BS 7671 Appendix 4 — Cable Current Rating (Exam Prep)

The cable-type tables you will use most: 4D1A/4D1B single-core 70 °C thermoplastic (PVC) cables; 4D2A/4D2B multicore thermoplastic non-armoured cables; 4D4A/4D4B multicore thermoplastic armoured (SWA) cables; 4D5 flat twin and earth, with current rating and voltage drop in one table; and the 4E tables for 90 °C thermosetting (XLPE) cables, such as 4E4A/4E4B for armoured XLPE. In each pair the A table gives current-carrying capacity and the B table voltage drop. Table 4A2 lists the installation methods and their reference methods; Tables 4B and 4C give the rating factors.

Reference Method A: enclosed in conduit in a thermally insulating wall. B: enclosed in conduit or trunking on a wall. C: clipped direct. D: in the ground, direct (D2) or in ducts (D1). E: multicore cable in free air, for example on perforated tray. F: single-core cables touching in free air. G: single-core cables spaced in free air. Table 4A2 maps each real installation method to its reference method, and twin and earth in insulation has its own methods in Table 4D5.

Ambient temperature (Ca) from Table 4B1, with 30 °C as the reference; grouping (Cg) from Table 4C1; thermal insulation (Ci) from Appendix 4 section 2.6, where a cable totally surrounded for 50, 100, 200 or 400 mm is derated by 0.88, 0.78, 0.63 or 0.51 (0.5 beyond that). Divide the protective device rating by the product of the factors to find the tabulated rating you need: It ≥ In ÷ (Ca × Cg × Ci).

Volt drop (V) = mV/A/m × Ib × L ÷ 1000. Appendix 4 Table 4Ab sets 3% for lighting and 5% for other uses where the supply is public low voltage, which at 230 V is 6.9 V and 11.5 V. Example: 2.5 mm² twin and earth (18 mV/A/m, Table 4D5) carrying 32 A over 30 m drops 18 × 32 × 30 ÷ 1000 = 17.3 V, or 7.5%, so it fails the 5% limit and needs a larger cable.

The three-phase mV/A/m values in the tables are already line (line-to-line) values, so multiply them by Ib and length as usual and compare the result with 5% of 400 V, which is 20 V. Do not multiply by √3 again; that double-counts and oversizes the cable.

Tab Table 4A2 (installation methods), 4B1 (ambient), 4C1 (grouping), 4D5 (twin and earth) and 4E4A/4E4B (armoured XLPE). Practise cable-sizing questions end to end: design current, device rating, rating factors, tabulated current, then voltage drop.

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