Before sizing a cable, you need to determine the load requirements of the circuit. This includes:
Consult standard cable tables (e.g., IEC 60364, NEC Chapter 9, or BS 7671) to find a cable size whose standard current rating is greater than or equal to $I_z$.
Sizing a cable for a three-phase motor.
Sizing a cable correctly is a systematic process that balances thermal limits, voltage regulation, fault survival, and protection coordination. Always start from the load current, apply realistic derating, check voltage drop for long runs, and verify short-circuit withstand. When in doubt, the next larger standard size is a low-cost safety margin. Document every assumption – temperature, grouping, soil conditions – because field conditions rarely match the ideal table.
Example : 2.5 mm² PVC copper (30 A tabulated), ambient 45°C (k₁=0.79), 4 circuits (k₂=0.65) → effective = 30×0.79×0.65 = 15.4 A. For a 16 A load, this cable fails. Increase to 4 mm². how to size a cable
If $\textVoltage Drop % > \textAllowable %$, select the next larger cable size and re-check.
Cable sizing is not merely about matching a conductor to a load current. It is a multi-variable optimization problem that ensures safety, reliability, efficiency, and longevity of an electrical installation. An undersized cable causes overheating, voltage drops, energy losses, and fire hazards. An oversized cable wastes material, increases installation costs, and may create termination difficulties. Before sizing a cable, you need to determine
$$K = k_1 \times k_2 \times k_3$$
$L_max = \frac0.8 \times U_0Z_loop \times I_a$ Sizing a cable correctly is a systematic process