--- title: "Single phase vs 3-phase distribution transformer" subject: "Transmission and Distribution System Design" chapter: 7 key: "semester_8/transmission_and_distribution_system_design:single-vs-three-phase-transformer" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/single-vs-three-phase-transformer site: BCT Engineering --- # Single phase vs 3-phase distribution transformer Transmission and Distribution System Design · Chapter 7: Distribution System Design - Asked in: 2073 Magh, 6 marks; 2067 Chaitra (old course), 6 marks (asked 2 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/single-vs-three-phase-transformer - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-7 ## Question Explain the suitability of single phase and 3-phase distribution transformer. ## Short answer Single-phase transformers suit small scattered domestic loads in rural areas, being cheap and light. Three-phase transformers suit high-density urban or industrial areas with three-phase motors, offering lower cost per kVA and balanced loading. ## Answer Distribution transformers may be **single-phase** (11 kV/230 V, or SWER/phase-to-phase primary) or **three-phase** (11/0.4 kV). Each suits a different type of load and area. #### Single-phase distribution transformer Suitable where: - Loads are **small, scattered and mainly domestic** (lighting, fans, TV): rural villages, hilly settlements. - Load per cluster is small (5–25 kVA), so a three-phase unit would be lightly loaded with high no-load loss. - The HT line can be a **single-phase tap** (two wires or SWER), much cheaper than three-phase. Advantages: low cost, light (easy pole mounting and transport in hills), lower no-load loss, can be placed very close to small clusters, reducing LT length. Disadvantages: cannot supply three-phase motors; may unbalance the three-phase feeder unless taps are distributed across phases; lower capacity. #### Three-phase distribution transformer Suitable where: - Load density is **high** (urban, semi-urban areas, market centres). - There are **three-phase loads**: motors, mills, pumps, workshops, industries, irrigation. - Larger capacities (25–500 kVA) are needed. Advantages: supplies both 3-phase (400 V) and 1-phase (230 V) loads; about 15–20% less material and cost per kVA than three single-phase units; balanced loading of the HT system; better efficiency at large ratings. Disadvantages: higher cost for small loads, heavier, needs a three-phase HT line, high no-load loss when lightly loaded. | Point | Single-phase | Three-phase | |---|---|---| | Typical rating | 5–50 kVA | 25–500 kVA | | Area | Rural, scattered | Urban, dense | | Load type | Domestic lighting | Motors + mixed | | HT line needed | 1-phase / SWER | 3-phase | | Cost per kVA | Higher | Lower | | Cost for small load | Lower | Higher | In practice, single-phase units are used for rural electrification of small, dispersed settlements, and three-phase units for urban, commercial and industrial areas. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Where is a single-phase distribution transformer most suitable? A. Urban market centres with motors B. Small scattered domestic loads in rural or hilly areas C. Industries with 3-phase pumps D. Dense cities needing 500 kVA Answer: B. Small dispersed domestic loads (5-25 kVA) with a cheap single-phase or SWER HT tap suit single-phase units. (Key point: Single-phase suitability) 2. Which is an advantage of a three-phase unit over three single-phase units? A. Can feed both 400 V 3-phase and 230 V loads, with about 15-20% less material per kVA B. Cheaper for very small loads C. Needs only a single-phase HT line D. Lower no-load loss when lightly loaded Answer: A. A three-phase unit supplies both load types and is cheaper per kVA at larger ratings. (Key point: Three-phase advantage) 3. What is a drawback of single-phase transformers? A. They cannot supply 3-phase motors and may unbalance the feeder B. They are heavy for hill transport C. They have higher no-load loss than 3-phase units D. They need a three-phase HT line Answer: A. They cannot feed three-phase motors and may unbalance the three-phase feeder unless taps are spread across phases. 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