Bs7671 Cable Sizing < 720p >

The first step is to establish the design current ($I_b$) of the circuit. For final circuits, this is typically the rated current of the load. For distribution circuits, it is the maximum demand of the downstream 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. bs7671 cable sizing

Where ( R_1 + R_2 ) is the resistance per metre of line and earth/cpc conductors. If ( Z_s ) exceeds the maximum permitted for the protective device, the cable size must increase to reduce ( R_1 + R_2 ), or an RCD must be added. The first step is to establish the design

For a circuit protected by a 32A MCB, if ( C_a = 0.87 ) and ( C_g = 0.8 ), then ( I_t ) must be at least ( 32 / (0.87 \times 0.8) = 46A ) from the table – a much larger cable than intuition suggests. This piece is for educational and reference purposes

10mm² twin and earth PVC copper.