Cable calculation, steps 1 and 2: cable type and installation methodWhich reference method is cable in trunking?

Step 1: cable type (Table 4A1)

In a dwelling we'll use twin and earth. It's the most versatile cable on the market. T&E isn't "domestic only". You can use it in commercial and industrial work too, and you can use singles or flex in a house. Chapter 52 covers the do's and don'ts of wiring systems.

The route: use the existing trunking (there's room, and the client is happy with it). That means less lifting floorboards, chasing walls or going through ceilings. Above the cooker position, the cable comes out of the back of the trunking into a chase straight down.

Table 4A1 tells you whether each cable type is permitted (P) or not permitted (NP) with each installation method.

• Non-sheathed cables (singles: a conductor with one layer of coloured insulation) are NP clipped direct or on cable tray, but permitted in conduit, trunking and ducting and on insulators.
• Sheathed cables are split into multicore and single-core. A single-core sheathed cable is like a meter tail. T&E is multicore: line, neutral and CPC in one sheath.
• Multicore sheathed cable is permitted almost everywhere, including in trunking. Clipped inside the chase counts as clipped direct, which is also permitted.

70 °C or 90 °C? 70 °C thermoplastic (PVC) is fine in a house. Think about 90 °C thermosetting or other cables where there's an increased risk of fire, a combustible building, or flammable materials stored or processed.

70 °C thermoplastic twin and earth cable.
Table 4A1 suggests this cable type is suitable for trunking,
and clipped inside the chase.

BS 7671 · Appendix 4 · Table 4A1 · cable types vs installation methods (P / NP) · p.460 BS 7671 · Chapter 52 · selection and erection of wiring systems

Step 2: installation method (Table 4A2)

• In the trunking: a multicore cable in trunking on a wooden or masonry wall (run horizontally or vertically) is Reference Method B.
• In the chase: we assume the wall is thermally insulating. A multicore cable in a thermally insulating wall is Reference Method A (at the top of Table 4A2).

If the wall isn't thermally insulating, stick with Method B throughout. The books give little guidance for this case, and it depends on the age of the house.

Installation method:
  Ref Method B  (whilst in trunking)
  Ref Method A  (whilst in chase)

BS 7671 · Appendix 4 · Table 4A2 · installation methods · p.461