What size should main bonding conductors be?

For a TN-S or TT supply, BS 7671 Section 544 requires a main protective bonding conductor of at least half the cross-sectional area of the earthing conductor, with a minimum of 6 mm² copper. For a TN-C-S (PME) supply, Table 54.8 sets a minimum of 10 mm² copper for supply neutrals up to 35 mm², rising with larger neutrals.

What is an extraneous-conductive-part?

A conductive part that is not part of the electrical installation but could introduce a potential, usually earth potential, into the building: a metal gas or water pipe entering from the ground, an oil line, structural steel or a metal-sheathed cable. It must be bonded to the main earthing terminal so it cannot be at a different voltage from the electrical earth during a fault.

Do plastic incoming gas and water pipes need bonding?

No. A plastic service pipe cannot introduce a potential from outside, so it is not an extraneous-conductive-part and BS 7671 does not require bonding. Where the internal pipework is metal but the incoming service is plastic, bonding is normally unnecessary, though a test of resistance to the main earthing terminal confirms it.

Where should the main bonding connection be made?

As near as practicable to the point where the service enters the building, on the consumer's side of any meter or insulating section, and before any branch. IET guidance is within 600 mm of the meter outlet where practicable. The clamp must carry the label 'Safety Electrical Connection – Do Not Remove'.

What EICR code is missing main bonding?

Electrical Safety First's Best Practice Guide 4 classifies absent or inadequate main protective bonding to gas or water as C2, potentially dangerous, because a fault could leave pipework at a dangerous voltage. Undersized bonding that is present and sound, for example 6 mm² on a PME supply, is generally C3.

What is main protective bonding? Explained

Main protective bonding is the connection, using a conductor of at least 6 mm² copper (10 mm² on a PME supply), between the main earthing terminal of an installation and the extraneous-conductive-parts entering the building, such as metal gas, water and oil pipes and structural steel, so that during an earth fault all of them rise to the same potential and no dangerous voltage can appear between a person's hands. It is required by BS 7671 Regulation 411.3.1.2 and sized under Section 544, and its absence is classified C2 on an EICR by Electrical Safety First's Best Practice Guide 4.

Key takeaways

Bonding equalises potential; it is not there to carry fault current back to the supply.
Metal gas, water, oil and heating pipes and structural steel entering the building must be bonded.
Minimum 6 mm² copper on TN-S and TT; minimum 10 mm² copper on TN-C-S (PME).
Missing main bonding is normally coded C2 on an EICR; undersized but sound bonding is generally C3.

What is main protective bonding in practice?

Automatic disconnection under BS 7671 Chapter 41 assumes that, in the fraction of a second before the protective device operates, a person cannot touch two things at different voltages. The installation's metalwork is at the supply earth potential through the circuit protective conductors; a metal water pipe from the ground is at the potential of the ground outside, which under fault conditions is not the same.

Main protective bonding ties them together at the main earthing terminal (MET). If the installation's earth rises to 100 V during a fault, the bonded gas pipe rises with it and the voltage between a tap and a radiator stays at zero. On a PME supply a broken network neutral can lift the whole installation earth well above true earth, which is why the bonding conductor is larger.

When is main protective bonding needed?

Every installation needs main bonding to every extraneous-conductive-part present. The usual list is:

Service	Bond required?
Metal gas pipe entering the building	Yes
Metal water pipe entering the building	Yes
Metal oil supply pipe	Yes
Structural steel frame	Yes
Lightning protection system	Yes, where present
Plastic incoming gas or water pipe	No, not an extraneous-conductive-part
Internal copper pipework fed by a plastic incomer	Usually no; test to confirm

Where it is unclear whether internal metalwork is extraneous, IET Guidance Note 8 describes a test: measure the resistance between the part and the MET with the bonding disconnected; above about 22 kΩ the part cannot introduce a dangerous potential. Main bonding is checked on every EIC and Minor Works Certificate, which both ask whether it is present and adequate, and inspected on every EICR.

What does main protective bonding consist of and how is it measured?

Conductor size, under BS 7671 Section 544:

Supply	Minimum bonding conductor
TN-S or TT	Half the earthing conductor CSA, minimum 6 mm² copper, need not exceed 25 mm²
TN-C-S (PME), supply neutral up to 35 mm²	10 mm² copper (Table 54.8)

Connection: a BS 951 earth clamp on the pipe, as near as practicable to the point of entry, on the consumer's side of the meter and before any branch or insulating section; IET guidance suggests within 600 mm of the meter outlet. The clamp must carry the label "Safety Electrical Connection – Do Not Remove" required by Regulation 514.13.1. The other end terminates at the MET.

Testing: continuity is measured with the low-ohm function of a multifunction tester between the MET and the far end of the bonding conductor at the clamp. IET Guidance Note 3 gives a guide value of 0.05 Ω or less for a sound conductor.

Common mistakes with main protective bonding

Bonding after a branch, so the first tee off the incoming pipe is left unbonded.
6 mm² on a PME supply, common in 1980s and 1990s installations; sound but undersized, usually C3.
Missing that a metal incomer has been replaced with plastic, or bonding a plastic incomer's internal copper unnecessarily.
No label on the clamp, a C3 but an easy one to avoid.
Confusing main bonding with supplementary bonding. Supplementary bonding in bathrooms under Section 701 is a separate requirement.
Ticking "adequate" on a Minor Works Certificate without looking. Go and look at the meter box.

Related terms

How Certio helps

Certio records this on the BS 7671 model forms without the typing: photograph the board to draft the circuit schedule, point the camera at a Megger MFT-X1 to capture readings, or dictate them. Starter is free for 7 days.

Written by the Certio team. Last checked 14 September 2026.