BS 7671:2018+A2 (NICEIC) Cable Sizing — Method
BS 7671:2018+A2:2022 (IET Wiring Regulations, 18th Edition) is the UK national wiring code, certified by NICEIC, NAPIT, ELECSA and STROMA. Cable sizing follows Appendix 4 — pick the row from Tables 4D1A–4J5A (selected by conductor material, insulation, reference method and number of cores), apply Ca, Cg, Ci correction factors, then verify voltage drop and 0.4 s / 5 s disconnection.
Where:
- Ib — design current of the circuit (A), from the load calculation
- Ca — ambient temperature correction (1.00 at 30 °C reference)
- Cg — grouping / bunching factor (1.00 for a single circuit)
- Ci — thermal-insulation factor (1.00 if the cable is in free air; 0.50 if fully buried in insulation)
Then pick the smallest cable cross-section in BS 7671 Appendix 4 Table 4D2A (single-core PVC, Ref Method B) whose tabulated ampacity Iz ≥ It.
Related cable sizing calculators
Other standard- and method-specific cable-sizing calculators in the same series — same procedure, different reference tables and defaults:
- Cable Sizing Calculator (universal) — the seed page covering all standards in one tool
- IEC 60364 Cable Sizing Calculator — IEC 60364-5-52 · International
- NEC Cable Sizing Calculator — NEC NFPA 70 · USA
- Single-Phase Cable Sizing Calculator — Single-Phase · 230 V / 240 V
- All Electrical Engineering Calculators →
💡 Sizing the earth / protective conductor? Use the dedicated Earthing Cable Size Calculator — BS 7671 Table 54.7, IEC 60364-5-54, NEC 250.122.
Frequently Asked Questions
Tables 4D1A–4J5A in Appendix 4. Most common: 4D2A (single-core 70 °C PVC, methods A–E), 4D5 (multi-core 70 °C PVC), 4E2A (90 °C thermosetting). The 'A' tables give tabulated current-carrying capacity Iz; 'B' tables give voltage drop in mV/A/m.
From Appendix 4: Ca (ambient temperature, Table 4B1 — 0.87 at 40 °C for PVC), Cg (grouping, Table 4C1 — 0.80 for 3 circuits enclosed), Ci (thermal insulation, Table 52.2 in BS 7671 — 0.5 if fully buried in insulation > 0.5 m). Required tabulated current It = Ib / (Ca × Cg × Ci). Then pick a cable whose Iz ≥ It.
BS 7671 Table 4Ab: 3 % for lighting and 5 % for power circuits, measured from the origin of the installation to any consuming point. For installations supplied directly from the public network, the same limit applies to the whole installation. For HV installations or longer runs, refer to BS 7671 Section 525.
Yes. BS 7671 Tables 41.2–41.4 give the maximum earth-fault loop impedance Zs that ensures disconnection within the required time (0.4 s for 32 A or below in TN systems; 5 s for distribution circuits). Cable size and length, breaker characteristic (B/C/D) and circuit Zs together must satisfy this check, often more restrictive than ampacity.
Amendment 2 (March 2022) added: mandatory AFDDs in HMOs, hotels, care facilities and student accommodation (Section 421.1.7); revised Section 411 and 722 EV-charger requirements; updated Surge Protection Device (SPD) coordination; expanded use of low-smoke and halogen-free cable in escape routes. Cable-sizing tables are unchanged from 2018.
NICEIC inspectors verify on the Electrical Installation Certificate (EIC): correct cable type for the location, correct CSA from BS 7671 Appendix 4, voltage-drop calculation, Zs measurement vs Table 41 limit, RCD type & rating, and segregation from Cat-5/data cable per BS 6701. The cable-sizing calculator can populate the EIC's design-data section.