Earthing and bonding conductor sizes: a BS 7671 guide to Tables 54.7 and 54.8

Published 27 June 2026 · SparkCerts guides for UK electricians

Earthing and bonding sizing is one of those jobs that should be quick and often is not, because two separate conductors, three earthing arrangements and two BS 7671 tables all share the same corner of the consumer unit. Get the rule for each one straight and the sizes fall out in seconds. This guide does that, then covers the supplementary bonding and circuit protective conductor sizing the calculators usually skip.

Earthing and bonding: two jobs, two rules

The main earthing conductor connects the installation's main earthing terminal (MET) to the means of earthing, the path back to the source that lets a protective device clear an earth fault. The main protective bonding conductors connect extraneous-conductive-parts, the metal that can introduce a potential from outside the installation, such as the incoming gas and water pipes and any structural steel, to that same MET. Their job is to hold all that metalwork at roughly the same potential during a fault so nobody gets a shock across two simultaneously touchable surfaces.

Because the two conductors do different jobs, they are sized from different things. The earthing conductor is sized from the line conductor of the supply. The main bonding is sized from the supply itself, and how it is sized depends on whether the supply is PME. Mixing those two rules is the most common sizing slip on the bench.

Main earthing conductor (Table 54.7)

The simplest method is the selection table, BS 7671 Table 54.7, which sizes the earthing conductor straight from the line conductor where both are the same material (copper here):

So 25 mm² tails, the usual domestic single-phase supply, need a 16 mm² earthing conductor. The selection table is deliberately conservative; the adiabatic equation can sometimes justify a smaller conductor where the fault current and disconnection time are known, but the table is the fast, defensible answer and the one most inspectors expect to see.

One extra rule catches people out on TT systems. Where the earthing conductor is buried in the ground (a connection to an earth electrode), Table 54.1 sets minimums against corrosion and mechanical damage: 16 mm² copper if protected against corrosion but not mechanical damage, and 25 mm² copper if protected against neither. That overrides the smaller numbers Table 54.7 might otherwise allow.

Main protective bonding (Table 54.8)

On a PME (TN-C-S) supply, the combined neutral and earth means a broken supply neutral can push current down the bonding, so BS 7671 sizes main bonding more generously, from the copper-equivalent cross-section of the supply neutral conductor, using Table 54.8:

The everyday domestic supply sits in the top row, so the main bonding to gas and water is 10 mm². Two caveats. First, the table works on the copper-equivalent area, so an aluminium supply neutral has to be converted before you read the table. Second, the local distributor (DNO) can require a larger minimum, often a 10 mm² floor regardless, so check their requirements if in doubt.

Bonding on TN-S and TT supplies

Table 54.8 is only for PME. On a TN-S supply (separate earth, often the old lead sheath or an SWA earth) or a TT supply (your own earth electrode), main bonding is sized by Reg 544.1.1 instead: not less than half the cross-sectional area of the earthing conductor, with a minimum of 6 mm², and it need not exceed 25 mm² copper. So a 16 mm² earthing conductor on TN-S gives 10 mm² bonding (half of 16, rounded to the next standard size), and a small installation might land on the 6 mm² floor. Identifying the earthing arrangement before you size anything is therefore the first step, not an afterthought.

The quick domestic answer

For the supply you meet most days, single-phase PME with 25 mm² tails:

• Main earthing conductor: 16 mm² (Table 54.7).
• Main protective bonding: 10 mm² to gas and water (Table 54.8).

That is the pairing people garble as "is it 10 or 16?". It is both: 16 mm² earthing, 10 mm² bonding. If you want the numbers for any other supply or earthing type without reaching for the book, the free earthing and bonding conductor size calculator gives both at once from the supply and the earthing arrangement.

Worth a sentence: bonding is only required to extraneous-conductive-parts that can actually introduce a potential. A property fed by a plastic incoming water main and a plastic gas pipe may need no main bonding to those services at all, because the plastic does not import an earth potential. Bond the metalwork that qualifies, and note in the certificate why anything was left unbonded.

Supplementary bonding and the bathroom rule

Supplementary bonding ties together simultaneously-accessible exposed-conductive-parts and extraneous-conductive-parts within a location, classically the pipework and accessories in a room containing a bath or shower. It is sized from the circuit protective conductors, not the supply (Reg 544.2):

• Between two exposed-conductive-parts: not less than the smaller of the two circuit cpcs.
• Between an exposed-conductive-part and an extraneous-conductive-part: not less than half the cpc of the exposed-conductive-part.

Whatever those rules give, there is a mechanical floor: a supplementary bonding conductor must be at least 2.5 mm² if it has mechanical protection (for example contained in a wall or in conduit), or 4 mm² if it does not. In practice a length of 4 mm² green-and-yellow run clipped on the surface covers nearly every domestic case.

The more useful point on a modern installation is that you can often leave supplementary bonding out. Reg 701.415.2 allows it to be omitted in a location containing a bath or shower when all three of these are true: every final circuit meets the disconnection times for fault protection, every circuit has additional protection by a 30 mA RCD, and all extraneous-conductive-parts of the location are effectively connected to the main protective bonding. On a recent installation on a modern RCD-protected board with sound main bonding, all three usually hold, so the supplementary bonding most older bathrooms have is no longer required (though removing existing, sound bonding is rarely worth it).

Circuit protective conductors and the adiabatic method

The circuit protective conductor (cpc) is the earth inside each final circuit. It can be sized by the same selection table, Table 54.7, against that circuit's line conductor, or by calculation with the adiabatic equation, which usually permits a smaller conductor:

This matters because the cpc built into a twin-and-earth cable is a reduced size (a 2.5 mm² cable carries a 1.5 mm² cpc, and 1.0 mm² carries 1.0 mm²), and the adiabatic check is how you confirm that reduced cpc will survive the fault long enough for the device to clear it. Our adiabatic calculator and cable size calculator do the arithmetic, but the principle is the same as the main conductors: size for the fault that has to flow down it.

The details that fail an EICR

Sizing is half the job; the rest is the detail an inspector codes against. The recurring ones, and how they tend to fall on an EICR:

Two practical habits keep these off your reports. Land bonding connections within 600 mm of a meter or at the point of entry, before any branch or isolating valve, and fit the warning label at every earthing and bonding connection. And record the supply type, tails size and the resulting conductor sizes on the certificate, so the next inspector can see the sizing was deliberate rather than guessed.

Common questions

What size earthing conductor do I need?

By BS 7671 Table 54.7 the main earthing conductor equals the line conductor up to 16 mm², is 16 mm² for line conductors over 16 and up to 35 mm², and is half the line conductor above 35 mm². A typical 25 mm² domestic supply gives a 16 mm² earthing conductor. The adiabatic equation can sometimes justify a smaller size.

What size is the main bonding for a domestic supply?

On a PME (TN-C-S) supply with a neutral up to 35 mm², which covers most homes on 25 mm² tails, the main protective bonding is 10 mm² copper from Table 54.8. It rises to 16, 25, 35 and 50 mm² as the supply grows.

Is the bonding 10mm or 16mm?

On a normal domestic PME supply the main protective bonding is 10 mm² and the main earthing conductor is 16 mm². People mix the two up because both appear on the same job. Bonding goes to the gas and water; the earthing conductor goes from the main earthing terminal to the supply earth.

What size is supplementary bonding?

A supplementary bonding conductor is sized from the circuit protective conductors it connects: at least the smaller cpc between two exposed-conductive-parts, or at least half the cpc between an exposed-conductive-part and an extraneous-conductive-part. The floor is 2.5 mm² with mechanical protection or 4 mm² without.

Can I leave out supplementary bonding in a bathroom?

Yes, supplementary bonding in a location containing a bath or shower may be omitted under Reg 701.415.2 when all three conditions are met: every final circuit meets the disconnection times for fault protection, all circuits have 30 mA RCD additional protection, and all extraneous-conductive-parts are effectively connected to the main protective bonding.

Is undersized main bonding a C2 on an EICR?

Main protective bonding that is missing or undersized is normally coded C2, because it is potentially dangerous: a fault could leave extraneous metalwork at a dangerous voltage. First confirm whether bonding is required at all, since a property with plastic incoming water and gas may not need it.

Does the bonding size change on TN-S or TT?

Yes. Without PME, Reg 544.1.1 sets the main bonding at not less than half the cross-section of the earthing conductor, with a minimum of 6 mm², rather than the PME values in Table 54.8. So a 16 mm² earthing conductor gives 10 mm² bonding on TN-S or TT.