The most common mistake we encounter is cable undersizing based on rated current alone without accounting for derating factors. In Nigeria's climate, cables laid in conduit or grouped in trunking can operate at significantly higher ambient temperatures than the baseline 30°C assumed in most tables. A 6mm² cable rated for 40A in free air may only carry 28A when grouped with five other cables in a 40°C environment. Under-specified cables overheat, degrade insulation, and create fire risks — yet this error is frequently not caught until installation is complete.

The second common error is failing to coordinate protective device settings in distribution systems. When an overload occurs, the breaker closest to the fault should trip first — without the upstream breaker tripping and cutting power to the whole building. This requires careful selectivity analysis of trip curves and time-current characteristics. We also frequently see main distribution boards specified for an incoming supply that has not been agreed with the distribution company, resulting in expensive redesign when the actual available fault level differs from assumptions.

The remaining top mistakes include: specifying voltage levels incorrectly for generator-backed systems where the voltage tolerance differs from mains; omitting surge protection on buildings in high lightning-risk zones (most of Nigeria's coastal belt); and sourcing products from unverified suppliers to save cost, only to face rejection at the inspection stage when certification documents cannot be produced. Engaging a technical consultant at specification stage — before procurement begins — typically costs a fraction of the remediation work these errors generate.