--- title: "Selecting load centre and transformer, rural/urban" subject: "Transmission and Distribution System Design" chapter: 7 key: "semester_8/transmission_and_distribution_system_design:load-centre-transformer-rural-urban" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/load-centre-transformer-rural-urban site: BCT Engineering --- # Selecting load centre and transformer, rural/urban Transmission and Distribution System Design · Chapter 7: Distribution System Design - Asked in: 2079 Chaitra, 6 marks; 2072 Magh, 6 marks; 2067 Mangsir (old course), 8 marks (asked 3 times) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/load-centre-transformer-rural-urban - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-7 ## Question Discuss the primary consideration to be given when selecting a load centre and distribution transformer while designing a grid extended rural and urban distribution. ## Short answer Place the transformer at the load centre of gravity to minimise LT feeder length and voltage drop, near an accessible 11 kV line tap on safe ground. Select rating to meet design-year peak kVA using standard sizes, with single-phase for rural clusters and three-phase for urban loads. In rural areas, voltage drop governs; in urban areas, thermal limits govern. ## Answer A **load centre** is the point from which a distribution transformer feeds a group of consumers through LT feeders. Its location and the transformer size decide the LT voltage drop, losses and cost of the scheme. The considerations differ for rural and urban grid-extension schemes. #### Selecting the load centre - **Centre of gravity of load:** place the transformer at or near the load-weighted centre, $x = \dfrac{\sum P_i x_i}{\sum P_i}$, so that LT feeders are short and balanced. - **LT feeder length limit:** keep feeder lengths within the voltage drop limit (about ±5–10% in Nepal; LT reach typically 0.5–1 km urban, up to about 1.5–2 km rural). - **Accessibility:** near roads/tracks for transport, installation, maintenance and replacement. - **Short HT tap:** close to the 11 kV line route, so the HT tap line is short. - **Safe and clear site:** away from schools, playgrounds, flooding, landslide areas; land availability (government or community land). - **Future growth** direction of the settlement. #### Selecting the distribution transformer - **Design-year peak demand:** rating ≥ forecast 5th-year (or 10th-year) peak kVA including losses; choose the next standard size (10, 16, 25, 50, 100, 200, 300, 500 kVA). - **Loading for minimum loss:** avoid gross oversizing (high no-load loss) and overloading (high copper loss, ageing). - **Phase:** single-phase for scattered small domestic loads; three-phase where motors or larger loads exist. - **Standardisation:** few sizes to reduce spare stock. - **Efficiency/loss capitalisation:** low-loss (e.g. amorphous core) units where load factor is low. #### Rural vs urban | Point | Rural | Urban | |---|---|---| | Load density | Low, scattered | High, concentrated | | Design limit | Voltage drop | Thermal (current) | | Transformer size | Small (10–50 kVA), often 1-phase | Large (100–500 kVA), 3-phase | | Number of transformers | One per cluster/village | Many, closely spaced | | LT feeder | Long, few feeders | Short, many feeders, often ABC/cable | | Growth | Slow, coverage rises | Fast, high load growth | | Site | Open land, cheap | Space scarce, pole-mounted/compact | In rural schemes, it is often cheaper to extend the 11 kV line and use several small transformers near each cluster than one large transformer with long LT lines; in urban schemes, transformers are placed so that each covers a compact block within thermal and voltage limits. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Where should the distribution transformer be placed ideally? A. At the 11 kV substation B. At the load-weighted centre, x = sum(P x)/sum(P) C. At the farthest consumer D. At the centre of the map regardless of load Answer: B. Placing it at the centre of gravity of load keeps LT feeders short and balanced, limiting voltage drop and loss. (Key point: Location of transformer) 2. How should the transformer rating be chosen? A. Equal to present load B. Much larger than needed to be safe C. Next standard size above forecast design-year peak kVA including losses D. The smallest available Answer: C. Oversizing raises no-load loss and overloading raises copper loss and ageing, so choose the next standard size. (Key point: Transformer sizing) 3. Which design limit usually governs rural and urban feeders respectively? A. Rural thermal, urban voltage drop B. Both thermal C. Both voltage drop D. Rural voltage drop, urban thermal Answer: D. Rural lines are long with low density (voltage drop governs); urban feeders are short and heavily loaded (thermal limit). (Key point: Rural versus urban design limit) 4. In rural schemes it is often cheaper to: A. Use one large transformer with long LT lines B. Extend the 11 kV line and use several small transformers near clusters C. Use only three-phase 500 kVA units D. Avoid HT lines Answer: B. Several small transformers near clusters avoid long LT lines with high drop and loss. (Key point: Rural strategy) ## Related in Transmission and Distribution System Design - [Selecting distribution transformer size and location](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atransformer-size-and-location) — Transmission and Distribution System Design, chapter 7, asked 2× - [T/F: higher primary voltage in rural distribution](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-rural-higher-primary-voltage) — Transmission and Distribution System Design, chapter 7, asked 2× - [Selecting transformer location, voltage and layout](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atransformer-location-voltage-layout) — Transmission and Distribution System Design, chapter 7 - [T/F: thermal limit design urban, losses rural](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-thermal-limit-urban-loss-rural) — Transmission and Distribution System Design, chapter 7, asked 3× - [T/F: rural design, transformer size first](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-rural-transformer-size-first) — Transmission and Distribution System Design, chapter 7 - [T/F: urban design, transformer location first](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-urban-transformer-location-first) — Transmission and Distribution System Design, chapter 7 ## Related in other subjects - [Selecting distribution transformer size using Ku and Ks](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Atransformer-size-ku-ks) — Industrial Power Distribution and Illumination, chapter 4, asked 3× - [25 kVA transformer: load voltage and primary current](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Atf-25kva-11kv-load-voltage) — Electrical Machines, chapter 2 - [Rural and urban scheme layout diagram](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arural-urban-scheme-diagram) — Water Supply Engineering, chapter 1, asked 6× - [Primary current and efficiency of 11 kV/380 V Δ/Y transformer](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adelta-star-11kv-380v-40kw-efficiency) — Electrical Machines I, chapter 2, asked 3×