Original ARAN VISION analysis and writing based on the Electrical Installation Guide and IEC 60364 concepts; reviewed on 14 Shahrivar 1405. Final project requirements must be checked against current local regulations and calculations by a qualified engineer.
Start with the load list and operating modes
Before selecting a transformer, cable or protective device, record each load's location, rated power, load type, starting method, operating duration, duty cycle and process importance. A large motor's starting load differs from its steady-state load. Emergency, critical and interruptible loads should not be combined into one calculation group.
Installed power is not simultaneous demand
Simply adding all equipment nameplate ratings rarely identifies the actual capacity required. Utilization and simultaneity factors must reflect real production scenarios, shifts and startup sequences. Using an unsupported generic factor can cause expensive overestimation or dangerous underestimation. Record assumptions, factor sources and peak-load scenarios in the calculations.
Supply architecture follows demand and reliability requirements
After estimating actual demand, assess voltage level, the number and capacity of sources, transformer arrangements, main switchboards, emergency generation and UPS systems. Architecture is not determined by power alone: consider downtime impact, maintainability, future expansion, power quality and separation of sensitive loads.
Cable cross-section is a multi-criteria decision
Ampacity is only one conductor selection constraint. Installation method, ambient temperature, cable grouping, insulation type, loaded conductor count, voltage drop, prospective short-circuit current and protective operating time can all affect the final cross-section. A cable suitable for continuous current may fail voltage-drop or short-circuit thermal withstand requirements.
Assess protection and cables as one system
The circuit breaker or fuse rating and operating curve must coordinate with circuit design current, cable ampacity and load starting conditions. Protective device breaking capacity must not be below the calculated short-circuit current at its installation point. Check upstream and downstream protection coordination using manufacturer data and a network study to achieve selectivity and service continuity.
Consider neutral, protective conductors and harmonics separately
Do not infer neutral and PE cross-sections from a simple phase-conductor rule alone. Nonlinear loads, third harmonics, phase imbalance and fault current can change neutral and protective conductor requirements. Verify and document protective-path continuity, earthing arrangement and automatic disconnection time along the entire path to the load.
Keep assumptions and acceptance criteria traceable
A reliable design package includes not only single-line diagrams and cable schedules but also load bases, demand factors, voltage-drop and short-circuit calculations, protection settings, manufacturer data and document revisions. Capacity or cable-route changes must pass through the same checks again. Traceability makes independent reviews and future expansion faster and less risky.
Limits of general guides
The Electrical Installation Guide is useful for conceptual learning and structuring a review, but does not replace project design, mandatory local regulations or approval by a qualified engineer. Verify standards editions, supply voltage, environmental conditions, fire-safety requirements and utility rules independently for each project.