Electrical Installation and Wiring Design
This domain tests the design calculations behind a safe, compliant installation: load estimation, conductor sizing, voltage drop, conduit, and earthing. Expect numerical answers with units and code clauses.
Core rules
- Calculate connected load, apply demand and diversity factors, then size the protective device and conductor.
- Continuous loads are taken at 125% for branch-circuit and feeder sizing in NEC-style questions; IEC practice uses utilization and diversity factors instead.
- Conductor ampacity must be corrected for ambient temperature and grouping, and the design must use the derated value, not the table value.
- Three-phase voltage drop: V_d = √3 × I × L × (R cos φ + X sin φ), with R and X in Ω per unit length.
- Protect conductors against short-circuit heating by checking the let-through energy (I²t) against the cable withstand rating.
Worked example
A 400 V three-phase feeder carries 100 A over 80 m. Cable data: R = 0.15 Ω/km, X = 0.08 Ω/km, load power factor 0.85. Per 80 m: R = 0.012 Ω, X = 0.0064 Ω. V_d = 1.732 × 100 × (0.012 × 0.85 + 0.0064 × 0.527) = 1.732 × 100 × 0.01357 = 2.35 V. As a percentage of 400 V this is 0.59%, well inside a typical 5% limit, so the cable passes on voltage drop and the decision falls to ampacity and short-circuit withstand.
Traps
Using the single-phase voltage-drop formula (2 × I × L × R) on a three-phase circuit, or forgetting that tabulated current-carrying capacity assumes reference conditions, are common. Keep units consistent: mm², metres, and Ω/km are easily mixed.