Earthing & bonding
Earthing and bonding conductor size
Find the minimum main earthing conductor and main protective bonding conductor for a supply, from the supply conductor size and the earthing arrangement.
Main earthing from BS 7671 Table 54.7, main bonding from Table 54.8 (PME) or Reg 544.1 (TN-S / TT). Copper. Guide only; the adiabatic method may permit a different size.
Two different conductors, two different rules
The main earthing conductor connects the installation's main earthing terminal to the supply earth. The main protective bonding connects extraneous parts (gas, water, structural steel) to that terminal. They are sized differently, and mixing the rules is a common slip.
Main earthing conductor
BS 7671 Table 54.7 gives the simple sizing: equal to the line conductor up to 16 mm², 16 mm² for line conductors between 16 and 35 mm², and half the line conductor above 35 mm². The adiabatic equation can sometimes justify a smaller conductor, but the table is the quick, safe answer.
Main protective bonding
On a PME (TN-C-S) supply, Table 54.8 sets the minimum from the supply neutral: 10 mm² up to 35 mm² tails, then 16, 25, 35 and 50 mm² as the supply grows. The typical domestic 25 mm² tails give 10 mm² bonding and a 16 mm² earthing conductor. On TN-S or TT the bonding is at least half the earthing conductor, minimum 6 mm². Confirm the result on the EICR where undersized bonding is a common C2.
FAQs
What size is the main protective bonding for a 25mm2 supply?
On a PME (TN-C-S) supply with tails up to 35 mm2, the main protective bonding is 10 mm2 copper, from BS 7671 Table 54.8. Larger supplies need 16, 25, 35 or 50 mm2.
How big should the main earthing conductor be?
By BS 7671 Table 54.7 it equals the line conductor up to 16 mm2, is 16 mm2 for line conductors of 16 to 35 mm2, and half the line conductor above 35 mm2. A 25 mm2 supply gives a 16 mm2 earthing conductor.
Is bonding different on TN-S or TT?
Yes. Without PME, the main bonding is at least half the cross-section of the earthing conductor, with a minimum of 6 mm2, rather than following the PME Table 54.8 values.