--- title: "Underground feeder economical in urban areas" subject: "Transmission and Distribution System Design" chapter: 5 key: "semester_8/transmission_and_distribution_system_design:justify-underground-urban-economical" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/justify-underground-urban-economical site: BCT Engineering --- # Underground feeder economical in urban areas Transmission and Distribution System Design · Chapter 5: Electric Power Distribution - Asked in: 2072 Magh, 2 marks; 2070 Bhadra, 1+3 marks; 2069 Bhadra (old course), 4 marks; 2068 Bhadra (old course), 4 marks (asked 4 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/justify-underground-urban-economical - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-5 ## Question Justify that the underground distribution feeder may not only be a necessity but also economical in urban area with respect to rural area. ## Short answer In urban areas, underground feeders are necessary for space and safety, and become economical because high load density spreads the high installation cost over many kilowatts, short feeder lengths reduce cable cost, and greater reliability lowers outage costs. Rural areas, with low density and long distances, remain cheaper with overhead lines. ## Answer In dense urban areas, underground distribution is often **unavoidable** for space and safety reasons. When the full costs are counted, it can also be **cheaper per unit of load served** than overhead lines, while in rural areas the opposite holds. **Why it is a necessity in urban areas:** - Streets are narrow and buildings close, so it is hard to keep pole and conductor clearances from balconies and windows. - The public safety risk of bare conductors is high. - Appearance matters (city centres, heritage zones such as Kathmandu Durbar Square). - Many feeders and transformers must fit into a small space. **Why it can be economical in urban areas:** 1. **High load density** (MW/km²): one cable route serves a large load over a short length. The high cost per km is spread over many kW, so the cost per kW is low. In rural areas the load is small and scattered, so the cost per kW of cable is very high. 2. **Short feeder lengths:** urban feeders are a few km long, while rural feeders run tens of km. Cable cost grows with length. 3. **Overhead is costly in cities too:** overhead lines there need short spans, special poles, covered conductors or ABC, tree trimming and more frequent repairs. 4. **Reliability value:** fewer faults and less outage time mean less lost revenue (kWh not sold). Urban commercial and industrial customers have a high cost of interruption, and loop or ring-main cables restore supply quickly. 5. **Lower losses and maintenance:** cables have lower reactance, less theft and fewer weather-related faults. These savings over 40 years or more offset the higher first cost. 6. **Land value:** no pole space or right-of-way is needed on costly urban land. So, for the same total load, an underground network has a much lower life-cycle cost per kW in an urban area than it would in a rural area. Rural areas, with low density, long distances and cheap open land, remain economical with overhead lines. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why can underground be economical in urban areas? A. Cable cost per km is lower there B. Cables do not need insulation C. High load density spreads the cost over many kW D. Overhead is not allowed Answer: C. One cable route serves a large load, so the cost per kW is low. (Key point: Load density) 2. How does feeder length matter? A. Urban feeders are short, rural ones long, and cable cost grows with length B. Urban feeders are longer C. Length has no effect on cost D. Cable cost is independent of length Answer: A. Short urban feeders keep the total cable cost low. (Key point: Feeder length) 3. What does reliability add economically? A. Higher losses B. Less lost revenue and lower cost of interruption for commercial and industrial customers C. Lower land value D. Less safety Answer: B. Fewer faults and ring-main restoration reduce outage cost. 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