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Reading the BS 7671 cable tablesWhat must the table rating reach?

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What it's for: finding a cable size whose current rating covers your It. Once you know It, you go to the right cable table for your cable type and read down the column for your installation method.

  • The cable tables ("current-carrying capacity" tables) fill over 40 pages of Appendix 4. The top-right corner of each says copper or aluminium; the aluminium tables start with Table 4H1A.
  • Tables ending in A hold the current ratings. The matching B table on the next page holds the voltage drop (mV/A/m). That's why you skip every other page as you search.
  • The one exception is Table 4D5 (flat twin and earth, p.484). It fits ratings and volt drop on a single table.
TableCable
4D1ASingle-core 70 °C thermoplastic (not ours)
4D2AMulticore 70 °C thermoplastic (not ours)
4D3ASingle-core armoured 70 °C thermoplastic (not ours)
4D4A / 4D4BMulticore armoured 70 °C: ratings / mV/A/m (not ours)
4D570 °C flat twin and earth, ratings and mV/A/m on one table, p.484 (not ours)
4E1ASingle-core 90 °C thermosetting, non-sheathed: our cable
4E1BIts voltage drop (mV/A/m) table

BS 7671 · Appendix 4 · cable tables · copper first, then aluminium (from Table 4H1A) BS 7671 · Table 4E1A · 90 °C thermosetting single-core, current ratings · p.486 BS 7671 · Table 4D5 flat T&E · p.484

Worked example: choosing the cable

Table 4E1A · first column = conductor CSA (mm²)
Use the Reference Method B, single-phase (2 cables) column
Read down it for the first value ≥ 20.83 A:
   1.5 mm² → 23 A  ✓

→ 1.5 mm² 90 °C thermosetting singles, in conduit (Method B),
  can carry up to and including 23 A.

In summary: know In, the cable type, the installation method and the correction factors. Do the It calculation. Then find a table value that's bigger.

10 A15 A20 A25 A Ib 13.2 Aactual current Iz 17.66 A23 × 0.96 × 0.8 (real rating) 23 A in the table1.5 mm², Table 4E1A, Method B In 16 Adevice rating It ≥ 20.83 A16 ÷ (0.96 × 0.8) Ib ≤ In ≤ Iz → 13.2 ≤ 16 ≤ 17.66 ✓
The numbers from our example on one line. The Ib of 13.2 A is the made-up design current used in the voltage drop step next. Dividing In by the factors (20.83 A) and then multiplying the table value by the same factors (17.66 A) are two views of the same check.
The letters It and Iz
It ≥ In ÷ (factors) = 20.83 A is the smallest tabulated current-carrying capacity you can accept, so you pick the first table value that's bigger: 23 A. Iz is the cable's rating once the factors are applied: 23 × 0.96 × 0.8 = 17.66 A, which is ≥ In (16 A) ✓.

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