--- title: "Office lighting design with CFL luminaires" subject: "Industrial Power Distribution and Illumination" chapter: 7 key: "semester_6/industrial_power_distribution_and_illumination:office-lighting-design-cfl" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/office-lighting-design-cfl site: BCT Engineering --- # Office lighting design with CFL luminaires Industrial Power Distribution and Illumination · Chapter 7: Illumination Design Principles - Asked in: 2080 Chaitra, 10 marks - Video: [Lumen method of Interior lighting](https://www.youtube.com/watch?v=I5jayDiwEsk) — Electrical Learning Aids - 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/office-lighting-design-cfl - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-7 ## Question An office has length of 20m, width of 10m and height of 3m. The ceiling to desk height is 2m and the area to be illuminated to a general is 250 lux using CFL Luminaire with a SHR(max) of 1.25. The maintenance factor is 0.63, Utilization factor is 0.69 and Efficiency of Lamp is 120lumen/Watt. Design a lighting system for this office with number of Lamps, sizing of lamps, luminaries layout, wiring layout and distribution system layout. ## Short answer 32 CFL luminaires of 30 W each (3600 lm) in an 8×4 grid, spacing 2.5 m. Total lighting load 960 W, achieving 250.4 lux with UF=0.69, MF=0.63, and 120 lm/W efficacy. ## Answer **Given:** room 20 m × 10 m × 3 m, mounting height above desk Hm = 2 m, E = 250 lux, SHR(max) = 1.25, MF = 0.63, UF = 0.69, lamp efficacy 120 lm/W. **Assumptions:** ceiling-mounted CFL/LED-type luminaires, pf 0.9; 6 power sockets (16 A, 1 kW each) for office equipment; 3-phase, 400/230 V, 4-wire supply. #### Number and size of lamps The number is fixed by the maximum spacing (for uniformity), then the lamp size is fixed by the lumens needed. ```text Max spacing S = SHR × Hm = 1.25 × 2 = 2.5 m Along length: 20/2.5 = 8 rows; along width: 10/2.5 = 4 Number of luminaires N = 8 × 4 = 32 Total lumens = E × A / (UF × MF) = 250 × 200 / (0.69 × 0.63) = 115022 lm Lumens per luminaire = 115022 / 32 = 3594.4 lm Lamp wattage = 3594.4 / 120 = 29.95 W → choose 30 W lamp Lumens per 30 W lamp = 30 × 120 = 3600 lm ``` **Answer: 32 luminaires of 30 W each (3600 lm), in 8 × 4 grid, total lighting load 960 W.** #### Arrangement and spacing ```text Spacing along length = 20/8 = 2.5 m Spacing along width = 10/4 = 2.5 m End spacing (to wall) = half of these Max spacing = SHR × Hm = 1.25 × 2 = 2.5 m 2.5 m and 2.5 m ≤ 2.5 m → OK Actual E = 32 × 3600 × 0.69 × 0.63 / 200 = 250.4 lux (≥ 250 lux required) ``` #### Layout diagram ```text Plan 20 m × 10 m (letter = phase of lamp circuit) +-----------------+ | R R R R R R Y Y | | B B B B Y Y Y Y | | B B B B B B R R | | Y Y Y Y Y R R R | +-----------------+ [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 30 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 | 180 | 0.87 | R | 6 A | | L2 | 6 | 180 | 0.87 | Y | 6 A | | L3 | 5 | 150 | 0.72 | B | 6 A | | L4 | 5 | 150 | 0.72 | B | 6 A | | L5 | 5 | 150 | 0.72 | R | 6 A | | L6 | 5 | 150 | 0.72 | Y | 6 A | Power sub-circuits (assumed): 6 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 | #### Distribution board design | Phase | Circuits | Load (W) | Current (A) | |---|---|---|---| | R | L1, L5, P1 | 2330 | 11.26 | | Y | L2, L6, P2 | 2330 | 11.26 | | B | L3, L4, P3 | 2300 | 11.11 | ```text Connected load = 960 W light + 6000 W power = 6960 W Most loaded phase = 2330 W Iph = 2330 / (230 × 0.9) = 11.26 A Main MCB ≥ 1.25 × 11.26 = 14.07 A → 16 A TPN MCB ``` **DB specification:** 4-way-per-phase TPN DB (12 ways in all, spares included) with 16 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 and 20 A SP MCBs for power circuits. Incoming cable: 4-core 1.5 mm² Cu PVC (rating 21 A ≥ 16 A), plus earth continuity conductor. ```text 3-phase, 400/230 V, 4-wire supply | [Energy meter] | [16 A TPN MCB] | [25 A 4P RCCB] | ==R====Y====B====N== busbars | | | | | L1 L2 L3 P1 P2 ... ``` #### Wiring layout ```text 20 m +-----------------------------------+ | x x x x x x x x [P]| | x x x x x x x x | 10 m | x x x x x x x x [P]| | x x x x x x x x | +[P]-----[P]----[SB][DB]-------[P]--+ door x = 30 W luminaire [P] = 16 A socket Conduit runs along rows; each row-group is looped back to its switch on SB. ``` Wiring is in concealed PVC conduit: 1.5 mm² Cu (phase, neutral, earth) for light circuits and 2.5 mm² Cu for socket circuits, all drawn from the DB near the door. #### Distribution system layout ```text NEA LT supply 400 V, 3φ 4-wire | [Energy meter] → [Main switch / TPN MCB] | Office DB (TPN) ── L circuits → luminaires └─ P circuits → sockets | Earth electrode (pipe/plate) ── ECC to all points ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is the maximum luminaire spacing found in this design? A. S = SHR x mounting height = 1.25 x 2 = 2.5 m B. S = room length / 2 = 10 m C. S = SHR / Hm = 0.625 m D. S = Hm + SHR = 3.25 m Answer: A. Maximum spacing is SHR x Hm = 1.25 x 2 = 2.5 m, which gives 8 rows along 20 m and 4 along 10 m, i.e. 32 luminaires. (Key point: Maximum spacing) 2. Which expression gives the total lumens required? A. E x A x UF x MF B. E x A / (UF x MF) C. E x A / (UF + MF) D. E / (A x UF x MF) Answer: B. Total lumens = E x A / (UF x MF) = 250 x 200 / (0.69 x 0.63), about 115022 lm. (Key point: Lumen method formula) 3. Lumens per luminaire are about 3594 lm and efficacy is 120 lm/W. What lamp is chosen? A. 20 W B. 25 W C. 30 W D. 60 W Answer: C. 3594.4 / 120 = 29.95 W, so the nearest standard size above is 30 W (3600 lm). Total lighting load is 32 x 30 = 960 W. (Key point: Lamp size) 4. What limit applies to a light sub-circuit in this answer? A. At most 2 points B. At most 10 points and 800 W C. At most 20 points and 2000 W D. Only one luminaire Answer: B. A light sub-circuit carries at most 10 points and 800 W, and the number of circuits is a multiple of 3 for phase balance. (Key point: Sub-circuit rule) ## 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× - [Conference hall 2×36 W CFL lighting](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aconference-hall-cfl) — Industrial Power Distribution and Illumination, chapter 7 - [Flood lighting design and luminaire selection](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aflood-design-luminaire-selection) — Industrial Power Distribution and Illumination, chapter 8 - [Ten factors for flood, street lighting, luminaires](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aten-factors-flood-street-luminaires) — Industrial Power Distribution and Illumination, chapter 8 - [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 - [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× ## Related in other subjects - [Nichrome strip resistor length and width](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Anichrome-strip-resistor) — Electric Machine Design, chapter 2, asked 2× - [Crest width, cistern length of vertical drop, Q 1.55](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Avertical-drop-q155-drop075) — Irrigation and Drainage Engineering, chapter 7, asked 2× - [Field and office work; length of offsets](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Achain-field-office-work-offset-length) — Surveying I, chapter 3 - [Cluster analysis; K-means on height/width](https://www.bctengineer.com/question.md?key=semester_7%2Fdata_mining%3Akmeans-height-width) — Data Mining, chapter 5