How To Calculate Cable Size -

: Short-circuit withstand. ( k=115 ). ( S_min = \sqrt10000^2 \times 0.4/115 = \sqrt40\times 10^6/115 = 6324/115 = 55 mm^2 ). Required minimum 70 mm². Our 50 mm² fails. Therefore increase to 70 mm².

For a three-phase circuit (line-to-line): [ V_d = \sqrt3 \times I_b \times (R \cos\phi + X \sin\phi) \times L ] how to calculate cable size

: Table for 70°C PVC copper, Method C: 50 mm² → 193A base. So select 50 mm². : Short-circuit withstand

Total cost over ( N ) years at interest rate ( r ): [ PV_loss = E_loss \times \textelectricity price \times \frac(1+r)^N - 1r(1+r)^N ] Optimum conductor size is where incremental capital cost equals incremental loss savings. Following IEC/BS 7671 approach: Step 1: Determine Design Current ( I_b ) [ I_b = \fracP\sqrt3 \times V_L \times \cos\phi \ \text(three-phase) \quad \textor \quad \fracPV_P \times \cos\phi \ \text(single-phase) ] Add margin for future expansion (typically 20%). Step 2: Select Protective Device Rating ( I_n ) Choose standard rating (e.g., 16, 20, 25, 32, 40, 50, 63A) such that ( I_n \ge I_b ). Step 3: Determine Installation Method and Correction Factors From standards tables (e.g., IEC 60364-5-52 Table B.52.1 for installation methods: clipped direct, trunking, buried, etc.). Apply ( k_temp ), ( k_group ), ( k_soil ), etc. Step 4: Calculate Required Tabulated Ampacity ( I_tab ) [ I_tab \ge \fracI_nk_total ] Where ( k_total ) = product of all correction factors. Step 5: Select Cable Size from Manufacturer Table Pick smallest standard size (mm²) with base rating ≥ ( I_tab ). Step 6: Check Voltage Drop If drop exceeds limit, increase size until compliant. Step 7: Check Short-Circuit Withstand Compute ( S_min ) from adiabatic equation. If selected size < ( S_min ), increase size. Step 8: Check Earth Fault Loop Impedance (for protection against indirect contact) Ensure disconnection time < 0.4s (for final circuits) or 5s (distribution). This may require larger CPC (circuit protective conductor) or larger line conductor. 7. Worked Example System : 400V three-phase, 50 kW motor, ( \cos\phi = 0.85 ), efficiency 0.94, length 120m, PVC insulated copper cable, clipped direct to wall (Reference Method C), ambient 40°C, grouped with 3 other circuits (4 cables touching), prospective short-circuit 10 kA, permissible voltage drop 3% (12V), disconnection time 0.4 sec. Required minimum 70 mm²