Free notes
Sizing earthing and bonding conductors: the quick ruleHow big should main bonding be?
Free from the notes: Sparky’s video on it, the section word for word as members see it, and a PDF to keep.
The notes
Exactly as they are on the site.
One last point: the size of the bonding conductors matters, and there are tables for it in your books.
- Main protective bonding depends on the size of the incoming supply (the tails and the earthing conductor).
- Supplementary bonding depends on the conductors at the exposed conductive parts being bonded.
Earthing conductor (Electrician's Guide Tables 3.3.2a/b, p.59): tails 6 → 6 mm² (10 on PME), 10 → 10, 16 → 16, 25 → 16, 35 → 16, 50 → 25, 70 → 35. Main bonding: TN-S 6 mm² for 6 and 10 mm² tails; PME 10 mm² for tails up to 35 mm². The distributor may ask for a 16 mm² earthing conductor.
Example (PME): 25 mm² tails → 16 mm² earthing conductor → 10 mm² main bonding
A common mix-up is that supplementary bonding is sized from the "live cables" at the exposed parts. In BS 7671 (544.2) it's actually sized from the protective conductors (CPCs) connected to those parts, and whether the bonding conductor is mechanically protected. Use Table 3.3.3 in the Electrician's Guide. Also, BS 7671's general minimum for main bonding (544.1.1) is 6 mm². The 10 mm² minimum applies to PME (TN-C-S) supplies (Table 54.8), which is why 10 mm² is the standard size in homes.
Electrician's Guide to the Building Regs · Tables 3.3.2a and 3.3.2b earthing and main bonding conductor sizes · p.59 Electrician's Guide to the Building Regs · 3.3.3 supplementary bonding · Table 3.3.3 BS 7671 · Reg 544.1.1 · Table 54.8 · Reg 544.2 · check your copy for the pages
Don't ignore main or supplementary bonding. When you price a job, check the bonding is there. If it's needed, include it and explain why. Plenty of electricians don't.
Free section from Sparky Study Notes. The full Level 3 notes, 1,463 practice questions and the video lessons: sparkystudynotes.co.uk
Test yourself on it
The tests are in the members’ area. Get one wrong and it shows you why, and where to revise.
Get new notes and exam tips from Sparky
About one email a fortnight. Unsubscribe any time. Privacy noticeThat was one section. There are 583.
Sparky Study Notes is the whole Level 3 syllabus written up like this: the explanation, the worked example, and where it is in your book. With a video lesson for every part and a practice paper for every exam.
- 583sections of written notes
- 35video lessons
- 1,463practice questions
- 146diagrams, redrawn
- 18th Edition
- Building Regs
- PAT
- Fundamental Inspection & Testing
- Initial Verification
- Periodic Inspection & Testing