---
title: "Flood lighting a 50 × 16 m building"
subject: "Industrial Power Distribution and Illumination"
chapter: 8
key: "semester_6/industrial_power_distribution_and_illumination:floodlight-building-50x16"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/floodlight-building-50x16
site: BCT Engineering
---

# Flood lighting a 50 × 16 m building

Industrial Power Distribution and Illumination · Chapter 8: Outdoor Lighting Design

- Asked in: 2076 Bhadra, 4 marks
- Video: [ILLUMINATION ENGINEERING - PART -12 - FLOOD LIGHTING - IMPORTANT TERMS & PRBOLEMS](https://www.youtube.com/watch?v=xOpYxgnD9kg) — zenmurali Sachithanandam
- 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/floodlight-building-50x16
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-8

## Question

A building elevation is 50m x 16m is to be illuminated by flood lighting projectors situated at 25 meters away. If the illumination is 100 lumen/m², coefficient of utilization is 0.5, depreciation factor 1.5, waste light factor 1.2, estimate the number and size of projectors. Also find out angle of spread.

## Short answer

Total lumens = 100×800×1.5×1.2/0.5 = 288,000 lm. Assuming 1000 W tungsten-halogen (20 lm/W): 15 projectors. Beam spread ≈ 35°. Alternative: 400 W metal-halide (80 lm/W) yields 9 projectors.

## Answer

**Given:** building face 50 m × 16 m, projectors 25 m away, E = 100 lm/m² (lux), CU = 0.5, depreciation factor DF = 1.5, waste light factor WLF = 1.2.

#### Total lumens required
```text
Area A   = 50 × 16 = 800 m²
Total lm = E × A × DF × WLF / CU
         = 100 × 800 × 1.5 × 1.2 / 0.5
         = 288 000 lm
```

#### Number and size of projectors
Size is not given, so assume **1000 W tungsten-halogen floodlight projectors** with efficacy about 20 lm/W (standard textbook assumption):
```text
Lumens per lamp = 1000 × 20 = 20 000 lm
N = 288 000 / 20 000 = 14.4 → 15 projectors
```
(If 400 W metal-halide lamps of 80 lm/W = 32 000 lm are used instead, N = 9.)

#### Angle of spread
With projectors 25 m away, aimed at the middle of the face (8 m high), the vertical beam must cover 16 m:
```text
θ = 2 tan⁻¹(8 / 25) = 35.5°
```
(If projectors are at ground level aimed upward, the face subtends tan⁻¹(16/25) = 32.6°.)
So projectors with a **medium beam of about 35°** are chosen. Horizontally the 15 projectors are spread along the 50 m frontage so that their beams overlap.

**Answer: total 288 000 lm; 15 projectors of 1000 W (20 lm/W assumed); beam spread ≈ 35°.**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the total lumen requirement for 100 lux on 800 m^2 with DF 1.5, WLF 1.2, CU 0.5?
   A. 288000 lm
   B. 80000 lm
   C. 28800 lm
   D. 144000 lm
   Answer: A. 100 x 800 x 1.5 x 1.2/0.5 = 288000 lm. (Key point: Total lumens)

2. With 1000 W projectors of 20 lm/W assumed, how many projectors?
   A. 9
   B. 15
   C. 144
   D. 29
   Answer: B. 288000/20000 = 14.4, rounded up to 15. (Key point: Number of projectors)

3. How is the angle of spread found for aiming at the middle of the face (half height 8 m, distance 25 m)?
   A. 2 tan^-1(8/25) = 35.5°
   B. tan^-1(25/8) = 72°
   C. tan^-1(16/25) x 2 = 65°
   D. 8 x 25 = 200°
   Answer: A. The beam must cover 8 m on each side of the aim line at 25 m, giving about 35.5°. (Key point: Angle of spread)

## Related in Industrial Power Distribution and Illumination

- [Flood lighting procedure and 50 × 15 m frontage](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aflood-procedure-building-50x15) — Industrial Power Distribution and Illumination, chapter 8
- [Industry room 30 × 20 m LED lighting and DB](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aindustry-room-30x20-led) — Industrial Power Distribution and Illumination, chapter 7, asked 2×
- [Waste light factor and floodlight calculation](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Awaste-light-factor-floodlight) — Industrial Power Distribution and Illumination, chapter 8
- [Hall 16 × 12 m LED lighting](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Ahall-16x12-led) — Industrial Power Distribution and Illumination, chapter 7, asked 4×
- [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
- [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×