--- title: "Department store 30 × 15 m twin 55 W LED" subject: "Industrial Power Distribution and Illumination" chapter: 7 key: "semester_6/industrial_power_distribution_and_illumination:department-store-30x15-twin-55w" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/department-store-30x15-twin-55w site: BCT Engineering --- # Department store 30 × 15 m twin 55 W LED Industrial Power Distribution and Illumination · Chapter 7: Illumination Design Principles - Asked in: 2073 Magh, 12 marks - 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/department-store-30x15-twin-55w - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-7 ## Question A department store 30 m × 15 m is illuminated by twin 55W LED luminaries of output 4950 lumens. The lamp being mounted at a height of 3 m from the work plane, the average illumination required is 300 lux. Calculate and arrange the number of luminaries required to be fitted in the department store, assuming the coefficient of utilization to be 0.8 and the maintenance factor to be 0.8. Also design the number of sub-circuit, switches and distribution board, if the supply system is 3-phase, 400V, 50Hz. ## Short answer 45 twin 55W LED luminaires arranged in 9×5 grid provide 316.8 lux. Nine 6A lighting sub-circuits (5 luminaires each) balanced across three phases. Distribution board: 10A TPN MCB incomer, 25A 4P RCCB (100mA), 6A SP MCBs for lights, 4-core 1.5mm² Cu incoming cable. ## Answer **Given:** store 30 m × 15 m, twin 55 W LED luminaire (110 W per luminaire) giving 4950 lm, Hm = 3 m, E = 300 lux, CU = 0.8, MF = 0.8, supply 400 V, 3-phase, 50 Hz. **Assumptions:** 4950 lm is the output of one twin luminaire; SHR ≤ 1.5 (spacing ≤ 4.5 m); pf 0.9. #### Number of luminaires Lumen method: N = (E × A) / (φ × UF × MF), where φ is the output of one luminaire. ```text Area A = 30 × 15 = 450 m² φ per luminaire = 4950 lm Total lumens = E × A / (UF × MF) = 300 × 450 / (0.8 × 0.8) = 210938 lm N = 210938 / 4950 = 42.61 ``` Rounding up and fitting a regular grid: **9 × 5 = 45 luminaires** (9 along the 30 m side, 5 along the 15 m side). #### Arrangement and spacing ```text Spacing along length = 30/9 = 3.33 m Spacing along width = 15/5 = 3 m End spacing (to wall) = half of these Max spacing = SHR × Hm = 1.5 × 3 = 4.5 m 3.33 m and 3 m ≤ 4.5 m → OK Actual E = 45 × 4950 × 0.8 × 0.8 / 450 = 316.8 lux (≥ 300 lux required) ``` #### Layout diagram ```text Plan 30 m × 15 m (letter = phase of lamp circuit) +-------------------+ | R R R R R Y Y Y Y | | R R R B B B B B Y | | R R Y Y Y Y Y B B | | Y R R R R R B B B | | Y Y Y Y B B B B B | +-------------------+ [DB] [SB] <- entrance: DB and switch board S S S S ... one switch per sub-circuit ``` Each light sub-circuit is controlled by one 6 A one-way switch; all switches are grouped on a switch board (SB) next to the entrance, and the DB is mounted beside it. #### Sub-circuits Rule (IS 732 / NEA practice): a light sub-circuit carries at most 10 points and 800 W. Here one luminaire is 110 W, so up to 7 luminaires per sub-circuit. Number of circuits is made a multiple of 3 for phase balance. | Circuit | Luminaires | Load (W) | Current (A) | Phase | MCB | |---|---|---|---|---|---| | L1 | 5 | 550 | 2.66 | R | 6 A | | L2 | 5 | 550 | 2.66 | Y | 6 A | | L3 | 5 | 550 | 2.66 | B | 6 A | | L4 | 5 | 550 | 2.66 | R | 6 A | | L5 | 5 | 550 | 2.66 | Y | 6 A | | L6 | 5 | 550 | 2.66 | B | 6 A | | L7 | 5 | 550 | 2.66 | R | 6 A | | L8 | 5 | 550 | 2.66 | Y | 6 A | | L9 | 5 | 550 | 2.66 | B | 6 A | #### Distribution board design | Phase | Circuits | Load (W) | Current (A) | |---|---|---|---| | R | L1, L4, L7 | 1650 | 7.97 | | Y | L2, L5, L8 | 1650 | 7.97 | | B | L3, L6, L9 | 1650 | 7.97 | ```text Total connected load = 4950 W (lighting) Most loaded phase = 1650 W Iph = 1650 / (230 × 0.9) = 7.97 A Main MCB ≥ 1.25 × 7.97 = 9.96 A → 10 A TPN MCB ``` **DB specification:** 4-way-per-phase TPN DB (12 ways in all, spares included) with 10 A TPN MCB as incomer, 25 A 4-pole RCCB (100 mA) for earth-leakage protection, 6 A SP MCBs (B-curve) for light circuits. Incoming cable: 4-core 1.5 mm² Cu PVC (rating 21 A ≥ 10 A), plus earth continuity conductor. ```text 3-phase, 400/230 V, 4-wire supply | [Energy meter] | [10 A TPN MCB] | [25 A 4P RCCB] | ==R====Y====B====N== busbars | | | | | L1 L2 L3 L4 L5 ... ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In this problem, what is used as phi in the lumen method? A. 4950 lm for one twin luminaire B. 4950 lm for each 55 W lamp, so 9900 lm per luminaire C. 55 x 4950 lm D. 4950 / 2 lm per luminaire Answer: A. The answer takes 4950 lm as the output of one twin luminaire (110 W). (Key point: Output of twin luminaire) 2. 300 x 450 / (0.8 x 0.8) = 210938 lm; dividing by 4950 gives 42.61. Which layout is chosen? A. 43 luminaires in any arrangement B. 9 x 5 = 45 luminaires in a regular grid C. 6 x 7 = 42 luminaires D. 9 x 4 = 36 luminaires Answer: B. 42 would fall short, and 43 does not fit a regular grid; 9 x 5 = 45 gives 316.8 lux, meeting 300 lux. (Key point: Number of luminaires and grid) 3. With a limit of 800 W per light sub-circuit and 110 W per luminaire, what is the maximum number of luminaires per circuit? A. 10 B. 7 C. 8 D. 5 Answer: B. 800 / 110 = 7.27, so at most 7 luminaires; the design uses 5 per circuit, in 9 circuits. (Key point: Luminaires per sub-circuit) 4. Total lighting load is 4950 W, shared over three phases. What incomer results? A. Each phase carries 1650 W, 7.97 A at pf 0.9, so a 10 A TPN MCB B. Each phase carries 4950 W, so a 32 A MCB C. Each phase carries 1650 W, 1650/230 = 7.17 A, so a 6 A MCB D. Each phase carries 550 W, so a 6 A MCB Answer: A. Iph = 1650 / (230 x 0.9) = 7.97 A; 1.25 x 7.97 = 9.96 A, so 10 A. (Key point: Phase load and incomer) ## Close variants set in other exams - [Department store 30 × 15 m twin 40 W fluorescent](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Adepartment-store-twin-40w-fluorescent) — Industrial Power Distribution and Illumination, chapter 7, asked 3× - [Department store 36 × 15 m LED panel lighting](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Adepartment-store-36x15-led) — Industrial Power Distribution and Illumination, chapter 7, asked 3× ## Related in Industrial Power Distribution and Illumination - [Drawing hall 40 × 25 × 6 m, 250 lux LED](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Adrawing-hall-40x25-250lux) — Industrial Power Distribution and Illumination, chapter 7, asked 3× - [Factory hall 55.75 m × 40.25 m LED lighting](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Afactory-hall-led-55x40) — Industrial Power Distribution and Illumination, chapter 7, asked 4× - [Lamp suspension height and illumination at offset](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Alamp-height-250cp) — Industrial Power Distribution and Illumination, chapter 7 - [Process room 30 × 15 × 5 m fluorescent lighting](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aprocess-room-30x15-fluorescent) — Industrial Power Distribution and Illumination, chapter 7 - [Room 40 × 15 m, 75 lux FTL design](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aroom-40x15-ftl-75lux) — Industrial Power Distribution and Illumination, chapter 7 - [Industrial shop 30 × 25 m LED lighting](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aindustrial-shop-30x25-led) — Industrial Power Distribution and Illumination, chapter 7