Cable calculation, step 6: correction factors, Ca and CgWhat do Ca and Cg do to a cable?

Correction factors (some books call them derating factors) tailor the design to the real situation: buried in the ground, grouped with other circuits, passing through thermal insulation, and so on. Two apply here.

Ambient temperature, Ca

There's always an ambient temperature. A house is usually about 20 °C, but summers are getting hotter (remember the 40 °C heat of summer 2022). Good insulation keeps a house cool in summer too. 35 °C is a sensible, cautious middle ground. And a cooker circuit only works hard while someone is cooking.

Table 4B1, 70 °C thermoplastic, ambient 35 °C → Ca = 0.94

BS 7671 · Appendix 4 · Table 4B1 · ambient temperature factors (Ca)

Grouping, Cg

Count your circuit too: 2 already in the trunking + ours = 3 circuits. Table 4C1 doesn't print the symbol "Cg" in its heading, but the notes underneath mention Cg several times.

• Columns = number of circuits or multicore cables: 1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 16, 20. We use 3.
• Rows (items) = the arrangement:
  1. Bunched in air, on a surface, embedded or enclosed ← ours (in trunking)
  2. Single layer on a wall or floor
  3. Single layer on a perforated cable tray (tray with slots cut in the bottom)
  4. Single layer on a cable ladder (big SWA runs, industrial)

Table 4C1, item 1 (bunched/enclosed), 3 circuits → Cg = 0.7

The higher the factor, the less effect it has on your design.

BS 7671 · Appendix 4 · Table 4C1 · grouping factors (Cg in the notes underneath)