What is supplementary bonding in a bathroom?

Supplementary bonding is an additional equipotential bonding connection that links all extraneous-conductive-parts (metalwork that is not part of the electrical installation but can introduce earth potential, such as metal water pipes, radiator pipes, and metal baths) and exposed-conductive-parts (metalwork that is part of the electrical installation, such as an earthed light fitting or shower unit) within the bathroom. The purpose is to equalise the electrical potential between all accessible metalwork, so that if a fault occurs, the voltage difference between any two pieces of metalwork that a person could simultaneously touch is reduced to a safe level. This is particularly important in bathrooms because the human body resistance is lower when wet, increasing the risk of electric shock from even relatively low voltages.

Is supplementary bonding still required in bathrooms?

Not always. BS 7671:2018+A4:2026 Regulation 701.415.2 sets out a prerequisite and three conditions. The prerequisite is that the building has a protective equipotential bonding system in accordance with Regulation 411.3.1.2 — that is, main bonding conductors are connected from the main earthing terminal to incoming metallic services (gas, water, oil). Once that prerequisite is met, supplementary bonding may be omitted only if all three conditions are also satisfied: (d) all final circuits of the location comply with automatic disconnection requirements per Regulation 411.3.2; (e) all final circuits have additional protection by a 30mA RCD in accordance with Regulation 415.1.1; and (f) all extraneous-conductive-parts in the location are effectively connected to the protective equipotential bonding per Regulation 411.3.1.2. Note that condition (f) is not merely a restatement of the prerequisite — it requires that each individual extraneous-conductive-part (metal pipe, metal bath, etc.) within the bathroom is itself effectively connected into the main bonding system. If the prerequisite or any of the three conditions cannot be confirmed, supplementary bonding is still required.

What size conductor is used for bathroom bonding?

Supplementary bonding conductors in bathrooms must be at least 4mm2 copper if they are not mechanically protected (for example, run along the surface of a wall or behind a bath panel). If the conductor is enclosed in conduit, trunking, or otherwise mechanically protected, 2.5mm2 copper is permitted. The conductor can be single-core green/yellow PVC insulated cable. It must be securely connected to the metalwork using BS EN 60998-compliant clamps — purpose-made earth clamps that grip the pipe firmly and are labelled "Safety Electrical Connection — Do Not Remove." Wrap-around pipe clamps are the most common type. The bonding conductor should be as short as practicable and routed to minimise the risk of mechanical damage.

Do plastic pipes need bonding in a bathroom?

No. Plastic pipes (including plastic water supply pipes, plastic waste pipes, and plastic push-fit plumbing) are non-conductive and are therefore not extraneous-conductive-parts. They do not require bonding. This is increasingly relevant because modern properties often have plastic plumbing throughout — in which case there may be no metalwork in the bathroom that requires supplementary bonding. However, be careful to check the full run of the pipework: if plastic pipes are connected to metal pipes elsewhere in the system (for example, the plastic supply pipe connects to a copper riser in the airing cupboard), the metal section could still introduce earth potential into the bathroom. Check what is accessible within the bathroom itself and whether any metal pipework enters the room.

What happens if bonding is missing on an EICR?

If supplementary bonding is required (because the conditions for omission under Regulation 701.415.2 are not met) and it is found to be absent or inadequate during an EICR, the inspector should record this as an observation. The classification depends on the circumstances. If the bathroom has no RCD protection and no bonding, this is typically a C2 (Potentially Dangerous) observation because the risk of electric shock is significantly elevated. If RCD protection is present but one of the other conditions for omission is not met (for example, the disconnection time cannot be verified), it might be classified as C3 (Improvement Recommended) or C2 depending on the overall risk assessment. If main bonding is also missing, this is likely C1 (Danger Present). The observation text should reference Regulation 701.415.2 and clearly state which condition is not met.

Do I need to bond a metal bath?

A metal bath is an extraneous-conductive-part if it is connected to the metalwork of the building (for example, via metal waste pipes or metal water supply pipes). If the bath has metal waste and supply connections, it should be included in the supplementary bonding — unless the conditions for omission under Regulation 701.415.2 are met. If the metal bath has plastic waste and plastic water supply connections, and there is no other metalwork connecting it to earth, it may not be an extraneous-conductive-part — but this requires careful assessment. In practice, if there is any doubt, bond it. A bonding clamp on the underside of a metal bath (accessible behind the bath panel) is straightforward to install and eliminates any risk. Modern acrylic and fibreglass baths are non-conductive and do not require bonding.

Does the 30mA RCD requirement apply to circuits that only pass through the bathroom?

Yes. Regulation 701.411.3.3 requires 30mA RCD additional protection for all low voltage circuits passing through the bathroom zones — not only circuits that serve the bathroom. This means, for example, that a lighting circuit running through the bathroom ceiling void to supply an adjacent room must also be RCD-protected, even if it supplies no bathroom fittings. When designing or inspecting, check the routing of all LV circuits in the vicinity. A NOTE within Regulation 701.411.3.3 cross-refers to Regulations 314.1(d) and 531.3.2, which address the avoidance of unwanted tripping. Where multiple circuits are protected by a single RCD, consider discrimination and selectivity to prevent nuisance tripping of unrelated circuits.

Where do I connect the bonding conductor at the other end?

The supplementary bonding conductor connects each piece of extraneous-conductive-part metalwork to the nearest exposed-conductive-part earth terminal — typically the earth terminal of a socket, light switch, or the bathroom circuit junction box. Alternatively, all the bonding conductors can be brought together at a common bonding point (a connector strip or terminal block) that is connected to the circuit protective conductor. The important thing is that all the metalwork in the bathroom is connected together and connected to the circuit protective conductor, creating an equipotential zone. The bonding does not need to run back to the consumer unit — it connects locally within or near the bathroom.

INSTALLATION GUIDE

Bonding in a Bathroom: When Is It Required?

Supplementary bonding in bathrooms is one of the most common questions in domestic electrical work. This guide explains when bonding is required, when it can be omitted under Regulation 701.415.2, what to bond, conductor sizing, testing procedures, and how to record bonding observations on the EICR.

Start 7-day free trial

Free for 7 days · No charge until day 8 · Cancel anytime · Used by 1,000+ UK electricians

11 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.

ShareX in W

Do you need supplementary bonding in a bathroom?

Not always. Under BS 7671 Reg 701.415.2, supplementary equipotential bonding in a room containing a bath or shower can be omitted only if all three conditions are met: (a) every circuit meets its required disconnection time, (b) every circuit has 30mA RCD additional protection, and (c) all extraneous-conductive-parts are connected to the main earthing terminal by main protective bonding. If any one is not met, supplementary bonding is required.

1,000+

UK electricians

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer — DP Electrical

Key Takeaways

1Supplementary bonding in bathrooms connects all simultaneously accessible extraneous-conductive-parts and exposed-conductive-parts to equalise potential and reduce electric shock risk.
2Under BS 7671 Regulation 701.415.2, supplementary bonding can be omitted where the building has main protective bonding per Reg 411.3.1.2 (the prerequisite) AND all three conditions are met: (d) 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 the main protective equipotential bonding.
3Where supplementary bonding is required, the conductor must be at least 4mm2 copper (or 2.5mm2 if mechanically protected in conduit or trunking).
4Typical items to bond include metal pipework (water, gas), metallic central heating pipes and air conditioning systems (Reg 701.415.2(b)), metal baths, metal waste pipes, and any other extraneous-conductive-parts accessible from within the bathroom zones.
5Elec-Mate AI tools help electricians decide whether supplementary bonding can be omitted, and the EICR certificate records the bonding status with correct observation codes.

01 · Installation Guide

What Is Bonding and Why Does It Matter in Bathrooms?

Bonding is the practice of connecting metalwork together with a conductor to create an equipotential zone — an area where all accessible metalwork is at the same electrical potential. If a fault occurs and one piece of metalwork becomes live, the bonding ensures that the voltage difference between that metalwork and any other metalwork a person could simultaneously touch is minimised.

In bathrooms, this is particularly important because water reduces the skin resistance of the human body. A voltage that might cause a mild tingle on dry skin can cause a dangerous electric shock when a person is wet. The risk is highest when a person is in a bath or shower and can simultaneously touch metalwork connected to different earth potentials — for example, a metal bath connected to a water pipe and a metal radiator connected to a heating pipe.

BS 7671:2018+A4:2026 addresses this in Section 701, which covers the particular requirements for electrical installations in locations containing a bath or shower. The key regulation for bonding is Regulation 701.415.2.

Free download

Get the BS 7671 A4:2026 Cheat Sheet — free

Every key change in the 2026 amendment on one page. AFDDs, TN-C-S protection, new schedule columns, model forms. Pinned on your van dash.

Every regulation change summarised
New model forms (EIC + MEIWC)
Free PDF — no subscription

02 · Installation Guide

Main Bonding vs Supplementary Bonding

There are two types of equipotential bonding, and it is important to understand the difference because they serve different purposes and have different requirements.

Main Protective Bonding

Main bonding (Regulation 411.3.1.2) connects the main earthing terminal of the installation to the incoming metallic services — gas, water, and oil supply pipes — at or near the point of entry to the building. Main bonding conductors are typically 10mm2 or 16mm2 copper depending on the size of the earthing conductor. Main bonding is required in every installation and cannot be omitted. Its purpose is to connect all incoming metallic services to the earthing system of the installation, ensuring they are at the same potential as the electrical earth.

Supplementary Bonding

Supplementary bonding (Regulation 415.2 and 701.415.2 for bathrooms) is an additional local bonding connection within a specific area — in this case, the bathroom. It connects all extraneous-conductive-parts (metal pipes, metal baths, etc.) and exposed-conductive-parts (earthed electrical equipment) within the bathroom together. Supplementary bonding conductors are typically 4mm2 copper. Unlike main bonding, supplementary bonding in bathrooms can be omitted if certain conditions are met.

Both types of bonding are essential parts of the fault protection strategy defined in BS 7671. Main bonding must always be in place. Supplementary bonding in bathrooms is additional protection that can be omitted only when the three conditions of Regulation 701.415.2 are satisfied.

03 · Installation Guide

When Is Supplementary Bonding Required in a Bathroom?

Supplementary bonding is required in a bathroom when any of the three conditions for omission under Regulation 701.415.2 cannot be confirmed. If you cannot verify all three conditions, supplementary bonding must be installed.

No RCD protection on bathroom circuits: If any circuit serving the bathroom (lighting, socket, shower, underfloor heating) is not protected by a 30mA RCD, supplementary bonding is required.
Disconnection time cannot be verified: If you cannot confirm that all circuits serving the bathroom comply with Regulation 411.3.2, supplementary bonding is required. Table 41.1 applies to: (i) socket-outlet final circuits rated up to 63A, and (ii) fixed-load-only final circuits rated up to 32A (Regulation 411.3.1.2). Verification requires measuring earth fault loop impedance (Zs) and confirming it is within the maximum Zs values for the protective device.
Main bonding is absent or inadequate: If main protective bonding to the incoming gas, water, and oil pipes is not in place or is undersized, supplementary bonding is required. In practice, if main bonding is missing, this should be rectified as a priority — it is a C1 or C2 observation on an EICR.

In older properties with rewirable fuse boards (no RCDs), metal pipework, and no main bonding, supplementary bonding in the bathroom is essential — and is often the only protection against electric shock from a fault on the bathroom circuits.

04 · Installation Guide

When Can Supplementary Bonding Be Omitted?

BS 7671:2018+A4:2026 Regulation 701.415.2 permits supplementary bonding to be omitted where two things are established: a prerequisite, and three named conditions (d), (e), and (f).

Prerequisite: The location is in a building that has a protective equipotential bonding system in accordance with Regulation 411.3.1.2. In practice this means main bonding conductors connect the main earthing terminal to the incoming metallic services (gas, water, oil) at or near their point of entry to the building. Without this main bonding in place, supplementary bonding cannot be omitted regardless of the conditions below.

Once the prerequisite is established, ALL three conditions must also be met:

Condition (d) — Automatic disconnection (Reg 411.3.2): All final circuits of the location comply with the automatic disconnection requirements. Table 41.1 disconnection times apply to socket-outlet circuits up to 63A and to fixed-load-only circuits up to 32A (Regulation 411.3.1.2). Verify by measuring earth fault loop impedance (Zs) and confirming it is within the maximum Zs values for the protective device.
Condition (e) — Additional RCD protection (Reg 415.1.1): All final circuits of the location have additional protection by a 30mA RCD in accordance with Regulation 415.1.1 (Regulation 701.411.3.3). This includes the lighting circuit, any socket circuit, the shower circuit, electric towel rail circuit, and any underfloor heating circuit. Note that Regulation 701.411.3.3 also requires 30mA RCD protection for LV circuits that merely pass through bathroom zones — not just circuits serving the bathroom.
Condition (f) — All extraneous-conductive-parts connected (Reg 411.3.1.2): All extraneous-conductive-parts within the location are effectively connected to the protective equipotential bonding. This goes beyond confirming that main bonding exists at the incoming services — it requires each individual piece of accessible metalwork within the bathroom (metal pipes, metal bath, etc.) to be effectively connected into the bonding system.

In a modern domestic installation with a dual-RCD or RCBO consumer unit and main bonding in place, all three conditions are typically met — and supplementary bonding in the bathroom can be omitted. This is now the norm in new-build properties and properties with recently upgraded consumer units.

However, the electrician must verify each condition — not assume. During an EICR, check for the presence and adequacy of main bonding, confirm RCD protection on all bathroom circuits, and verify disconnection times through Zs testing. Only if all three are confirmed can you record that supplementary bonding is not required.

05 · Installation Guide

4mm Conductor: Size and Installation Requirements

Where supplementary bonding is required, the conductor must meet specific size and installation requirements set out in BS 7671.

4mm2 copper: The minimum cross-sectional area for a supplementary bonding conductor that is not mechanically protected. This is single-core green/yellow PVC insulated cable. It must be routed to minimise the risk of mechanical damage — typically behind bath panels, under the floor, or clipped along skirting boards.
2.5mm2 copper: Permitted where the conductor is mechanically protected — enclosed in conduit, trunking, or a recognised protective enclosure. In practice, 4mm2 is used in most bathroom bonding installations because it is easier to run a single 4mm2 cable than to install conduit for 2.5mm2.
Clamp connections: Bonding connections to pipes must use purpose-made earth clamps that conform to BS EN 60998. Each clamp must be labelled "Safety Electrical Connection — Do Not Remove." The clamp must make a secure, low-resistance connection to the pipe — clean the pipe surface before fitting the clamp to ensure good metal-to-metal contact.
Accessibility: Bonding connections should be accessible for inspection and testing. Connections hidden behind permanently sealed panels or buried in walls cannot be inspected during future EICRs. Where access is limited (for example, behind a fitted bath panel), ensure the panel can be removed.

Try Elec-Mate free for 7 days

16 certificate types, 70+ calculators, RAMS, quoting, invoicing, AI agents, and 46+ training courses — from £6.99/mo.

Start free trial

06 · Installation Guide

What to Bond in a Bathroom

When supplementary bonding is required, you must bond all extraneous-conductive-parts and exposed-conductive-parts that are simultaneously accessible within the bathroom. Here is what to look for:

Metal water pipes: Hot and cold supply pipes entering the bathroom. Bond at the point of entry to the room or as close as practicable.
Metal central heating pipes and air conditioning systems: Radiator flow and return pipes, and any metallic air conditioning pipework or fan-coil units accessible within the bathroom (Regulation 701.415.2(b)). Bond at the point where the pipes enter the bathroom or at the radiator/unit.
Metal waste pipes: If the bath, basin, or shower waste pipe is metal (typically chrome or brass), bond it. Plastic waste pipes do not require bonding.
Metal baths: A steel or cast iron bath connected to metal pipework is an extraneous-conductive-part and should be bonded. Acrylic and fibreglass baths are non-conductive and do not require bonding.
Metal towel rails: If the towel rail is plumbed into the central heating system with metal pipe connections, it is an extraneous-conductive-part. Electric towel rails are exposed-conductive-parts and should be earthed via the circuit protective conductor.
Structural metalwork: Any accessible structural steelwork, metal window frames (if conductive and earthed), or metal door frames within the bathroom. This is uncommon in domestic properties but should be checked.

Items that do not need bonding: plastic pipes, plastic baths, plastic cisterns, ceramic tiles, glass shower screens, and any non-conductive material. If the entire plumbing system in the bathroom is plastic (increasingly common in modern and renovated properties), there may be no extraneous-conductive-parts to bond at all.

07 · Installation Guide

Testing Bonding Connections

Bonding connections must be tested as part of any periodic inspection (EICR) or initial verification (EIC). The key test is the continuity of supplementary bonding conductors.

Continuity test: Using a low-resistance ohmmeter, measure the resistance between each bonded item and the nearest exposed-conductive-part earth terminal (or between any two simultaneously accessible bonded items). The resistance should be low — typically well under 1 ohm for short bonding conductors. A high resistance indicates a loose clamp, corroded connection, or broken conductor.
Visual inspection: Check that all bonding clamps are tight, correctly labelled, and making good contact with the metalwork. Check the conductor for damage — kinks, cuts, or signs of overheating. Verify the conductor size is adequate (4mm2 minimum unprotected).
Recording results: Record the bonding test results on the EICR schedule of test results or in the general condition section. Note which items are bonded and confirm the adequacy of the connections. If supplementary bonding has been omitted under Regulation 701.415.2, note this and confirm that all three conditions for omission are met.

A common EICR finding is bonding that was adequate when installed but has been compromised by subsequent plumbing work. A plumber replacing a section of copper pipe with plastic effectively breaks the bonding path — even though the bonding clamp is still attached to the remaining copper section. Always trace the full path from the bonding clamp to the incoming metal service to confirm continuity.

Bathroom Bonding Regulations: Do You Need It? (BS 7671)

Supplementary bonding in bathrooms explained: when it is required and when it can be omitted under BS 7671 Section 701, with the exact conditions.

Try it free for 7 days

08 · Installation Guide

For Electricians: Bonding Issues on EICRs

Bonding deficiencies are one of the most common observations on domestic EICRs, particularly in older properties. Missing main bonding, missing supplementary bonding in bathrooms, and bonding that has been broken by plumbing alterations are all frequent findings.

AI Observation Coding

Describe the bonding deficiency in plain English — "no supplementary bonding in bathroom, metal pipes accessible" — and Elec-Mate's AI returns the correct observation code with the BS 7671 regulation reference. No more looking up codes in the book.

Remedial Quoting

Turn the bonding observation into a priced remedial quote. Elec-Mate's estimator prices the bonding work — 4mm2 cable, earth clamps, labour — and generates a quote for the customer. Send the EICR and the remedial quote in the same visit.

Certificate the Remedial Work

After installing the bonding, issue a Minor Works Certificate for the remedial work using Elec-Mate. Record the continuity test results and the items bonded. Send the certificate to the customer from your phone.

701.415.2 Bathroom Bonding | BS 7671:2018+A4:2026

701.415.2 supplementary bonding in bathrooms: meet BS 7671:2018+A4:2026 requirements. Instant compliance checks, observation coding…

Try it free for 7 days

Frequently Asked Questions About Bathroom Bonding

What electricians say

Verified reviews from the UK App Store.

One App for Everything!

Elec-Mate is my go to app for business and electrical work. It's feature rich without feeling cluttered. A true all in one app for quotes, certs, calculations, RAMS, EICRs, and more. I use it every day without fail, and it makes my workflow much smoother since I'm not jumping between apps anymore. The price-to-feature ratio is excellent. Any issues I've had, the developer responds within the hour and usually fixes them the same day. 100% recommend.

Fantastic app for electricians

I've used the app and the web based version for a while now and it's well worth the investment. If you're an apprentice or experienced Spark give it a go, you won't be disappointed.

Absolutely amazing

I've been using Elec-Mate for a while now, and honestly, it's one of the best apps I've ever downloaded. Every aspect of it feels thoughtfully designed, from the clean and intuitive interface to the powerful features that make everything so easy to manage. It's clear that a lot of care and attention went into building this app, and it shows in every detail.

Trusted by electricians across the UK

Real feedback from real sparks

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer

Sole Trader · DP Electrical

“I've won two contracts this month because I could turn quotes around same-day with the AI cost engineer.”

Nathan Perry

Electrician · NP Electrical Services

“The study centre got me through my AM2. Mock exams and flashcards are brilliant.”

Jake Pizey

3rd Year Apprentice · Apprentice

7-Day Free Trial — Cancel Anytime, No Hassle

Record Bonding Observations on Your Phone

EICR certificates with AI observation coding, remedial quoting, and instant PDF delivery. Join 1,000+ UK electricians using Elec-Mate. 7-day free trial, cancel anytime.

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer, DP Electrical

From £6.99/mo after trial — less than a coffee a week

or download the app

7 days free, then from £6.99/moCancel in one tap — no calls, no hassleiOS, Android & WebBS 7671 compliant

Bonding in a Bathroom: When Is It Required?

Do you need supplementary bonding in a bathroom?

Key Takeaways

What Is Bonding and Why Does It Matter in Bathrooms?

Get the BS 7671 A4:2026 Cheat Sheet — free

Main Bonding vs Supplementary Bonding

Main Protective Bonding

Supplementary Bonding

When Is Supplementary Bonding Required in a Bathroom?

When Can Supplementary Bonding Be Omitted?

4mm Conductor: Size and Installation Requirements

Try Elec-Mate free for 7 days

What to Bond in a Bathroom

Testing Bonding Connections

Bathroom Bonding Regulations: Do You Need It? (BS 7671)

For Electricians: Bonding Issues on EICRs

AI Observation Coding

Remedial Quoting

Certificate the Remedial Work

701.415.2 Bathroom Bonding | BS 7671:2018+A4:2026

Frequently Asked Questions About Bathroom Bonding

What is supplementary bonding in a bathroom?

Is supplementary bonding still required in bathrooms?

What size conductor is used for bathroom bonding?

Do plastic pipes need bonding in a bathroom?

What happens if bonding is missing on an EICR?

Do I need to bond a metal bath?

Does the 30mA RCD requirement apply to circuits that only pass through the bathroom?

Where do I connect the bonding conductor at the other end?

Continue reading

EICR Certificate App

Observation Codes Explained

Bathroom Electrical Regulations

Earthing Arrangements

Supplementary Bonding Guide

18th Edition Course

What electricians say

One App for Everything!

Fantastic app for electricians

Absolutely amazing

Trusted by electricians across the UK

Record Bonding Observations on Your Phone