Supplementary Bonding Explained
Supplementary bonding ties exposed and extraneous metal parts together locally so no dangerous voltage can appear between them during a fault.
In a location where the shock risk is higher — a bathroom is the classic example — it can matter that all the touchable metal is at the same potential, so a person bridging two parts never sees a dangerous voltage. Supplementary bonding is the local connection that guarantees this, and BS 7671 sets out both how to size it and, importantly, when it can be left out.
This guide explains what supplementary bonding does, the conductor sizes required by Regulation 415.2, the three conditions in Regulation 701.415.2 that allow it to be omitted in a bathroom, and how it differs from main protective bonding.
Key takeaways
- Supplementary bonding connects exposed-conductive-parts and extraneous-conductive-parts together locally.
- It ensures no dangerous potential difference can arise between simultaneously accessible parts during a fault.
- Regulation 415.2 sets minimum conductor sizes, with a lower limit of 2.5 mm² if protected or 4 mm² if not.
- Regulation 701.415.2 allows omission in a bathroom only if all three stated conditions are met together.
- It is different from main protective bonding, which is a single connection at the origin, not a local link.
What supplementary bonding is
Supplementary bonding is a conductor that links together the exposed-conductive-parts of equipment (metal that becomes live only under fault, like an appliance casing) and the extraneous-conductive-parts of the location (metal that can introduce a potential from elsewhere, like metal pipework). By tying them together, it makes sure that if a fault raises the voltage on one part, all the connected parts rise together — so a person touching two of them never experiences a dangerous difference.
It is a local measure, applied within the specific location that needs it, and it supplements the fault protection already provided by automatic disconnection. It does not replace disconnection; it backs it up where the consequences of a lingering touch voltage would be more serious.
Conductor sizes under Regulation 415.2
Regulation 415.2 sets the minimum cross-sectional area of a supplementary bonding conductor by relating it to the protective conductors of the equipment being bonded. Where the conductor connects two exposed-conductive-parts, it must be at least the size of the smaller protective conductor connected to them. Where it connects an exposed part to an extraneous part, it need be no smaller than half the protective conductor’s size.
There are absolute lower limits regardless of that calculation: a supplementary bonding conductor must be at least 2.5 mm² if it has mechanical protection (for example run in conduit or within a cable), or at least 4 mm² if it does not. In practice a 4 mm² conductor is the common choice for exposed runs.
| Situation | Minimum size |
|---|---|
| With mechanical protection | 2.5 mm² |
| Without mechanical protection | 4.0 mm² |
| Between two exposed parts | ≥ smaller associated CPC |
| Exposed part to extraneous part | ≥ half the associated CPC |
Omitting supplementary bonding in a bathroom
For years every bathroom got supplementary bonding as a matter of course. BS 7671 now allows it to be omitted in a room containing a bath or shower — but only under Regulation 701.415.2, and only when all three of its conditions are satisfied at the same time. Meet two of the three and you must still install it.
The three conditions are: the disconnection times required for the location are met on all circuits; all the circuits of the location are protected by a 30 mA RCD; and main protective bonding is in place for the installation. If any one of those is missing — an old board without RCD protection, say — supplementary bonding stays.
All three, or it stays
The three conditions in 701.415.2 are cumulative, not a menu. Disconnection times met, every circuit on 30 mA RCD protection, and main bonding present — miss any one and supplementary bonding is still required in that bathroom.
Supplementary vs main protective bonding
The two are easy to confuse but do different jobs. Main protective bonding is a single connection made once, at the origin of the installation, joining incoming metallic services — water, gas, structural steel and so on — to the main earthing terminal. It puts the whole installation at broadly the same potential as earth.
Supplementary bonding is local and additional: it links parts together within a specific higher-risk location, on top of the main bonding. The bathroom omission rule only works because main bonding is one of its three conditions — the installation-wide protection has to be there before you can rely on it in place of the local links.
Record the condition on the report
When you inspect a bathroom for a TradePlanr EICR, note whether the three 701.415.2 conditions are met — if supplementary bonding has been omitted without all of them satisfied, that is an observation to record and code.
Frequently asked questions
When can supplementary bonding be left out of a bathroom?
Only when all three conditions of Regulation 701.415.2 are met together: the required disconnection times are achieved on every circuit, all circuits serving the location have 30 mA RCD protection, and main protective bonding is in place for the installation. If any one is missing, supplementary bonding is still required.
What size should a supplementary bonding conductor be?
Under Regulation 415.2, at least 2.5 mm² if it has mechanical protection or 4 mm² if it does not. Beyond that, a conductor between two exposed parts must be at least the size of the smaller associated protective conductor, and one between an exposed and an extraneous part at least half that size.
What is the difference between supplementary and main bonding?
Main protective bonding is a single connection at the origin, joining incoming metallic services to the main earthing terminal for the whole installation. Supplementary bonding is a local link between metal parts within a specific higher-risk location, added on top of main bonding to equalise potential where the shock risk is greater.
From guidance to action
Related guides
Main Protective Bonding Explained
What main protective bonding does, conductor sizes, and where bonding is required to BS 7671.
Earthing Systems: TN-S, TN-C-S and TT Explained
How TN-S, TN-C-S (PME) and TT earthing arrangements work, and how to identify each on site.
Special Locations (Part 7) Explained
Why Part 7 exists, the locations it covers, and how bathroom zones and supplementary bonding work.
