Appendix 4 is the most-used part of BS 7671 in the exam — current-carrying capacity, installation methods, grouping factors, voltage drop. Master Appendix 4 navigation and you save minutes per question. This guide is for electricians revising Appendix 4 for the 18th Edition exam.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
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Method A — in conduit in a thermally insulated wall. Method B — in conduit on a wall. Method C — clipped direct. Method D — buried direct in ground. Method E — multi-core in free air. Method F — multi-core in free air with spacing. Method G — single-core in free air with spacing. Reference Method is the SINGLE biggest factor in cable current-carrying capacity — wrong method gives wildly wrong answers.
Ambient temperature (Ca) — 30°C is the reference; higher ambient reduces capacity. Grouping (Cg) — Table 4C1 / 4C2 for multiple cables in containment. Thermal insulation (Ci) — Table 52.2; cables in or surrounded by insulation are reduced (e.g. 0.78 for cable above insulating ceiling). Multiply ALL applicable factors together: I_z_actual = I_z_tabulated × Ca × Cg × Ci.
Voltage drop = (mV/A/m × I_b × L) / 1000 in volts. Compare to the 3% / 5% limits in Reg 525.202 (3% for lighting, 5% for other circuits, both at 230V nominal). For example: 2.5mm² T&E Method C, 32A circuit, 30m length, mV/A/m = 18 (from Table 4D2B). Volt drop = (18 × 32 × 30) / 1000 = 17.3V = 7.5% — fails the 5% limit. Increase cable size.
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For three-phase, the voltage drop in mV/A/m tables is the per-phase value. The line-to-line voltage drop = √3 × per-phase drop. The 5% limit applies to the line-to-line nominal voltage (400V), so 5% = 20V line-to-line = 11.5V per phase. Get this mixed up and you under-size three-phase cables.
Tab Table 4D2A (twin and earth current carrying), Table 4D2B (twin and earth voltage drop), Table 4A2 (installation method), Table 4C1 (grouping factors), and Table 4B1 (ambient correction). Practice 20+ cable-sizing scenarios before the exam — given the load, length, method, ambient and grouping, work out the cable size to comply with both current capacity AND voltage drop.
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