--- title: "Selecting distribution transformer size and location" subject: "Transmission and Distribution System Design" chapter: 7 key: "semester_8/transmission_and_distribution_system_design:transformer-size-and-location" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/transformer-size-and-location site: BCT Engineering --- # Selecting distribution transformer size and location Transmission and Distribution System Design · Chapter 7: Distribution System Design - Asked in: 2074 Magh, 8 marks; 2071 Bhadra, 6 marks (asked 2 times) - Video: [Site selection for Distribution transformer](https://www.youtube.com/watch?v=Lrs4yqrz1Bg) — ELECTRICAL ENGINEER - 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/transformer-size-and-location - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-7 ## Question Discuss the primary considerations to be given while selecting a distribution transformer size and its location. ## Short answer Size is based on design-year peak demand, standard ratings, load growth, losses, overload capability, load type, and number of units. Locate at the load centre, near HT line, accessible, safe, with firm dry ground and room for future expansion. ## Answer The **distribution transformer** steps 11 kV (or 33 kV) down to 400/230 V for consumers. Its size and location decide the voltage at consumers' terminals, the LT and transformer losses and the cost of the scheme. #### Considerations for size (rating) 1. **Design-year peak demand:** rating is based on the forecast peak (usually 5th year, sometimes 10th) in kVA, including diversity, contribution factors, power factor and loss allowance: $S = P_{peak}/\cos\phi$. 2. **Standard ratings:** choose the next standard size (e.g. 10, 16, 25, 50, 100, 200, 300, 500 kVA) to keep spares and stock limited. 3. **Load growth:** allow for future growth, but avoid gross oversizing. A unit loaded at about 50–80% of rating at peak is usual. 4. **Losses:** an oversized unit has high no-load (core) losses throughout the year, which matters when load factor is low (rural). An undersized unit has high copper loss and overheating. Loss capitalisation (total owning cost) decides the economic size. 5. **Overload capability:** short-time overloads allowed by IEC 60076-7 loading guides, considering ambient temperature. 6. **Type of load:** motor starting, single- vs three-phase load, harmonics. 7. **Number of units:** one large unit vs several small units; small units near loads reduce LT length. 8. **Transport and mounting:** pole-mounted units are limited to about 200–300 kVA; larger ones need plinth mounting. #### Considerations for location 1. **Load centre:** at or near the centre of gravity of the load, $\bar{x} = \sum P_i x_i/\sum P_i$, to minimise LT length, voltage drop and loss. 2. **Voltage drop limit:** all consumers should be within the allowable LT feeder length. 3. **Proximity to HT line:** short 11 kV tap line. 4. **Accessibility:** near roads for installation, maintenance and replacement. 5. **Safety:** away from schools, playgrounds and crowded places; proper clearances, fencing and earthing. 6. **Site conditions:** firm ground, free from flooding, landslide and waterlogging; space for earthing pits. 7. **Land availability and public acceptance.** 8. **Future expansion** of the settlement. ```text 11 kV line ======+========================= | HT tap (short) [T] at load centre ____|____________ | | | | LT feeders o o o o o o o o consumers ``` The final choice balances capital cost against losses and voltage quality. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a distribution transformer placed at the centre of gravity of its load? A. To keep the HT line as long as possible B. To shorten LT feeders, cutting voltage drop and loss C. To reduce the core loss of the transformer D. To avoid the need for earthing Answer: B. Locating at the load centre, x = sum(Pi xi)/sum(Pi), minimises LT length and so voltage drop and I^2R loss. (Key point: Load centre) 2. What is the main energy penalty of a grossly oversized distribution transformer, especially at low load factor? A. High copper loss B. High no-load (core) loss all year C. Poor overload capability D. Higher voltage drop on the HT side Answer: B. Core loss exists whenever energised, so an oversized unit wastes energy throughout the year. (Key point: Oversizing) 3. On what is the transformer rating mainly based? A. Present average demand only B. Forecast design-year peak kVA, taken to the next standard size C. The largest single consumer D. The length of the HT line Answer: B. Rating comes from the forecast peak (S = P/cos phi) with growth allowance, rounded to a standard size, usually loaded about 50-80%. (Key point: Sizing basis) 4. Which siting practice is recommended for safety? A. Beside schools so supply is close B. On low waterlogged ground near the stream C. Away from schools and crowds, with clearances, fencing and earthing D. Anywhere, as long as it is near the load Answer: C. Transformers should be away from crowded places, with fencing and earthing, on firm flood-free ground. 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