Protective Conductor Sizes…
Regulation 543.1.1: This can be achieved by either calculation (543.1.3) or selected (543.1.4) from Table 54.7.
The formula used in 543.1.4 for calculating the size of a protective conductor is covered in great detail in a session inside our 17th Edition online course.
Selecting from Table 54.7
The protective conductor size is relative to the size of the line conductor which is represented by the letter ‘S’.
Also, the far right column can usually be ignored as this is only used when the protective conductor and line conductor are of different materials.
So, it’s just the first two columns and the rows are…
1. Where the line conductor ‘S’ is less than or equal to 16mm then the protective conductor is the same size as ‘S’ your line conductor.
2. Where the line conductor ‘S’ is greater than 16mm but less than or equal to 35mm then the protective conductor size is 16mm
3. Where the line conductor ‘S’ is greater than 35mm then the protective conductor is half the line conductor size S/2
Remember when you get a non standard conductor size using this table you must round up to the next size available.
Main Bonding Conductor Sizes…
For protective bonding conductors in a TN-S or TT supply it’s (Regulation 544.1.1) half the size of the main earth with a minimum size being 6mm and also if using copper it does not need to exceed 25mm
For protective bonding conductors in a TN-C-S supply it’s Table 54.8 and is based on the size of the neutral of the supply 35mm or less = 10mm etc
Supplementary Bonding Conductor Sizes…
See Regulation 544.2.1 to 544.2.3: If mechanical protection is provided then it’s 2.5mm, all other instances 4mm minimum ‘which is usually the case although a lot of contractors use 6mm’
For more information please see BS7671 Chapter 54 or enrol on our 17th Edition Online City & Guilds 2382 Course