Current-carrying capacityHow do you read Appendix 4?

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External influences: 522 and Appendix 5

Section 522 is the selection of wiring systems for the external influences they'll face. It gives the codes and examples; Appendix 5 gives the detail.

Code	Influence	Notes
AA	Ambient temperature	Leads to the C_a factor
AE	Presence of solid foreign bodies	Expressed as IP ratings — dust, fingers, tools
AG	Impact / mechanical stress	AG1 low (household) · AG2 medium (industrial)
AH	Vibration
AK	Presence of flora / mould
BA	Capability of persons	BA1 ordinary · BA2 children · BA4 instructed · BA5 skilled

↔Which book, which question. Asked what a code means (what is AG2?) → Appendix 5. Asked what influences you must consider for a wiring system → 522. The two interlock, and that's deliberate.

The AE row is a neat example of the book joining up: "presence of solid foreign bodies" is expressed as IP ratings — the same IP2X and IP4X from Section 416 earlier, arriving from a completely different direction.

522 · selection for external influences Appendix 5 · classification codes in detail

Current-carrying capacity and Appendix 4

Section 523 covers current-carrying capacity, and sends you to Appendix 4 for the numbers.

Cable	Max operating temperature
Thermoplastic (PVC) — standard grey twin & earth	70 °C
Thermosetting	90 °C

523 also explains why the neutral counts as a live conductor: it carries current in normal service, so it has to be allowed for. Harmonics are the case where the neutral current can exceed the line current.

Finding your way round Appendix 4

≡Tables ending in A = current-carrying capacity. Tables ending in B = voltage drop. The exception is Table 4D5 — flat twin & earth — which gives both in one table.

Also check whether you're on copper or aluminium conductors, and match the installation reference method.

10 mm² twin & earth, copper, reference method C

Appendix 4 → Table 4D5
"70 °C thermoplastic sheathed and insulated
 flat cable with protective conductor" = twin & earth

10 mm² row, reference method C column
It = 64 A

Volt drop, from the same appendix:

Volt drop = (mV/A/m × I_b × L) ÷ 1000

Correction factors live here too. For a cable up to 10 mm² surrounded by thermal insulation, Appendix 4 section 2.6 (p.451) gives C_i by length: 50 mm 0.88, 100 mm 0.78, 200 mm 0.63, 400 mm 0.51, and 0.5 for 0.5 m or more — in the notes ahead of the tables, not in the tables themselves. Anything to do with cable rating factors, current capacity or volt drop is Appendix 4.

523 · current-carrying capacity Table 52.2 · maximum operating temperatures Appendix 4 · It values, volt drop, correction factors Table 4D5 · flat twin & earth

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