--- title: "T/F: thermal limit design urban, losses rural" subject: "Transmission and Distribution System Design" chapter: 7 key: "semester_8/transmission_and_distribution_system_design:tf-thermal-limit-urban-loss-rural" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-thermal-limit-urban-loss-rural site: BCT Engineering --- # T/F: thermal limit design urban, losses rural Transmission and Distribution System Design · Chapter 7: Distribution System Design - Asked in: 2078 Chaitra, 1+3 marks; 2075 Bhadra, 1+3 marks; 2068 Bhadra (old course), 4 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/tf-thermal-limit-urban-loss-rural - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-7 ## Question State and justify whether the following statement is TRUE or FALSE: Though thermal limit design for distribution feeder is applicable in highly dense urban areas, but the power loss are more in rural areas. ## Short answer TRUE. Urban feeders carry large currents over short lengths, so thermal (ampacity) is the design limit. Rural feeders are very long with small loads, designed by voltage drop. Their high resistance due to length, low load factor, and unbalanced loading cause higher percentage power loss than in urban feeders. ## Answer **TRUE.** **Thermal limit in urban areas:** - Urban areas have **high load density** (many kW per km²), so feeders are short but carry large currents. - Over a short length, the voltage drop ($\Delta V \propto I \cdot l$) stays within limits even at high current. The limiting factor is the **current-carrying (thermal) capacity** of the conductor or cable: overheating, sag and insulation damage. - So urban feeders are designed by the thermal (ampacity) limit. **Higher losses in rural areas:** - Rural loads are **scattered, with low load density**, so feeders (11 kV and LT) are very long for small loads. They are designed by the **voltage drop limit**, not the thermal limit. - Power loss $= 3I^2R$, and $R$ is proportional to length. For the same delivered power, long feeders with small conductors give a much higher **percentage loss**. - Rural distribution transformers are lightly loaded for most of the day (low load factor), so no-load (core) losses form a large share of the energy supplied. - Poor maintenance, unbalanced single-phase loading, long service drops and theft add to the losses. **Example:** a 10 kW load at the end of a 0.5 km LT feeder has only one-quarter of the loss of the same load at the end of a 2 km feeder (loss ∝ length for a fixed current). Thus urban feeders reach their thermal limit first, while rural feeders, though lightly loaded, have a higher percentage loss than urban feeders. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The statement (thermal limit in dense urban areas, but higher power loss in rural areas) is: A. False B. True C. False, because urban losses are higher D. True only for 33 kV Answer: B. Urban feeders are short and heavily loaded so thermal capacity limits them, while long rural feeders have higher percentage loss. (Key point: Verdict) 2. Why does thermal limit govern urban feeders? A. Short length keeps voltage drop small even at high current, so ampacity is limiting B. Voltage is very low C. Loads are small D. They are underground only Answer: A. Voltage drop is proportional to I x length; short feeders stay within limits, but large current heats the conductor. (Key point: Why thermal limit in urban) 3. Why are rural percentage losses higher? A. Resistance is zero B. Long feeders (R proportional to length) with small conductors and lightly loaded transformers with significant core loss C. Loads are very large D. Transformers are always overloaded Answer: B. I^2R with long length plus no-load loss share and poor maintenance give 15-25% loss in rural Nepal. 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