--- title: "Conductor sizing factors and armoured cable" subject: "Industrial Power Distribution and Illumination" chapter: 3 key: "semester_6/industrial_power_distribution_and_illumination:conductor-size-factors-armoured" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/conductor-size-factors-armoured site: BCT Engineering --- # Conductor sizing factors and armoured cable Industrial Power Distribution and Illumination · Chapter 3: Power Carrying Devices - Asked in: 2070 Magh, 8 marks - Video: [How to Calculate Cable Size | Cable Size Calculation | Step-by-Step](https://www.youtube.com/watch?v=g-t0cpEakNk) — Electrical lectures - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/conductor-size-factors-armoured - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-3 ## Question Point out factors to be considered while determining the sizes of conductor. Explain construction features of 3.5 core armoured cable. ## Short answer Conductor size is chosen for load current, derated for installation conditions, and must satisfy voltage drop within 2.5–5% and short-circuit withstand (A ≥ I_sc·√t/k). Additional factors: load type, future expansion, economy, and mechanical strength. A 3.5-core armoured cable has three full-size phase conductors and a reduced neutral (half size), each stranded Cu/Al with PVC/XLPE insulation, fillers, inner bedding, galvanized steel wire/strip armour, and outer PVC sheath. ## Answer The conductor size of a cable is chosen so that it carries the load current without overheating, keeps the voltage drop within limits and survives the short-circuit current until the protection clears the fault. #### Factors for deciding conductor size 1. **Load (design) current**: I = P / (√3 · V_L · cosφ · η) for 3-phase loads; the cable rating must be ≥ the design current and ≥ the rating of the protective device. 2. **Derating factors**: ambient temperature, grouping of cables, depth of laying, soil thermal resistivity and method of installation (air, duct, ground) reduce the rating given in the tables. 3. **Voltage drop**: the drop from supply to load should be within the permitted value (about 2.5–5 % of rated voltage). Long cables are often sized by voltage drop, not current. 4. **Short-circuit withstand**: A ≥ I_sc · √t / k, where k depends on conductor and insulation (about 115 for Cu-PVC, 143 for Cu-XLPE). 5. **Type of load**: motor starting current, harmonics (neutral current), duty cycle. 6. **Future expansion**: a margin of about 20–25 %. 7. **Economy and material**: copper vs aluminium, cost of energy losses over the cable life. 8. **Mechanical strength**: minimum sizes (e.g. 1.5 mm² Cu for lighting, 2.5 mm² Cu for power sockets). #### Construction of 3½-core armoured cable A 3½-core cable has three full-size phase conductors and one reduced-size neutral (about half the phase area), used for 3-phase 4-wire LT supply where neutral current is smaller than phase current. ```text outer PVC sheath / steel wire / strip armour / / bedding (inner sheath) / / / filler ( ( ( ___ ___ ) ) ) ( R ) ( Y ) \ / ( B ) (n) <- reduced neutral each core: conductor + PVC/XLPE insulation ``` Layers from inside to outside: 1. **Conductors**: stranded copper or aluminium, often sector shaped to reduce the overall diameter; three phase cores plus one half-size neutral. 2. **Insulation**: PVC or XLPE over each conductor, colour coded (red, yellow, blue, black). 3. **Fillers**: non-hygroscopic PVC or polypropylene fillers that make the laid-up cores round. 4. **Inner sheath (bedding)**: extruded PVC layer that protects the cores and gives a bed for the armour. 5. **Armour**: galvanised steel wires (round) or steel strips laid helically; gives mechanical protection and acts as an earth-continuity path. Single-core cables use non-magnetic (aluminium) armour. 6. **Outer sheath (serving)**: tough PVC (or FRLS) sheath against moisture, chemicals and corrosion, with voltage grade and size printed on it. Such cables (e.g. 1.1 kV, 3½ × 95 mm² Al XLPE armoured) are laid directly in the ground or on trays between the transformer, MDB and SDBs. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. When is a cable often sized by voltage drop rather than current rating? A. For short runs with low current B. For long runs, where the drop exceeds the permitted 2.5-5% C. Only for control cables D. Whenever the cable is armoured Answer: B. Voltage drop grows with length, so long cables are governed by it. (Key point: Long cables) 2. Which formula gives the minimum conductor area for short-circuit withstand? A. A >= Isc x t / k B. A >= Isc / (k x sqrt(t)) C. A >= Isc x sqrt(t) / k D. A >= P / (sqrt(3) x V) Answer: C. k depends on conductor and insulation, e.g. 115 for Cu-PVC and 143 for Cu-XLPE. (Key point: Short-circuit withstand) 3. What is the neutral size in a 3.5 core cable? A. Equal to the phase area B. Double the phase area C. One quarter of the phase area D. About half the phase area, since neutral current is smaller in a balanced 3-phase 4-wire system Answer: D. A half-size neutral saves conductor material. (Key point: Neutral size) 4. Why does a 3.5 core cable have steel armour? A. To give mechanical protection and act as an earth-continuity path B. To increase the current rating C. To insulate the cores from one another D. To fill gaps and make the cable round Answer: A. Fillers make it round; armour protects and carries earth fault current. 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