4 mm² copper, 70°C PVC.
PROCESS: Ib = current In = next std rating Apply Ca, Cg, Ci → It Select min mm² from table where Iz_corrected ≥ It Calculate Vdrop If Vdrop > limit: increase mm² Check Zs (if supply Ze given) If Zs too high: increase mm² or require RCD
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30 A shower, 230 V, 25 m run, 70°C PVC cable, clipped direct (Method C), ambient 30°C, no grouping.
Adiabatic check (optional if RCD present). Fine. 4 mm² copper, 70°C PVC
If (V_d) > limit → increase cable size and recheck.
(BS 7671 Appendix 4): [ I_z \ge I_t ] (I_t) = tabulated current-carrying capacity required. Adiabatic check (optional if RCD present)
[ I_t \ge \fracI_nC_g \times C_a \times C_ci \times C_temp ] Where: