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Cable calculation, step 7: It and choosing the cableWhy does a cooker need 16 mm² here?
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The question at the end: Why does a cooker need 16 mm² here? The answer is in the notes below.
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Exactly as they are on the site.
In these notes this worked-out number is called It. The formula is also printed in BS 7671 Appendix 4.
Working
It ≥ In ÷ (Ca × Cg) = 32 ÷ (0.94 × 0.7) = 32 ÷ 0.658 = 48.6 A
BS 7671 · Appendix 4 · the It ≥ In ÷ (Ca × Cg …) formula · p.453
Only Ib (24.5 A) actually flows. But grouped with two other circuits at 35 °C, the cable finds it harder to get rid of its heat. It's like wrapping insulation round a metal rod held over a match: you'd burn your fingers sooner. So the cable must be able to handle as much heat as 48.6 A would make in open air. The 70 °C rating is the insulation's limit. It's the conductor size that decides how much heat the cable can carry (a big serving spoon heats up more slowly than a teaspoon).
Finding the cable
BS 7671 Appendix 4 has over 40 pages of current-rating tables: copper first, then aluminium. Each table's heading gives the cable type, and the corner says copper or aluminium. For flat 70 °C T&E it's Table 4D5. Read down the column for your reference method until you reach a value at least 48.6 A.
| CSA | Ref Method A | Ref Method B | Ref Method C | mV/A/m |
|---|---|---|---|---|
| 10 mm² | (too small) | 52 A | – | – |
| 16 mm² | 57 A | 69 A | 85 A | 2.8 |
- Method B (trunking) alone: 10 mm² would do (52 A ≥ 48.6 A).
- Method A (insulating wall): 10 mm² isn't enough. You need 16 mm² (57 A).
- Worst case wins, so: 16 mm² T&E.
Our statement
Table 4D5 suggests that, using Ref Method A, a 16 mm² cable is to be used, which has a tabulated rating (It) of 57 A.
BS 7671 · Appendix 4 · current-rating tables (copper first, then aluminium) BS 7671 · Table 4D5 · 70 °C flat twin and earth (ratings and mV/A/m) · p.484 Use the mV/A/m column, not a rating column
It is the tabulated current-carrying capacity: the worked-out 48.6 A is the smallest It you can accept, so you choose the first table value above it, 57 A. Iz is the cable's rating once the factors are applied: 57 × 0.94 × 0.7 = 37.5 A. Step 9 uses Iz.
A 16 mm² cable is huge for a cooker. It's expensive and far harder to install than 10 mm². On a real job, this is the point where you'd question the route. For example, you might suggest keeping the cable in trunking all the way down (Method B), avoiding the insulating wall and Method A. We stick with 16 mm² here because it's a paper exercise.
Worth knowing: a cable buried directly in an ordinary (not insulating) masonry wall is normally Reference Method C, which rates higher than B. So in a normal wall, B in the trunking would still be the worst case. Check Table 4A2.
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