--- title: "Department store 36 × 15 m LED panel lighting" subject: "Industrial Power Distribution and Illumination" chapter: 7 key: "semester_6/industrial_power_distribution_and_illumination:department-store-36x15-led" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/department-store-36x15-led site: BCT Engineering --- # Department store 36 × 15 m LED panel lighting Industrial Power Distribution and Illumination · Chapter 7: Illumination Design Principles - Asked in: 2075 Baisakh, 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-36x15-led - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-7 ## Question A department store 36m × 15m is illuminated by 45W by LED panel light of output 4800 lumens. The lamp being mounted at a height of 3m from the working plane, the average illumination required is 200 lux. Calculate the number of luminaries required to be fitted in the department store, assuming the coefficient of utilization to be 0.8 and maintenance factor to be 0.8. Also design distribution board and layout of lamps, switches power sockets and sub-circuits, if the supply system is 3-Phase, 400V, 50 Hz. ## Short answer Mounting height3 m. N = (200 ×540) / (4800 ×0.64) =36 luminaires arranged 9 ×4. Distribution board: 32 A TPN MCB incomer, 40 A RCCB, 6 A SP MCBs for six lighting circuits, 20 A SP MCBs for power circuits. ## Answer **Given:** store 36 m × 15 m, 45 W LED panel giving 4800 lm, Hm = 3 m, E = 200 lux, CU = 0.8, MF = 0.8, supply 400 V, 3-phase, 50 Hz. **Assumptions:** SHR ≤ 1.5 (so spacing ≤ 4.5 m); pf 0.9; 12 power sockets of 16 A (1 kW each) along the walls. #### Number of luminaires Lumen method: N = (E × A) / (φ × UF × MF), where φ is the output of one luminaire. ```text Area A = 36 × 15 = 540 m² φ per luminaire = 4800 lm Total lumens = E × A / (UF × MF) = 200 × 540 / (0.8 × 0.8) = 168750 lm N = 168750 / 4800 = 35.16 ``` Rounding up and fitting a regular grid: **9 × 4 = 36 luminaires** (9 along the 36 m side, 4 along the 15 m side). #### Arrangement and spacing ```text Spacing along length = 36/9 = 4 m Spacing along width = 15/4 = 3.75 m End spacing (to wall) = half of these Max spacing = SHR × Hm = 1.5 × 3 = 4.5 m 4 m and 3.75 m ≤ 4.5 m → OK Actual E = 36 × 4800 × 0.8 × 0.8 / 540 = 204.8 lux (≥ 200 lux required) ``` #### Layout diagram ```text Plan 36 m × 15 m (letter = phase of lamp circuit) +-------------------+ | R R R R R R Y Y Y | | B B B B B B Y Y Y | | R R R R R R Y Y Y | | B B B B B B Y Y Y | +-------------------+ [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. Power sockets (16 A, 3-pin, with earth) are placed along the walls at about 1 m height, spaced roughly evenly. #### Sub-circuits Rule (IS 732 / NEA practice): a light sub-circuit carries at most 10 points and 800 W. Here one luminaire is 45 W, so up to 10 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 | 6 | 270 | 1.3 | R | 6 A | | L2 | 6 | 270 | 1.3 | Y | 6 A | | L3 | 6 | 270 | 1.3 | B | 6 A | | L4 | 6 | 270 | 1.3 | R | 6 A | | L5 | 6 | 270 | 1.3 | Y | 6 A | | L6 | 6 | 270 | 1.3 | B | 6 A | Power sub-circuits (assumed): 12 sockets of 16 A, each taken as 1 kW, at most 2 sockets per power sub-circuit (IS 732), 2.5 mm² Cu wire, 20 A MCB. | Circuit | Sockets | Load (W) | Phase | MCB | |---|---|---|---|---| | P1 | 2 | 2000 | R | 20 A | | P2 | 2 | 2000 | Y | 20 A | | P3 | 2 | 2000 | B | 20 A | | P4 | 2 | 2000 | R | 20 A | | P5 | 2 | 2000 | Y | 20 A | | P6 | 2 | 2000 | B | 20 A | #### Distribution board design | Phase | Circuits | Load (W) | Current (A) | |---|---|---|---| | R | L1, L4, P1, P4 | 4540 | 21.93 | | Y | L2, L5, P2, P5 | 4540 | 21.93 | | B | L3, L6, P3, P6 | 4540 | 21.93 | ```text Connected load = 1620 W light + 12000 W power = 13620 W Most loaded phase = 4540 W Iph = 4540 / (230 × 0.9) = 21.93 A Main MCB ≥ 1.25 × 21.93 = 27.42 A → 32 A TPN MCB ``` **DB specification:** 6-way-per-phase TPN DB (18 ways in all, spares included) with 32 A TPN MCB as incomer, 40 A 4-pole RCCB (100 mA) for earth-leakage protection, 6 A SP MCBs (B-curve) for light circuits and 20 A SP MCBs for power circuits. Incoming cable: 4-core 6 mm² Cu PVC (rating 45 A ≥ 32 A), plus earth continuity conductor. ```text 3-phase, 400/230 V, 4-wire supply | [Energy meter] | [32 A TPN MCB] | [40 A 4P RCCB] | ==R====Y====B====N== busbars | | | | | L1 L2 L3 P1 P2 ... ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which expression gives the number of luminaires in the lumen method? A. N = E x A x UF x MF / lumens per luminaire B. N = E x A / (lumens per luminaire x UF x MF) C. N = E x A x MF / (lumens per luminaire x UF^2) D. N = lumens per luminaire x UF x MF / (E x A) Answer: B. Required lumens on the plane (E x A) are divided by the useful lumens one luminaire delivers (phi x UF x MF). (Key point: Lumen method formula) 2. For 540 m^2, 200 lux, UF = MF = 0.8 and 4800 lm per luminaire, the calculation gives 35.16. What is done next? A. Round down to 35 and accept the small shortfall B. Round to a regular grid of 9 x 4 = 36 luminaires C. Use 40 luminaires to leave a 10 percent margin D. Use 18 luminaires because each is a panel of two LEDs Answer: B. N is rounded up to a number that fits a regular grid; 9 x 4 = 36 gives an actual illuminance of 204.8 lux, above 200. (Key point: Calculated N and grid) 3. Why is the 9 x 4 grid accepted (spacings 4 m and 3.75 m)? A. Both spacings are within SHR x Hm = 1.5 x 3 = 4.5 m, so lighting is uniform B. Both spacings are greater than 4.5 m, giving better coverage C. The spacings equal the room width divided by Hm D. Spacing is irrelevant once N is calculated Answer: A. Spacing must not exceed the spacing-to-height ratio times mounting height, otherwise illumination becomes uneven. (Key point: Spacing check with SHR) 4. Why is the number of light sub-circuits chosen as a multiple of 3 (6 circuits here)? A. Because each circuit can carry only 3 lamps B. To spread the load equally over R, Y and B phases C. To make each MCB rating 3 A D. Because 400 V divided by 3 gives the circuit voltage Answer: B. Equal circuits on each of the three phases give a balanced load and a small neutral current. (Key point: Sub-circuit phase balance) 5. The most loaded phase carries 4540 W at pf 0.9 (21.93 A). How is the incomer rated? A. At 21.93 A exactly, so a 20 A MCB B. At 1.25 x 21.93 = 27.4 A, so the next standard size, 32 A TPN MCB C. At 3 x 21.93 A = 65.8 A D. At 400 x 21.93 A Answer: B. The incomer is chosen at least 1.25 times the phase current and rounded up to a standard rating, giving 32 A. 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