WIRING GUIDE

Bonding Conductors UK Guide: Main and Supplementary Bonding Explained

A complete practical guide to protective bonding conductors in UK electrical installations — main bonding of gas, water and oil services, supplementary bonding in bathrooms, conductor sizing, when bonding can be omitted with RCD protection, and common bonding defects found on EICRs.

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12 min readUpdated 2026-06-10Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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What size main bonding conductor do I need?

Main protective bonding conductors must be at least half the cross-sectional area of the earthing conductor, subject to a minimum of 6mm² (BS 7671 Reg 544.1.1). For a typical domestic TN-C-S (PME) supply a minimum of 10mm² is required. Supplementary bonding conductors are sized separately under Reg 544.2.

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Key Takeaways

  • 1Main protective bonding connects all metallic services entering the building (gas, water, oil pipes) to the main earthing terminal, ensuring they are all at the same potential and preventing dangerous voltage differences between them.
  • 2BS 7671:2018+A4:2026 (Reg 544.1.1) requires main bonding conductors to be at least half the csa of the earthing conductor on non-PME supplies (minimum 6mm², maximum 25mm²), or sized per Table 54.8 against the PEN conductor on PME/TN-C-S supplies — typically 10mm² for domestic PME supplies (PEN ≤35mm²), rising to 16mm² where the PEN is over 35mm² up to 50mm².
  • 3Supplementary bonding in bathroom zones connects simultaneously accessible metallic parts (taps, pipework, radiators, baths, shower trays) to prevent dangerous potential differences within the bathroom.
  • 4Supplementary bonding in a bathroom can be omitted only when all three conditions in Reg 701.415.2 are simultaneously met: (d) all circuits comply with automatic disconnection per Reg 411.3.2; (e) all circuits have 30mA RCD additional protection per Reg 415.1.1; and (f) all extraneous-conductive-parts are effectively connected to main protective bonding per Reg 411.3.1.2.
  • 5Missing or inadequate main bonding is one of the most common C2 observations on EICRs, particularly in older properties where gas or water services have been renewed without reconnecting bonding.
  • 6Bonding conductors must be labelled with the safety label "Safety Electrical Connection | ~ Do Not Remove" (Reg 514.13.1) at: (a) the connection of every earthing conductor to an earth electrode; (b) the connection of every bonding conductor to an extraneous-conductive-part; and (c) the main earthing terminal where it is separate from the main switchgear.
01 · Wiring Guide

What Is Bonding and Why Is It Required?

Protective bonding connects metallic parts of services (gas, water, oil pipes) and structural elements to a common reference point — the main earthing terminal — to ensure they are all at the same electrical potential. Without bonding, a fault in the electrical installation could cause a dangerous voltage difference to appear between, for example, a metal bath (connected to water pipes) and a metal radiator (connected to gas pipework), creating a risk of electric shock to anyone touching both simultaneously.

  • Equipotential zone — bonding creates an equipotential zone within the building, where all metallic parts accessible to occupants are at the same potential. Even if that potential is raised above true earth during a fault, the absence of a potential difference between simultaneously accessible parts prevents current flowing through a person.
  • Two types of bonding — BS 7671 18th Edition distinguishes between main protective bonding (connecting services entering the building to the main earthing terminal) and supplementary bonding (connecting simultaneously accessible metallic parts within specific locations such as bathrooms). Both serve the same fundamental purpose but apply in different contexts.
  • Mandatory, not optional — bonding requirements in BS 7671 18th Edition are prescriptive requirements, not recommendations. Missing bonding is consistently recorded as a C2 (potentially dangerous) or C1 (danger present) observation on EICRs, depending on the specific circumstances.
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02 · Wiring Guide

Main Protective Bonding

Main protective bonding conductors connect all metallic services entering the building — gas, water, and oil pipes — to the main earthing terminal (MET) at the consumer unit. The bonding connection must be made as close as practicable to the point where the service enters the building, before any branch or appliance connection.

  • Gas service — bond to the gas installation pipework as close as practicable to the gas meter, on the consumer's side of the meter. The bonding must be before the first fitting or appliance. Under BS 7671:2018+A4:2026, where the incoming metallic gas pipe has a qualifying insulating section at the point of entry to the building, main protective bonding to that pipe is not required (see A4:2026 note below). Where no insulating section is present and plastic pipework gives way to metallic internally, bond to the first metallic section.
  • Water service — bond to the incoming water main as close as practicable to the water meter or the point of entry into the building. Where the incoming metallic water pipe has a qualifying insulating section at the point of building entry, main protective bonding is not required under A4:2026. If no insulating section is present, bond to the first metallic section of internal pipework before any branch.
  • Oil service — bond to the oil supply pipework entering the building, as close as practicable to the entry point or the oil storage tank supply connection.
  • Route to MET — main bonding conductors run directly from each service connection to the main earthing terminal at the consumer unit. They should be installed in a way that minimises damage risk and is accessible for inspection and testing. Where the run crosses walls or passes through hazardous locations, mechanical protection should be provided.

A4:2026 change — insulating section exemption

BS 7671:2018+A4:2026 introduced an important change to bonding requirements: a metallic service pipe (gas, water, oil) that enters the building via a qualifying insulating section at the point of entry need not be connected to the main protective equipotential bonding. The insulating section breaks any conductive path from the external network into the building, removing the hazard that bonding is intended to address. This exemption applies only where the insulating section is genuinely at the point of building entry — if it is located elsewhere in the run, the bonding requirement remains. Always verify the presence and location of the insulating section before deciding to omit bonding, and record the decision on the installation certificate as required by Reg 133.1.3.

03 · Wiring Guide

Main Bonding Conductor Sizing

The minimum cross-sectional area (csa) of main protective bonding conductors is governed by Reg 544.1.1. The rule differs depending on whether PME (TN-C-S) or non-PME earthing applies:

Table 54.8 — PME main bonding conductor sizing

Minimum main bonding conductor vs the supply PEN (neutral) conductor — BS 7671 Reg 544.1.2

Supply PEN (Cu equiv)
Min main bonding
≤ 35 mm²
10 mm²
over 35 up to 50 mm²
16 mm²
over 50 up to 95 mm²
25 mm²
over 95 up to 150 mm²
35 mm²
over 150 mm²
50 mm²

Most UK domestic PME supplies use a 16–25 mm² PEN, so 10 mm² is the standard main bonding size. Non-PME (TN-S/TT): at least half the earthing-conductor csa, minimum 6 mm² (Reg 544.1.1).

  • Non-PME supplies (TN-S, TT) — the bonding conductor must be at least half the csa of the earthing conductor of the installation, subject to a minimum of 6mm² and a maximum of 25mm² copper-equivalent (Reg 544.1.1).
  • PME / TN-C-S supplies — the bonding conductor is selected against the PEN conductor of the supply using Table 54.8: PEN ≤35mm² → 10mm²; PEN >35mm² up to 50mm² → 16mm²; PEN >50mm² up to 95mm² → 25mm² (Reg 544.1.2 / Table 54.8).

Most UK domestic properties are supplied on PME (TN-C-S) with a PEN conductor of 16mm² or 25mm², so 10mm² is the standard main bonding conductor size. The 6mm² minimum is the absolute floor and applies only to non-PME installations with a small earthing conductor — where in doubt, 10mm² should always be used.

  • Typical domestic PME supply (PEN ≤35mm²) — a standard 100A single-phase domestic TN-C-S supply uses a PEN conductor of 16mm² or 25mm². Table 54.8 (Reg 544.1.1) requires a minimum 10mm² main bonding conductor for these supplies. This is the most common bonding conductor size for domestic properties in the UK.
  • Larger PME supplies (PEN >35mm² up to 50mm²) — where the network PEN conductor exceeds 35mm² (common for larger commercial premises or upgraded domestic supplies), Table 54.8 requires 16mm² main bonding conductors. Using 10mm² in this situation would be undersized and is typically flagged C2 on an EICR.
  • Non-PME supplies (TN-S, TT) and the 6mm² minimum — on non-PME supplies the bonding conductor is sized at half the csa of the earthing conductor, with a floor of 6mm² copper. This applies only to TN-S or TT installations; the overwhelming majority of UK properties are PME (TN-C-S) and so use Table 54.8. Never specify 6mm² on a standard PME domestic supply: it will typically be flagged C2 on the next EICR.
  • Older installations with 6mm² — many older properties have 6mm² main bonding conductors installed to a previous edition's minimum. On a standard PME (TN-C-S) supply, Reg 544.1.1 / Table 54.8 requires 10mm² where the PEN conductor is ≤35mm². A 6mm² conductor on such a supply is undersized and is commonly observed as a C2 during EICRs.
04 · Wiring Guide

Supplementary Bonding in Bathrooms

Supplementary protective bonding is required in bathrooms and other specified locations to connect all simultaneously accessible metallic parts within the location, ensuring no dangerous potential differences exist between them. Bathrooms are the most common location requiring supplementary bonding in domestic installations.

  • What must be bonded — in a bathroom, supplementary bonding connects: metal baths and shower trays, exposed metallic taps and pipework, metal radiators and heated towel rails, and the earth terminals of all fixed electrical equipment within the bathroom (electric shower, extractor fan, shaver socket, towel rail). All parts that a person could simultaneously touch must be bonded together.
  • Conductor sizing — supplementary bonding conductors must be a minimum of 2.5mm² if mechanically protected (e.g. in conduit or under plaster) or 4mm² if not mechanically protected. This is smaller than the main bonding conductor requirement.
  • Zones — BS 7671 18th Edition defines zones within bathrooms (Zone 0, 1, 2, and outside zones) based on proximity to the water source. The zone designation affects what electrical equipment can be installed, but supplementary bonding applies to all metallic parts accessible within and near the bathroom regardless of zone.
  • Door architraves and window frames — not extraneous-conductive-parts — Reg 701.415.2 expressly states that metallic door architraves, window frames and similar parts are not considered extraneous-conductive-parts requiring supplementary bonding unless they are connected to metallic structural parts of the building. Do not bond these items unless they are genuinely structurally connected — unnecessary bonding of non-extraneous parts is a common on-site error and adds cost without safety benefit.
05 · Wiring Guide

When Supplementary Bonding Can Be Omitted

BS 7671:2018+A4:2026 (Reg 701.415.2) allows supplementary bonding to be omitted in a bathroom, but only where the building already has a main protective equipotential bonding system in accordance with Reg 411.3.1.2 and all three of the following conditions are simultaneously met:

  • Condition (d) — automatic disconnection (Reg 411.3.2) — all final circuits of the location shall comply with the requirements for automatic disconnection according to Reg 411.3.2 (i.e. fault loop impedance Zs is within the limit for the protective device).
  • Condition (e) — 30mA RCD additional protection (Reg 415.1.1) — all final circuits of the location shall have additional protection by a residual current device with a rated residual operating current not exceeding 30mA. This includes not just the bathroom lighting and shower circuits, but every circuit that could introduce a potential difference into the bathroom.
  • Condition (f) — main equipotential bonding (Reg 411.3.1.2) — all extraneous-conductive-parts of the location shall be effectively connected to the main protective equipotential bonding. Meeting this condition requires that main bonding of all incoming metallic services is intact and compliant.
  • All three conditions must be met simultaneously — failure on any one condition means supplementary bonding is still required. In a new installation with an all-RCBO consumer unit where all three conditions are satisfied, supplementary bonding can be omitted. In an older installation with any unprotected circuit, supplementary bonding remains necessary.
  • Main bonding still required — the omission of supplementary bonding does not affect the requirement for main protective bonding of incoming services (gas, water, oil). Main bonding is always required regardless of RCD protection.

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06 · Wiring Guide

Common Bonding Mistakes

Bonding errors are among the most frequently observed defects on EICRs. Understanding the common mistakes helps electricians avoid them in new work and identify them correctly during inspections.

  • Bonding after the meter — connecting the gas or water bond downstream of the meter means the meter body itself is not bonded. The bond must be applied to the consumer's pipework, typically just after the meter on the consumer's side.
  • Bonding to the wrong point — bonding the gas pipe to an earthing conductor in a socket circuit rather than to the main earthing terminal directly is incorrect. All main bonding conductors must run back to the MET at the consumer unit.
  • Broken continuity due to plastic inserts — plastic isolating sections in gas or water pipework (installed to prevent galvanic corrosion) break the metallic continuity of the pipe. Bonding must be applied to each section of metallic pipework separated by such inserts.
  • Missing safety labels — BS 7671 Reg 514.13.1 requires a warning notice "Safety Electrical Connection | ~ Do Not Remove" to be securely fixed at each bonding connection point (and at the earthing conductor-to-electrode connection and at any separate main earthing terminal). Missing labels are a commonly observed FI (Further Investigation) item on EICRs and a C3 observation in many cases.
  • Poor bonding clamp contact — bonding clamps that are incorrectly sized for the pipe diameter, poorly tightened, or applied over paint or corrosion may not make reliable electrical contact. The bonding continuity test will reveal high resistance at a defective clamp.
07 · Wiring Guide

Bonding Clamps and Connection Requirements

The quality and type of the bonding clamp is critical to reliable bonding. A poor connection creates high resistance in the bonding path, which defeats the purpose of the bonding installation.

  • Approved clamp types — bonding clamps for gas and water pipes must be approved BS 951 pattern clamps. They must be sized to suit the pipe diameter and must not damage the pipe. The clamp must make direct metal-to-metal contact with the pipe surface.
  • Surface preparation — paint, lacquer, or corrosion on the pipe surface at the clamp contact point must be removed before fitting the clamp. A poor surface contact is one of the most common reasons for high-resistance bonding connections found during EICR testing.
  • Gas safety — when fitting a bonding clamp to a gas pipe, ensure the work is carried out by a Gas Safe registered engineer or by an electrician working on the electrical bonding only (not the gas pipework itself). The clamp must not damage the pipe or its protective coating.
  • Safety labelling — fit a "Safety Electrical Connection | ~ Do Not Remove" notice to the bonding clamp and conductor at each connection point (Reg 514.13.1). The notice is also required at any earthing conductor-to-electrode connection and at the main earthing terminal if it is separate from the main switchgear. BS 951 clamps may incorporate the notice on the clamp itself. The label also alerts future workers (gas fitters, plumbers) not to disconnect the bonding.
08 · Wiring Guide

Bonding Defects on EICRs — Observation Codes

Bonding defects are among the most commonly observed items on EICRs for older domestic properties. Understanding how to classify bonding defects correctly ensures accurate and consistent reporting.

  • C2 — Missing main bonding — absent gas or water bonding conductors are typically C2 (potentially dangerous) as they leave metallic services at an indeterminate potential relative to the installation's earth. Under certain fault conditions, this could produce a dangerous touch voltage.
  • C2 — Undersized bonding conductors — bonding conductors below the minimum csa required by BS 7671 for the supply size are typically C2. The undersized conductor may not be capable of carrying the fault current required to operate the protective device within the disconnection time.
  • C3 — Missing safety labels — absent bonding conductor labels are typically C3 (improvement recommended) as the lack of labels does not immediately create an electrical hazard but could lead to the bonding being disconnected inadvertently by uninformed workers.
  • FI — High continuity resistance — where the bonding conductor and clamp appear to be present but the continuity resistance measurement is high (above approximately 0.05 ohms), this should be recorded as FI (Further Investigation) or C2 depending on the resistance value and circumstances.
09 · Wiring Guide

For Electricians: Testing and Certification of Bonding

Bonding continuity must be tested and recorded during both new installation commissioning and EICR inspections. The test verifies that a low-resistance path exists between bonded parts and the main earthing terminal.

Record Bonding Results on Certificate

Main bonding continuity resistance values must be recorded on the Schedule of Test Results on the EIC or EICR. Typical acceptable values are below 0.05 ohms for a well-installed bonding conductor. Use the Elec-Mate EICR app to record bonding test results on site and generate a compliant PDF instantly.

New Installation Bonding

When installing a new consumer unit, always check for and install main bonding conductors to all incoming metallic services. Quote for bonding as a standard item of every consumer unit replacement — the small additional cost prevents a C2 observation on the next EICR and ensures the installation is safe from day one.

Main Bonding Conductor Size Guide (BS 7671)

Main and supplementary bonding conductor sizes to BS 7671: minimum cross-sectional area by earthing arrangement and supply, with a sizing table.

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