Work out the running current for an appliance or circuit, then find the right copper cable size and breaker rating for it.
Based on 230V single-phase supply and a 25% safety margin over the running current — standard design practice. For cable runs longer than ~20m, go one cable size up to limit voltage drop.
This is the standard formula for single-phase current draw. Power factor accounts for the load type: purely resistive loads (bulbs, heaters) have a power factor of 1, while motors, fans and compressors draw more current for the same wattage because of their power factor — typically 0.75–0.85.
Sizing a circuit exactly to its expected running current leaves no margin for voltage sag, minor overloads, or future additions. Adding a 25% margin before selecting cable and breaker size is standard electrical design practice, not a Pakistan-specific rule.
The breaker protects the cable — it should trip before the cable's rated current is exceeded, so the breaker rating should be at or just below the cable's ampacity, both sized to safely carry your design current.