Earth Bonding Cable Size Selection

Line Conductor or Neutral Conductor of PME supplied	mm2	4	6	10	16	25	35	50	70
Earthing Conductor not buried or buried and protected against corrosion and mechanical damage	mm2	6	6	10	16	16	16	25	35
Main Protective Bonding Conductor	mm2	6	6	6	10	10	10	16	25
Main Protective Bonding Conductor for PME supplies (TN-C-S)	mm2	10	10	10	10	10	10	16	25

Notes:
1. Protective conductors (including earth and bonding conductors) of 10mm² cross-sectional area or less shall be of copper.

2. The distributor may require a minimum size of earth conductor at the origin of the supply of 16mm² copper or greater for TN-S and TN-C-S supplies.

3. Buried earth conductors must be at least:
• 25mm² copper if not protected against corrosion
• 50mm² steel if not protected against corrosion
• 16mm² copper is not protected against mechanical damage but protected from corrosion.
• 16mm² coated steel is not protected against mechanical damage but protected against corrosion.

4. The distributor should be consulted when in doubt.

Buried
Unprotected	Protected against corrosion	Protected against corrosion and mechanical damage
mm2	mm2	mm2
25	16	2.5
Not Buried
Unprotected	Protected against corrosion	Protected against corrosion and mechanical damage
mm2	mm2	mm2
4	4	2.5

Notes:
1. Assuming protection against corrosion by a sheath

2. The main protective bonding conductors shall have a cross-sectional area of not less than half that required for the earthing conductor and not less than 6mm².

Please be aware of the following:
a) Only copper conductors should be used: copper covered aluminium conductors or aluminium conductors or structural steel can only be used is special precautions outside the scope of the Wiring Guide are taken.

b) Bonding connections to incoming metal services should be made as near as practicable to the point of entry of the services into the premises, but on the consumer’s side of any insulating section.

c) Where practicable, the connection to the gas, water, oil, etc service should be within 600mm of the service meter, or a point of entry to the building if the service meter is external and must be on the consumer’s side before and branch pipework and after any insulating section in the service. The connection must be made to hard pipework, not to soft or flexible meter connections.

d) The connection must be made using clamps ( to BS 951) and be suitably protested against corrosion at the point of contact.

(taken from source: Onsite Guide : BS 7671:2008 +A3: 2015. 6th Edition)

 

Return to FAQs