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 location is in a building with a protective equipotential bonding system in accordance with Regulation 411.3.1.2 — main bonding conductors connecting the main earthing terminal to the extraneous-conductive-parts liable to introduce a dangerous potential difference, such as metallic water and gas installation pipes. 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 the requirements for automatic disconnection according to Regulation 411.3.2; (e) all final circuits of the location have additional protection by means of an RCD in accordance with Regulation 415.1.1; and (f) all extraneous-conductive-parts of the location are effectively connected to the protective equipotential bonding according to Regulation 411.3.1.2. Condition (f) is not a restatement of the prerequisite — it requires each individual extraneous-conductive-part inside the bathroom to be effectively connected. The NOTE to 701.415.2 says that effectiveness may be assessed, where necessary, by applying Regulation 415.2.2. If the prerequisite or any of the three conditions cannot be confirmed, supplementary bonding is still required.

What is supplementary bonding in a bathroom?

Supplementary bonding is an additional equipotential bonding connection. Regulation 415.2.1 requires it to include all simultaneously accessible exposed-conductive-parts of fixed equipment and extraneous-conductive-parts, and requires the bonding system to be connected to the protective conductors of all equipment including those of socket-outlets. In a bathroom, Regulation 701.415.2 puts it more specifically: it connects the terminals of the protective conductor of each circuit supplying Class I and Class II equipment to the accessible extraneous-conductive-parts within the room. The purpose is to equalise the potential between all accessible metalwork, so that if a fault occurs the voltage difference between any two parts a person could touch at the same time is limited. This matters most in bathrooms because body impedance falls when the skin is wet.

What size conductor is used for bathroom bonding?

Regulation 544.2 sizes it by what is being joined and whether the conductor is mechanically protected. Where mechanical protection is not provided, the minimum is 4 mm² in every case — which is why 4 mm² single-core green/yellow is the practical default for surface or behind-the-panel runs. Where the conductor is sheathed or otherwise mechanically protected: between two exposed-conductive-parts it must have a conductance not less than that of the smaller protective conductor connected to them (Reg 544.2.1); between an exposed-conductive-part and an extraneous-conductive-part, not less than half that of the protective conductor connected to the exposed-conductive-part (Reg 544.2.2); and between two extraneous-conductive-parts, not less than 2.5 mm² (Reg 544.2.3). Connections to pipes are made with clamps to BS 951, and Regulation 514.13.1 requires a warning notice marked "Safety Electrical Connection — Do Not Remove" at the point of connection of every bonding conductor to an extraneous-conductive-part; that notice may be provided on the BS 951 clamp itself.

Do plastic pipes need bonding in a bathroom?

No — plastic is not conductive, so a plastic pipe is not an extraneous-conductive-part and needs no bonding. There is a further point that is often got backwards. The NOTE to Regulation 411.3.1.2 states that where non-metallic pipes enter a building and are then connected to metallic pipes within the building, the metallic pipes within the building do not normally require protective bonding, as they are unlikely to be extraneous-conductive-parts. So a copper riser fed from a plastic incoming main is not automatically an extraneous-conductive-part. What decides it is whether that metalwork can introduce a potential — typically earth potential — into the location by some other route, for example a metallic gas installation pipe or metallic structural parts of the building. Where there is genuine doubt, measure rather than assume.

What happens if bonding is missing on an EICR?

If supplementary bonding is required (because the prerequisite and three conditions for omission under Regulation 701.415.2 are not all met) and it is found to be absent or inadequate during an EICR, the inspector records it 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 C3 (Improvement Recommended) or C2 depending on the overall risk assessment. Missing main protective bonding is a separate and more serious finding and should be coded on its own merits. The observation text should reference Regulation 701.415.2 and state clearly which condition is not met.

Do I need to bond a metal bath?

Only if it is an extraneous-conductive-part — that is, if it is liable to introduce a potential into the bathroom, typically through metal waste or metal supply pipework that is itself connected to earth. If the bath has metal waste and supply connections into a metallic system, include it 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 supply connections and nothing else connects it to earth, it is not an extraneous-conductive-part and bonding it achieves nothing. Modern acrylic and fibreglass baths are non-conductive and never require bonding. Where it is genuinely borderline, a bonding clamp on the underside of the bath behind the panel is cheap and removes the argument.

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

Yes, for zones 1 and 2. Regulation 701.411.3.3 requires additional protection by one or more RCDs having the characteristics specified in Regulation 415.1.1 (rated residual operating current not exceeding 30 mA) for low voltage circuits (a) serving the location, and (b) passing through zones 1 and/or 2 not serving the location. So a lighting circuit crossing the bathroom ceiling void within zone 1 or 2 to supply an adjacent room must be RCD-protected even though it feeds nothing in the bathroom. When designing or inspecting, check the routing of every LV circuit in the vicinity. The NOTE to 701.411.3.3 refers you to Regulations 314.1(d) and 531.3.2 concerning the avoidance of unwanted tripping — divide circuits between devices rather than putting everything behind one RCD.

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

To the protective conductor. Regulation 701.415.2 requires the bonding to connect the accessible extraneous-conductive-parts to the terminals of the protective conductor of each circuit supplying Class I and Class II equipment in the room — in practice the earth terminal of the shower switch, the light fitting, the fan connection unit or the socket, or a common connection point that is itself connected to those protective conductors. Regulation 415.2.1 reinforces this: the supplementary bonding system shall be connected to the protective conductors of all equipment including those of socket-outlets. It does not run back to the consumer unit. Regulation 701.415.2 also allows the bonding to be installed outside or inside the room, preferably close to the point of entry of the extraneous-conductive-parts into the room — which is usually a far easier place to work.

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 gives the omission test from Regulation 701.415.2 up front, then conductor sizes from Regulation 544.2, what to bond, how to test it, and how to record it on an EICR.

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12 min readUpdated 2026-08-07Andrew 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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The short answer

Is supplementary bonding still required in a bathroom?

Not always. Regulation 701.415.2 permits it to be omitted only where the building has protective equipotential bonding to Regulation 411.3.1.2 and all three conditions hold: (d) every final circuit of the location meets automatic disconnection per 411.3.2; (e) every final circuit has additional protection by an RCD per 415.1.1; and (f) every extraneous-conductive-part of the location is effectively connected to that bonding. If one fails, supplementary bonding is required.

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Key takeaways

01Reg 701.415.2 requires supplementary bonding in a room containing a bath or shower to connect the protective conductor terminals of every circuit supplying Class I and Class II equipment to the accessible extraneous-conductive-parts.
02It may be omitted only where the building has protective equipotential bonding to Reg 411.3.1.2 AND all three named conditions are met: (d) every final circuit of the location meets automatic disconnection per Reg 411.3.2; (e) every final circuit has additional protection by a 30 mA RCD per Reg 415.1.1; and (f) every extraneous-conductive-part of the location is effectively connected to that protective equipotential bonding.
03Where a supplementary bonding conductor is not mechanically protected, Reg 544.2 sets a floor of 4 mm². Where it is mechanically protected, the size depends on what is being joined — 2.5 mm² between two extraneous-conductive-parts, otherwise a conductance rule tied to the circuit protective conductor.
04Reg 701.415.2 names the parts to bond: (a) metallic pipes supplying services and metallic waste pipes, (b) metallic central heating pipes and air conditioning systems, and (c) accessible metallic structural parts of the building.
05Elec-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

Can Supplementary Bonding Be Omitted?

Yes — but only against a prerequisite plus three named conditions, all of which have to hold. This is the whole of the omission allowance in Regulation 701.415.2 of BS 7671:2018+A4:2026.

Prerequisite — before the conditions are even reached

The location containing the bath or shower is in a building with a protective equipotential bonding system in accordance with Regulation 411.3.1.2. Without that, the conditions below do not come into play at all and supplementary bonding stays.

Then all three conditions must be met

Condition	What Reg 701.415.2 requires	How you prove it
(d)	All final circuits of the location comply with the requirements for automatic disconnection according to Reg 411.3.2	Measure Zs and compare against the maximum Zs for the device
(e)	All final circuits of the location have additional protection by means of an RCD in accordance with Reg 415.1.1 (IΔn not exceeding 30 mA)	Identify the device on every circuit and verify it per Reg 643.8
(f)	All extraneous-conductive-parts of the location are effectively connected to the protective equipotential bonding according to Reg 411.3.1.2	The NOTE to 701.415.2 allows this to be assessed, where necessary, by applying Reg 415.2.2

Condition (f) is not a restatement of the prerequisite. The prerequisite is about the building; (f) is about this room. Each accessible piece of metalwork in the bathroom has to be effectively connected into the bonding system — which is exactly what the NOTE gives you a method for.

In a modern domestic installation with an RCBO or dual-RCD consumer unit and main bonding in place, all three conditions are typically met and supplementary bonding in the bathroom can be omitted. That is now the norm in new build and in properties with a recently upgraded consumer unit. It still has to be verified rather than assumed: check the presence and adequacy of main bonding, confirm RCD protection on every circuit of the location, and confirm disconnection through Zs testing. Only then can you record that supplementary bonding is not required.

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02 · Installation Guide

Conductor Sizes: What Regulation 544.2 Actually Says

The 4 mm² figure everyone quotes is the floor for an unprotected conductor, and it applies whatever is being joined. Where the conductor is sheathed or otherwise mechanically protected, the required size depends on which two parts it connects — and only one of those three cases gives you 2.5 mm².

The conductor connects…	Mechanically protected	Not mechanically protected
Two exposed-conductive-partsReg 544.2.1	Conductance not less than that of the smaller protective conductor connected to them	Not less than 4 mm²
An exposed-conductive-part to an extraneous-conductive-partReg 544.2.2	Conductance not less than half that of the protective conductor connected to the exposed-conductive-part	Not less than 4 mm²
Two extraneous-conductive-partsReg 544.2.3	Not less than 2.5 mm²	Not less than 4 mm²

Reg 544.2.3 carries an exception: where one of the two extraneous-conductive-parts is connected to an exposed-conductive-part in compliance with Reg 544.2.2, then 544.2.2 applies to the conductor joining the two extraneous parts as well.

In practice, 4 mm² single-core green/yellow is what gets used for bathroom bonding, because running one 4 mm² cable behind a bath panel is quicker than installing conduit to justify a smaller size. Route it to minimise the risk of mechanical damage.

Bonding need not always be a cable. Reg 544.2.4 allows supplementary bonding to be provided by a supplementary conductor, by a conductive part of a permanent and reliable nature, or by a combination of the two.
Clamps to BS 951. Connections to pipework are made with purpose-made earthing and bonding clamps to BS 951. Clean the pipe first so the clamp makes proper metal-to-metal contact.
The warning notice is a regulation, not a courtesy. Reg 514.13.1 requires a notice clearly and durably marked "Safety Electrical Connection — Do Not Remove", securely fixed in a visible position at the point of connection of every bonding conductor to an extraneous-conductive-part. It may be provided on the BS 951 clamp or on the label supplied with it.
Keep it accessible. Connections sealed behind permanent panels or buried in walls cannot be inspected on a future EICR. Where access is tight, make sure the bath panel can still come off.

03 · 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 sits at essentially the same potential. If a fault occurs and one piece of metalwork becomes live, bonding limits the voltage difference between it and any other metalwork a person could touch at the same time.

In bathrooms this matters more than anywhere else in a dwelling, because body impedance falls sharply when the skin is wet. A voltage that would give a dry hand a mild tingle can give a dangerous shock to someone standing in a bath. The worst case is a person in contact with two pieces of metalwork sitting at different potentials — a metal bath fed by a water pipe and a radiator fed by a heating pipe, for example.

BS 7671:2018+A4:2026 deals with this in Section 701, the particular requirements for locations containing a bath or shower. The bonding regulation is 701.415.2; the RCD regulation is 701.411.3.3.

04 · Installation Guide

Main Bonding vs Supplementary Bonding

Two different things share the word "bonding". They are sized differently, sited differently, and only one of them can ever be left out.

Main Protective Bonding

Reg 411.3.1.2 requires extraneous-conductive-parts liable to introduce a dangerous potential difference to be connected to the main earthing terminal. The regulation's own examples are metallic water installation pipes, metallic gas installation pipes, other metallic installation pipework and ducting, central heating and air conditioning systems, and exposed metallic structural parts of the building. Reg 544.1.2 requires the connection to be made as near as practicable to the point of entry, and within 600 mm of a meter outlet union where practicable. Main bonding cannot be omitted.

Supplementary Bonding

Reg 415.2, applied to bathrooms by Reg 701.415.2, is local bonding inside one location. It connects the protective conductor terminals of the circuits in the room to the accessible extraneous-conductive-parts, so that everything a person can touch at once is tied together. It is typically 4 mm² copper, and unlike main bonding it can be omitted where the prerequisite and the three conditions of 701.415.2 are all met.

Main bonding conductor size where PME conditions apply

Most UK domestic supplies are PME (TN-C-S). Where PME conditions apply, Reg 544.1.1 sizes the main protective bonding conductor against the PEN conductor of the supply using Table 54.8 — not against the earthing conductor.

Copper equivalent CSA of the PEN conductor	Minimum copper equivalent CSA of main bonding conductor
35 mm² or less	10 mm²
Over 35 mm² up to 50 mm²	16 mm²
Over 50 mm² up to 95 mm²	25 mm²
Over 95 mm² up to 150 mm²	35 mm²
Over 150 mm²	50 mm²

Table 54.8 carries a NOTE that local distributor's network conditions may require a larger conductor. Where PME conditions do not apply, Reg 544.1.1 instead requires not less than half the cross-sectional area required for the earthing conductor of the installation.

Both sit inside the same fault protection strategy. Main bonding is always there. Supplementary bonding is the layer you may be able to drop once the rest of the installation is doing its job.

05 · Installation Guide

When Is Supplementary Bonding Required in a Bathroom?

Whenever you cannot confirm the prerequisite and all three conditions of Reg 701.415.2. Three situations account for almost all of it in practice.

A circuit of the location has no 30 mA RCD. If any circuit serving the bathroom — lighting, sockets, shower, towel rail, underfloor heating — is not protected by an RCD to Reg 415.1.1, condition (e) fails and supplementary bonding is required.
Disconnection cannot be verified. Condition (d) is compliance with Reg 411.3.2. Reg 411.3.2.2 applies the maximum disconnection times in Table 41.1 to final circuits with a rated current not exceeding 63 A with one or more socket-outlets, and 32 A supplying only fixed connected current-using equipment. Verification means measuring earth fault loop impedance and confirming it is within the maximum Zs for the protective device. No test results, no condition (d).
Main bonding is absent, undersized or disconnected. The prerequisite fails, so the conditions never come into play. Missing main bonding is a finding in its own right on an EICR and should be put right as a priority rather than compensated for with supplementary bonding.

In an older property with a rewirable fuse board, metallic pipework and no main bonding, supplementary bonding in the bathroom is required and does real work: it holds the metalwork a bather can touch at a common potential while automatic disconnection deals with the fault.

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06 · Installation Guide

What to Bond in a Bathroom

Reg 701.415.2 names three categories. Everything else is a judgement about whether a given piece of metalwork is actually an extraneous-conductive-part.

(a) Metallic pipes supplying services and metallic waste pipes. The regulation's own examples are water and gas. Bond at the point of entry to the room or as close to it as practicable. Chrome and brass waste pipes count; plastic waste does not.
(b) Metallic central heating pipes and air conditioning systems. Radiator flow and return pipes, and any metallic air conditioning pipework or fan-coil unit accessible within the room.
(c) Accessible metallic structural parts of the building. Note the qualification the regulation itself adds: metallic door architraves, window frames and similar parts are not considered to be extraneous-conductive-parts unless they are connected to metallic structural parts of the building.

The two common judgement calls

Metal baths. A steel or cast iron bath is an extraneous-conductive-part only if something connects it to earth — usually metallic waste or supply pipework. Acrylic and fibreglass baths never are.
Towel rails. A rail plumbed into the heating system on metal pipe is an extraneous-conductive-part. An electric towel rail is Class I equipment: it is an exposed-conductive-part, earthed through its circuit protective conductor, and that protective conductor terminal is one of the points the supplementary bonding connects to.

Nothing non-conductive needs bonding: plastic pipes, plastic baths, plastic cisterns, ceramic tiles, glass shower screens. Where the whole bathroom is plumbed in plastic — increasingly common in new build and refurbishments — there may be no extraneous-conductive-parts in the room to bond at all.

07 · Installation Guide

Electric Showers and Bonding

"Electric shower earth bonding" is one of the most searched versions of this question, and it usually rests on a misunderstanding. An electric shower is Class I fixed equipment. Its earthing comes from the circuit protective conductor of the shower circuit — that is protective earthing under Reg 411.3.1.1, not bonding, and it is never optional.

Supplementary bonding is a different job. Where it is required, Reg 701.415.2 connects the terminals of the protective conductor of each circuit supplying Class I and Class II equipment to the accessible extraneous-conductive-parts. The shower's protective conductor terminal — normally at the shower's isolator or pull-cord switch — is therefore one end of the bond, not an extra thing to be bonded.

The shower circuit needs a 30 mA RCD regardless. Reg 701.411.3.3 requires additional protection by an RCD to Reg 415.1.1 for LV circuits serving the location. This applies whether or not supplementary bonding is present — RCD protection is not an alternative to it, it is one of the conditions for omitting it.
Metal shower pipework is bonded on its own merits. If the hot and cold feeds to the shower are copper and are extraneous-conductive-parts, they fall under 701.415.2(a).
Short flex to a nearby connection unit. Reg 544.2.5 covers this case: where supplementary bonding is applied to a fixed appliance supplied by a short length of flexible cable from an adjacent connection unit or accessory incorporating a flex outlet, the protective conductor within that flexible cable also provides the supplementary bonding connection to the appliance's exposed-conductive-parts. No separate bonding conductor is needed for that leg.

08 · Installation Guide

Testing Bonding Connections

Bonding is tested as part of initial verification and of any periodic inspection. The test is a continuity measurement with a low-resistance ohmmeter, and BS 7671 gives an explicit acceptance criterion for it.

The acceptance criterion — Reg 415.2.2

The resistance R between simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts shall satisfy:

R ≤ 50 V / I_a (AC systems)

where I_a is the operating current of the protective device: for RCDs, IΔn; for overcurrent devices, the 5 s operating current. In DC systems the numerator is 120 V. The NOTE to Reg 701.415.2 points at this same regulation as the way to assess whether extraneous-conductive-parts are effectively connected — which is how you evidence condition (f).

Continuity. Measure between each bonded part and the protective conductor terminal it is bonded to, or between any two simultaneously accessible bonded parts. Null the leads first and make clean metal-to-metal contact with the probes — paint, corrosion or a poor probe contact produces a falsely high reading and a wrong verdict against the 50 V / I_a criterion.
Visual inspection. Check clamps are tight, correctly labelled per Reg 514.13.1 and gripping bare metal. Check the conductor for kinks, cuts and signs of overheating, and confirm the size against Reg 544.2.
Recording. Record the results on the schedule of test results. Where supplementary bonding has been omitted under Reg 701.415.2, say so on the certificate and record the evidence for the prerequisite and each of conditions (d), (e) and (f).

The most common finding is bonding that was sound when installed and has since been broken by plumbing work. A plumber replacing a section of copper with plastic breaks the path even though the clamp is still tight on the remaining copper. Trace the full path rather than trusting the clamp.

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Elec-Mate records supplementary bonding against Reg 701.415.2 on the EICR — the omission conditions you verified, the continuity results, and the observation if it is missing.

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09 · Installation Guide

For Electricians: Bonding Issues on EICRs

Bonding deficiencies are among the most common observations on domestic EICRs. Missing main bonding, missing supplementary bonding where the omission conditions are not met, and bonding broken by later plumbing alterations account for most of them.

AI observation coding

Describe the deficiency in plain English — "no supplementary bonding in bathroom, metal pipes accessible, no RCD on lighting" — and Elec-Mate returns the observation code with the BS 7671 reference already attached.

Remedial quoting

Turn the observation into a priced remedial quote — 4 mm² cable, BS 951 clamps, labour — and send the EICR and the quote from the same visit.

Certificate the remedial work

Issue a Minor Works Certificate for the bonding once it is in, with the continuity results and the items bonded recorded against it. Sent to the customer from your phone.

Frequently Asked Questions About Bathroom Bonding

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