Table 54.7, and sizing the earthing conductorIs a CPC always half the line conductor?

A straight question: when do you actually use Table 54.7? The answer is a genuinely useful distinction.

You wouldn't reach for it to check the CPC in twin & earth — T&E only goes up to 16 mm², and the CPC in it is never larger than the line conductors. Where you do use it is choosing an earthing conductor from the size of the incoming tails.

25 mm² incoming tails
25 falls in the 16 < S ≤ 35 mm² band
Table 54.7 → 16 mm² earthing conductor

95 mm² incoming line conductor
Over 35 mm² → S ÷ 2 = 47.5 mm² — no such cable
Round up → 50 mm² earthing conductor

And bonding conductors — 544.1.1

Separate rule, separate place in the book, and a favourite exam question. Main protective bonding conductors are in 544.1.1:

• Not less than half the CSA required for the earthing conductor of the installation, and
• need not exceed 25 mm² for copper — that's the ceiling a typical question asks for on a TN-S system.

Table 54.7 · p.211 · protective conductor vs line conductor ★ 544.1.1 · main protective bonding · half the earthing conductor, max 25 mm² copper ★ 542.1.2 · supply arrangements · earthing terminal connected to the earth point of the source (TN-S)