S=I2×tkcap S equals the fraction with numerator the square root of cap I squared cross t end-root and denominator k end-fraction Minimum cross-sectional area ( mm2m m squared I: Fault current (Amps). t: Disconnection time of the protective device (Seconds).
Most guides present cable sizing as a neat two-step formula:
$$I_b \leq I_n \leq I_z$$
$$I_t \geq \fracI_nC_a \times C_g \times C_i \times C_t$$
If the cable you selected in Step 6 has an area smaller than the calculated $S$ above, you must upsize the cable. calculation of cable size formula
The current carrying capacity found in tables (e.g., IEC 60364) is based on a reference method (usually in free air at 30°C). Real-world conditions reduce this capacity.
Where:
A 100m, 32A single-phase circuit at 230V, 3% voltage drop (6.9V) → formula gives ~16mm² copper. But ampacity tables (30°C, clipped direct) allow 6mm² for 47A. So voltage drop forces 16mm², not current. Yet, a 16mm² cable in a hot attic with 3 other circuits might fail on thermal derating — a factor neither formula nor basic tables reveal without careful lookup.
S=I2×tkcap S equals the fraction with numerator the square root of cap I squared cross t end-root and denominator k end-fraction Minimum cross-sectional area ( mm2m m squared I: Fault current (Amps). t: Disconnection time of the protective device (Seconds).
Most guides present cable sizing as a neat two-step formula:
$$I_b \leq I_n \leq I_z$$
$$I_t \geq \fracI_nC_a \times C_g \times C_i \times C_t$$
If the cable you selected in Step 6 has an area smaller than the calculated $S$ above, you must upsize the cable.
The current carrying capacity found in tables (e.g., IEC 60364) is based on a reference method (usually in free air at 30°C). Real-world conditions reduce this capacity.
Where:
A 100m, 32A single-phase circuit at 230V, 3% voltage drop (6.9V) → formula gives ~16mm² copper. But ampacity tables (30°C, clipped direct) allow 6mm² for 47A. So voltage drop forces 16mm², not current. Yet, a 16mm² cable in a hot attic with 3 other circuits might fail on thermal derating — a factor neither formula nor basic tables reveal without careful lookup.
