--- title: "Transformer selection for apartment, residence, industry" subject: "Industrial Power Distribution and Illumination" chapter: 4 key: "semester_6/industrial_power_distribution_and_illumination:transformer-selection-apartment-industry" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/transformer-selection-apartment-industry site: BCT Engineering --- # Transformer selection for apartment, residence, industry Industrial Power Distribution and Illumination · Chapter 4: Distribution Substation of Industrial Plant - Asked in: 2072 Magh, 4 marks - Video: [Electrical Load Calculation for Buildings | Transformer Sizing, DB, SMDB, Main-DB Design Tutorial](https://www.youtube.com/watch?v=XGMUpguNzao) — Elegant Power Solutions - 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/transformer-selection-apartment-industry - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-4 ## Question Discuss the methodology of selection of transformer for apartment building, residential building and industry. ## Short answer Apartment: total demand = Σ(flat loads × simultaneity factor) + common services, margin, round up. Residential house/colony: after-diversity demand per house × number of houses + street lighting, standard pole-mounted rating. Industry: kVA = kW/(η·pf), apply utillization and diversity, check largest motor voltage dip and harmonics, add expansion margin, select standard rating at 60-80% loading. ## Answer The transformer for any building is chosen on its **maximum demand**, found from connected load reduced by utilization/demand factors and diversity (simultaneity) factors, plus a margin for growth, then rounded up to a standard rating. #### Apartment building - Find the load per flat (lights, sockets, kitchen, water heater, AC) e.g. 6–10 kVA. - Apply a **simultaneity factor that falls with the number of flats** (e.g. 1.0 for 2–4 flats, about 0.78 for 5–9, 0.53 for 15–19, 0.4 for 50 or more flats). - Add common services: lifts, water pumps, corridor lighting, fire pumps (with their own Ku and Ks). - Add 20–25 % margin; usually one transformer (or two for reliability) with standby generator for lifts and pumps. #### Residential building (individual house / colony) - Small loads, high diversity; demand per house ≈ 2–5 kVA after diversity. - Colony transformer = number of houses × after-diversity demand + street lighting; usually pole-mounted 50–250 kVA. #### Industry - Mostly motor loads: kVA = kW/(η·pf), Ku ≈ 0.75, plus lighting and heating (Ku = 1). - Apply Ks per SDB and MDB depending on number of circuits and shift pattern. - Check starting of the largest motor (voltage dip) and harmonics from drives. - Consider separate or duplicate transformers for critical processes, and future expansion. - Choose next standard rating (e.g. 250, 315, 400, 500, 630 kVA) so normal loading is about 60–80 %. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does the simultaneity factor vary with the number of flats? A. It rises with the number of flats B. It is constant at 1 C. It falls as flats increase (about 0.78 for 4, 0.4 for 50) D. It is independent of flats Answer: C. More consumers means more diversity. (Key point: Apartment Ks) 2. What is added to flat demand for an apartment building? A. Common services such as lifts, pumps and corridor lighting, plus 20-25% margin B. Nothing C. Only street lighting D. Only the largest flat load Answer: A. Common services have their own Ku and Ks. (Key point: Apartment commons) 3. What extra check is needed for an industrial transformer? A. Colour of the cables B. Roof height C. Starting of the largest motor (voltage dip) and harmonics D. Number of flats Answer: C. Normal loading around 60-80%. 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