Cable sizing to BS 7671 (the 18th Edition Wiring Regulations) comes down to three currents and a handful of correction factors. Get the relationship between Ib, In and Iz right and apply the correct factors, and the rest is arithmetic. This guide explains each term, walks through the correction factors Ca, Cg and Cc, and finishes with a full worked domestic example.
The three currents at the heart of BS 7671
- Ib - the design current: the current the circuit is actually intended to carry, derived from the connected load.
- In - the nominal rating of the protective device (the fuse or MCB).
- Iz - the effective current-carrying capacity of the cable in its installed conditions, after all correction factors are applied.
The fundamental requirement of Regulation 433.1.1 is a chain of inequalities:
Ib ≤ In ≤ Iz
In words: the device rating must be at least the design current, and the cable's installed capacity must be at least the device rating. There is also a second condition for ensuring the device actually operates on overload: I2 ≤ 1.45 × Iz, which is automatically satisfied by modern BS EN 60898 MCBs.
Working backwards to the tabulated capacity
To choose a cable you need its tabulated current-carrying capacity It (from BS 7671 Appendix 4, e.g. Tables 4D2A/4D5). Because the installed conditions reduce capacity, you size up the requirement before reading the table:
It ≥ In ÷ (Ca × Cg × Cc × Ci × Cf)
You then pick the smallest cable whose tabulated rating for the chosen installation method (reference method) meets or exceeds It.
The correction factors
Ca - ambient temperature
The tables assume a 30 °C ambient for cables in air and 20 °C for buried cables. If the surroundings are hotter, the cable carries less, so Cais below 1 (e.g. about 0.94 at 35 °C for 70 °C thermoplastic insulation). Values come from Tables 4B1 and 4B2.
Cg - grouping
Where several circuits are grouped together, mutual heating reduces each cable's capacity. Cg (Table 4C1 onwards) depends on the number of grouped circuits and the arrangement - for example, around 0.80 for two circuits touching, or 0.65 for four. Cables spaced apart are penalised less.
Cc - design / installation conditions
Cc handles two distinct situations. The first is protection by a BS 3036 semi-enclosed (rewirable) fuse, which fuses slowly and so requires a factor of 0.725. The second is for cables buried in the ground where the soil thermal resistivity or burial conditions differ from the table's assumptions.
Ci - thermal insulation
A cable surrounded by thermal insulation cannot dissipate heat. A cable totally enclosed in insulation for more than 0.5 m is derated to roughly 0.5; shorter lengths use the factors of Regulation 523.9 / Table 52.2.
Worked domestic example: a cooker circuit
A domestic cooker presents a 12 kW connected load on a 230 V single-phase supply. After applying the BS 7671 Appendix A diversity allowance (first 10 A at 100%, remainder at 30%, +5% if the unit has a socket):
- Full-load current = 12000 ÷ 230 = 52.2 A
- Diversity: 10 A + (52.2 − 10) × 0.30 = 10 + 12.7 ≈ 22.7 A → this is Ib.
Protect the circuit with a 32 A Type B MCB, so In= 32 A (which satisfies Ib ≤ In). The cable is 6 mm² twin-and-earth clipped direct (Reference Method C), run through a loft where the ambient reaches 35 °C (Ca = 0.94), grouped with one other circuit (Cg = 0.80), no rewirable fuse (Cc = 1) and clear of thermal insulation (Ci = 1):
- Required It= 32 ÷ (0.94 × 0.80 × 1 × 1) = 32 ÷ 0.752 ≈ 42.6 A
- Tabulated Itfor 6 mm² T&E, Method C = 47 A → this meets 42.6 A, so Iz= 47 × 0.752 ≈ 35.3 A.
- Check: Ib (22.7) ≤ In (32) ≤ Iz (35.3) ✓
Result:6 mm² is adequate for capacity. The final step is always to confirm voltage drop (≤ 5% for a power circuit) and the earth-fault loop impedance / disconnection time; on a long run those checks, not capacity, often decide the cable.
Verify with the calculators
The free Voltix cable sizing calculator applies these correction factors for BS 7671 and AS/NZS so you do not have to look up every table by hand. Use the cable derating calculator to isolate the effect of Ca and Cg, the voltage drop calculator for the final compliance check, and the earthing & grounding calculator to size the protective conductor. New to voltage drop limits? Start with voltage drop explained, and for the US equivalent of this whole process see our NEC wire sizing guide.