As the IET itself states: “The tables are a starting point, not the final answer.” Ignoring that principle is the fastest route to a non-compliant – and dangerous – installation. This piece is for educational and reference purposes. Always refer to the latest BS 7671 and consult a qualified electrical engineer for live designs.
[ S = \frac\sqrtI^2 tk ]
| Factor | BS 7671 Ref | Applies to | |--------|-------------|-------------| | ( C_a ) | Table 4B1 | Ambient temperature ≠ 30/40°C | | ( C_g ) | Table 4C1 | Grouping of circuits (mutual heating) | | ( C_d ) | Table 4B2 | Buried cables (soil thermal resistivity) | | ( C_i ) | Table 4B3 | Thermal insulation (e.g., in a stud wall) | | ( C_c ) | Regulation 433.1 | Protective device type (e.g., BS 3036 semi-enclosed fuse: 0.725) | bs7671 cable sizing
For any electrical installation operating in the UK, compliance with BS 7671 (IET Wiring Regulations) is not optional—it is a legal benchmark under the Electricity at Work Regulations 1989. At the heart of this compliance lies a process often misunderstood as simple table-lookup: cable sizing .
Where ( L ) is the cable length in metres (line + neutral – so for single-phase, use the tabulated mV/A/m directly; for three-phase, note correction). As the IET itself states: “The tables are
[ I_z = I_t \times C_a \times C_g \times C_d \times C_i \times C_c ]
If voltage drop exceeds the limit, the cable size must be increased – often overriding the thermal sizing for long runs. Even if a cable is correctly sized for load current, it must survive a short circuit fault without insulation damage. BS 7671 provides the adiabatic equation: [ S = \frac\sqrtI^2 tk ] | Factor
[ Z_s = Z_DB + (R_1 + R_2) \times L ]
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