--- title: "Room 40 × 15 m, 75 lux FTL design" subject: "Industrial Power Distribution and Illumination" chapter: 7 key: "semester_6/industrial_power_distribution_and_illumination:room-40x15-ftl-75lux" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/room-40x15-ftl-75lux site: BCT Engineering --- # Room 40 × 15 m, 75 lux FTL design Industrial Power Distribution and Illumination · Chapter 7: Illumination Design Principles - Asked in: 2070 Bhadra, 8 marks - Video: [Lumen Method Lighting Calculation Example](https://www.youtube.com/watch?v=Rn8wJRY-9WU) — D. Leith - EngTech MIET QTLS Electrical Tutor - 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/room-40x15-ftl-75lux - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-7 ## Question An illumination on the working plane of 75 lux is required in a room 40m × 15m in size. The luminaries are required to be hung 4m above the work bench. Assuming a suitable space height ratio, a utilization factor of 0.8, a lamp efficiency of 60 lumen per watt and maintenance factor is 0.7. Estimate the number and disposition of lamp and switches. Design the sub-circuits, distribution board if supply system is 3 phase 4 wire system and consider 1 × 36 watt FTL with electronic ballast. ## Short answer By lumen method, required luminaires N = 75×600/(2160×0.8×0.7)=37.2, rounded to 40 single 36 W FTL, arranged 10×4. Spacing 4 m×3.75 m within 6 m limit, actual illuminance 80.6 lux. ## Answer **Given:** room 40 m × 15 m, E = 75 lux, Hm = 4 m, UF = 0.8, MF = 0.7, 1 × 36 W FTL with electronic ballast at 60 lm/W, 3-phase 4-wire supply. **Assumptions:** suitable space–height ratio **1.5** → maximum spacing 1.5 × 4 = 6 m; pf 0.9 with electronic ballast, ballast loss neglected. #### Number of luminaires Lumen method: N = (E × A) / (φ × UF × MF), where φ is the output of one luminaire. ```text Area A = 40 × 15 = 600 m² φ per luminaire = 36 W × 60 lm/W = 2160 lm Total lumens = E × A / (UF × MF) = 75 × 600 / (0.8 × 0.7) = 80357 lm N = 80357 / 2160 = 37.2 ``` Rounding up and fitting a regular grid: **10 × 4 = 40 luminaires** (10 along the 40 m side, 4 along the 15 m side). #### Arrangement and spacing ```text Spacing along length = 40/10 = 4 m Spacing along width = 15/4 = 3.75 m End spacing (to wall) = half of these Max spacing = SHR × Hm = 1.5 × 4 = 6 m 4 m and 3.75 m ≤ 6 m → OK Actual E = 40 × 2160 × 0.8 × 0.7 / 600 = 80.6 lux (≥ 75 lux required) ``` #### Layout diagram ```text Plan 40 m × 15 m (letter = phase of lamp circuit) +---------------------+ | R R R R R R R Y Y Y | | B B B B B B Y Y Y Y | | B R R R R R R R Y Y | | B B B B B B Y 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. #### Sub-circuits Rule (IS 732 / NEA practice): a light sub-circuit carries at most 10 points and 800 W. Here one luminaire is 36 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 | 7 | 252 | 1.22 | R | 6 A | | L2 | 7 | 252 | 1.22 | Y | 6 A | | L3 | 7 | 252 | 1.22 | B | 6 A | | L4 | 7 | 252 | 1.22 | R | 6 A | | L5 | 6 | 216 | 1.04 | Y | 6 A | | L6 | 6 | 216 | 1.04 | B | 6 A | #### Distribution board design | Phase | Circuits | Load (W) | Current (A) | |---|---|---|---| | R | L1, L4 | 504 | 2.43 | | Y | L2, L5 | 468 | 2.26 | | B | L3, L6 | 468 | 2.26 | ```text Total connected load = 1440 W (lighting) Most loaded phase = 504 W Iph = 504 / (230 × 0.9) = 2.43 A Main MCB ≥ 1.25 × 2.43 = 3.04 A → 6 A TPN MCB ``` **DB specification:** 4-way-per-phase TPN DB (12 ways in all, spares included) with 6 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 ≥ 6 A), plus earth continuity conductor. ```text 3-phase, 400/230 V, 4-wire supply | [Energy meter] | [6 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 the lumen method, which expression gives the number of luminaires N? 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) D. N = A / (E x UF x MF x lumens per luminaire) Answer: B. Required lumens E x A are divided by the useful output per luminaire, which is reduced by the utilization factor and maintenance factor. (Key point: Lumen method formula) 2. The calculation gives N = 37.2. Why does the answer use a 10 x 4 = 40 grid? A. The number is rounded down to 36 to save cost B. It is the nearest regular grid not below 37.2 that keeps spacing within the limit C. 40 is always used for rooms of 600 m^2 D. Because the supply is 3-phase, N must be a multiple of 10 Answer: B. N is rounded up and fitted to a regular rows x columns grid; 10 x 4 gives 4 m and 3.75 m spacing, below the 6 m maximum. (Key point: Grid selection by calculation) 3. With space-height ratio 1.5 and Hm = 4 m, what is the maximum spacing and is 4 m x 3.75 m acceptable? A. 2.67 m; not acceptable B. 5.5 m; not acceptable C. 6 m; acceptable D. 6 m; not acceptable Answer: C. Maximum spacing = SHR x Hm = 1.5 x 4 = 6 m, and both 4 m and 3.75 m are below it. (Key point: Spacing limit) 4. How is the incomer MCB rating chosen for the most loaded phase (504 W)? A. Iph = 504/(230 x 0.9) = 2.43 A, then 1.25 x Iph rounded up to 6 A B. Iph = 504/400 = 1.26 A, then a 2 A MCB C. Iph = 1440/230 = 6.26 A, then a 6 A MCB D. Iph = 504 x 230 x 0.9, then a 25 A MCB Answer: A. Phase current uses phase voltage 230 V and pf 0.9; with a 1.25 margin it is 3.04 A, so the next standard 6 A TPN MCB is used. 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