Cable calculation, step 5: the protective device, InWhy a 32 amp breaker and not a 25?

In is the rating of the protective device, the number stamped on its front. It must be at least Ib.

You can "misuse" OSG Table B6 to see what sizes exist. Appendix B is really for maximum measured Zs values, but the column headings list every circuit-breaker size (BS 3871, BS EN 60898 circuit-breakers and BS EN 61009 RCBOs). Don't write the table number on your design, so nobody later thinks B6 is "the circuit-breaker size table".

• 25 A is the next standard size up from 24.5 A (Table 41.3 has a 25 A row), but it leaves almost no margin over Ib. There's no 30 A circuit-breaker; the next size is 32 A, which is what this design uses. So: In = 32 A.

Type B, C or D? Type B is for general use in domestic, commercial or industrial work. In a house you'll rarely need anything else, unless there's equipment with a high starting current (workshop machinery). Specialist equipment often states the type it needs. If a Type B keeps tripping on start-up, swapping it for a Type C is quick and cheap once you've checked the circuit's Zs against the lower Type C maximum (with an ordinary breaker at a few pounds, less so with a £140 AFDD RCBO).

Ib ≤ In    24.5 A ≤ 32 A ✓
In = 32 A Type B RCBO with AFDD (30 mA)

On-Site Guide · Appendix B · maximum measured Zs (Table B6 · p.159) On-Site Guide · Table B6 · circuit-breakers and RCBOs (lists the ratings)