--- title: "Coefficient of utilization and its factors" subject: "Industrial Power Distribution and Illumination" chapter: 7 key: "semester_6/industrial_power_distribution_and_illumination:coefficient-of-utilization" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/coefficient-of-utilization site: BCT Engineering --- # Coefficient of utilization and its factors Industrial Power Distribution and Illumination · Chapter 7: Illumination Design Principles - Asked in: 2071 Magh, 4 marks - Video: [Utilisation Factor | Maintenance Factor | Depreciation Factor | Part -08| Unit -01](https://www.youtube.com/watch?v=6OR0so4kiI0) — Leap 2 Learn - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/coefficient-of-utilization - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-7 ## Question What is the coefficient of utilization and explain the factors should be considered while selecting the above. ## Answer The **coefficient of utilization (CU)** or utilisation factor is the ratio of the lumens that actually reach the working plane to the total lumens emitted by the lamps: CU = lumens received on the working plane / total lumens emitted by the lamps It is always less than 1 (typically 0.4–0.8), because some light is absorbed by the luminaire, walls and ceiling or falls outside the working plane. It is used in the lumen method: N = E × A / (φ × CU × MF). #### Factors considered while selecting CU 1. **Type of luminaire and light distribution:** direct luminaires send most light downward and give a high CU; indirect lighting depends on ceiling reflection and gives a low CU. Luminaire efficiency (light output ratio) also matters. 2. **Room index (room dimensions and mounting height):** RI = L × W / [Hm (L + W)]. Large, low rooms (high RI) give high CU; narrow, tall rooms lose more light on walls (low CU). 3. **Reflection factors of ceiling, walls and floor:** light-coloured, brightly painted surfaces reflect more light back and raise CU; dark surfaces lower it. 4. **Mounting height:** higher mounting spreads more light onto walls, lowering CU. 5. **Colour and condition of surroundings and obstructions:** machinery and partitions block light. CU is read from manufacturers' tables for the chosen luminaire, entering with room index and reflectances. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the coefficient of utilization? A. Lumens on the working plane divided by total lumens emitted by lamps B. Lamp watts divided by room area C. Lumens emitted divided by watts D. Ratio of new to old output Answer: A. CU measures the fraction of lamp lumens that reach the working plane, always below 1. (Key point: Definition of CU) 2. How does room index affect CU? A. Large, low rooms (high room index) give higher CU; narrow, tall rooms lose more light on walls B. Tall narrow rooms give higher CU C. Room index has no effect D. CU depends only on lamp wattage Answer: A. In rooms with high RI, more light reaches the working plane directly; in tall narrow rooms more is absorbed by walls. (Key point: Room index effect) 3. Which change raises the coefficient of utilization? A. Painting ceiling and walls in dark colours B. Painting ceiling and walls in light colours C. Using indirect luminaires with a dark ceiling D. Mounting lamps higher Answer: B. Light surfaces reflect more light back onto the working plane; dark surfaces absorb it. 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