Supplementary: 4mm² (protected) or 6mm² (unprotected)

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.