--- title: "Transformer sizing and 11 kV feeder SLD" subject: "Industrial Power Distribution and Illumination" chapter: 4 key: "semester_6/industrial_power_distribution_and_illumination:transformer-size-sld-11kv-230v" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/transformer-size-sld-11kv-230v site: BCT Engineering --- # Transformer sizing and 11 kV feeder SLD Industrial Power Distribution and Illumination · Chapter 4: Distribution Substation of Industrial Plant - Asked in: 2074 Bhadra, 4 marks - 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-size-sld-11kv-230v - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-4 ## Question Explain with example, how does the proper size of transformer determined for a industry. Draw single diagram of electrical connection of 11KV feeder up to end user at 230 V. ## Short answer Size the transformer by plant maximum demand: sum (connected kVA × demand factor) divided by diversity factor plus20-25% margin. Example: 120 kVA(0,7), 20 kVA(0,9), 40 kVA(0,6) sum 126 kVA; /1,3 =96,9 kVA; +20% =116,3 kVA →160 kVA. SLD: 11 kV feeder, LA, AB switch, DO fuse, 11/0.4 kV Dyn11, LT MCCB, MDB, MCCB to SDB, MCB/RCCB to230 V. ## Answer #### Determining the proper transformer size The transformer is sized on the **maximum demand**, not on connected load: 1. List all connected loads in kVA (kVA = kW / (η·pf)). 2. Apply demand factor (or Ku) to each load group to get its maximum demand. 3. Divide the sum of group maximum demands by the diversity factor (or multiply by Ks). 4. Add 20–25 % for future expansion. 5. Choose the next higher standard rating (50, 100, 160, 200, 250, 315, 400 kVA ...). **Example**: an industry has motors 120 kVA (demand factor 0.7), lighting 20 kVA (DF 0.9) and other loads 40 kVA (DF 0.6); diversity factor 1.3. ```text Max demands: 120×0.7 = 84, 20×0.9 = 18, 40×0.6 = 24 Sum = 126 kVA Plant max demand = 126 / 1.3 = 96.92 kVA With 20 % margin = 96.92 × 1.2 = 116.31 kVA ``` **Selected: 160 kVA, 11/0.4 kV transformer.** #### Single line diagram: 11 kV feeder to 230 V end user ```text 11 kV feeder | [LA] lightning arrester | [AB] isolator | [DO] HT fuse | (TR) 11/0.4 kV, Dyn11 | [MCCB] LT main + meter | === MDB 400 V === | [MCCB] -> SDB (400/230 V) | [MCB + RCCB] | 230 V end user (phase-neutral) ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the correct order of sizing steps? A. Connected kVA, apply demand factor, divide by diversity, add margin, choose standard size B. Add margin, then apply demand factor, then pick any size C. Apply diversity first on connected load then ignore demand factor D. Sum kW and choose the nearest size Answer: A. Demand then diversity gives plant maximum demand. (Key point: Sizing steps) 2. With group maximum demands totalling 126 kVA and diversity factor 1.3, what is plant demand? A. 126 x 1.3 = 163.8 kVA B. 126 / 1.2 = 105 kVA C. 126 / 1.3 = 96.92 kVA D. 126 - 1.3 = 124.7 kVA Answer: C. Divide the sum of group demands by diversity factor. (Key point: Example demand) 3. With 20% margin (116.31 kVA), which transformer is selected? A. 160 kVA B. 100 kVA C. 250 kVA D. 116 kVA Answer: A. 160 kVA is the next standard rating above 116.31 kVA. 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