Electrical Installation and Distribution Systems
What this domain tests
Electrical Installation and Distribution Systems tests LV design inside buildings and industrial facilities: load estimation, diversity, distribution-board schedules, cable sizing, voltage drop, protective-conductor arrangement, and compliance with the Egyptian Code for Electrical Installations. The exam expects a designer who can justify a final circuit and a feeder, not merely quote a cable size.
Core rules and concepts
- Design current Ib comes from actual demand after diversity, not from the sum of connected loads. Applying diversity twice is a common error.
- Cable selection follows Ib ≤ In ≤ Iz. The protective device rating In must not exceed the corrected cable capacity Iz, and Iz must carry the design current.
- Correct the tabulated Iz with ambient-temperature factor Ca, grouping factor Cg, and where relevant thermal-insulation or protective-device factor Ci: Iz,corrected = Iz,tabulated × Ca × Cg × Ci.
- Voltage drop is checked at full load and during motor starting. Lighting and socket circuits normally have tighter limits than power feeders.
- Earthing arrangement matters: TN-S uses a separate PE throughout; TN-C-S combines neutral and PE upstream and separates downstream; TT relies on local electrodes and RCD protection.
- Balance single-phase loads across phases and keep neutral current within design assumptions.
Worked example
A 45 kW, 400 V, three-phase load has pf 0.85 and a demand factor of 0.8. Design current Ib = 45,000 × 0.8/(√3 × 400 × 0.85) = 61.1 A. A 63 A MCCB is selected. In a 45°C ambient with three grouped circuits, Ca = 0.87 and Cg = 0.80. The required tabulated cable capacity is 63/(0.87 × 0.80) = 90.5 A. A cable with tabulated Iz of at least 91 A is required; a smaller cable that only matches 63 A in free air is non-compliant. Then check voltage drop over the actual route length and the earth-fault loop impedance.
Traps that cost marks
Sizing from connected load; ignoring ambient and grouping derating; using 220 V in a three-phase current formula; assuming a 30 mA RCD removes the need for adequate cable capacity; mixing neutral and protective earth at a sub-board; and forgetting that a long feeder may fail voltage drop even when thermally adequate.
Exam-focused summary
Show the demand calculation, the correction factors, the device rating, and the voltage-drop check in that order.