---
title: "Department store 30 × 15 m twin 40 W fluorescent"
subject: "Industrial Power Distribution and Illumination"
chapter: 7
key: "semester_6/industrial_power_distribution_and_illumination:department-store-twin-40w-fluorescent"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/department-store-twin-40w-fluorescent
site: BCT Engineering
---

# Department store 30 × 15 m twin 40 W fluorescent

Industrial Power Distribution and Illumination · Chapter 7: Illumination Design Principles

- Asked in: 2068 Bhadra (old course), 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-twin-40w-fluorescent
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-7

## Question

A department store 30m ×15m is illuminated by twin 40W fluorescent luminaries of output 4800 lumens. The lamp being mounted at height of 3m from the work place, the average illumination required is 200lux. 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 the lamps, switches and sub-circuit. If the supply system is 3-phase, 400V, 50Hz.

## Short answer

Using lumen method: total lumens required = (200×450)/(0.8×0.8)=140625 lm; luminaire output 4800 lm, N=140625/4800≈29.3, rounded to 32 luminaires in 8×4 grid. Six 6 A SP MCB light circuits balanced across phases, 6 A TPN MCB incomer.

## Answer

**Given:** store 30 m × 15 m, twin 40 W fluorescent luminaire (80 W) 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 (spacing ≤ 4.5 m); electronic ballasts, 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 = 4800 lm
Total lumens  = E × A / (UF × MF)
              = 200 × 450 / (0.8 × 0.8)
              = 140625 lm
N             = 140625 / 4800 = 29.3
```

Rounding up and fitting a regular grid: **8 × 4 = 32 luminaires** (8 along the 30 m side, 4 along the 15 m side).

#### Arrangement and spacing

```text
Spacing along length = 30/8 = 3.75 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
3.75 m and 3.75 m ≤ 4.5 m  → OK
Actual E = 32 × 4800 × 0.8 × 0.8 / 450
         = 218.5 lux (≥ 200 lux required)
```

A 6 × 5 = 30 grid (5 m × 3 m) or 10 × 3 grid (3 m × 5 m) would put spacing above 4.5 m, so 8 × 4 = 32 is used.

#### Layout diagram

```text
Plan 30 m × 15 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.

#### Sub-circuits

Rule (IS 732 / NEA practice): a light sub-circuit carries at most 10 points and 800 W. Here one luminaire is 80 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 | 480 | 2.32 | R | 6 A |
| L2 | 6 | 480 | 2.32 | Y | 6 A |
| L3 | 5 | 400 | 1.93 | B | 6 A |
| L4 | 5 | 400 | 1.93 | B | 6 A |
| L5 | 5 | 400 | 1.93 | R | 6 A |
| L6 | 5 | 400 | 1.93 | Y | 6 A |

#### Distribution board design

| Phase | Circuits | Load (W) | Current (A) |
|---|---|---|---|
| R | L1, L5 | 880 | 4.25 |
| Y | L2, L6 | 880 | 4.25 |
| B | L3, L4 | 800 | 3.86 |

```text
Total connected load = 2560 W (lighting)
Most loaded phase    = 880 W
Iph = 880 / (230 × 0.9) = 4.25 A
Main MCB ≥ 1.25 × 4.25 = 5.31 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. Since the luminaire output of 4800 lm is given, how is N found?
   A. Multiply 80 W by 60 lm/W first
   B. N = E x A / (4800 x CU x MF) = 29.3
   C. N = A / 4800
   D. N = E x 4800 / A
   Answer: B. The given 4800 lm is used directly: 200 x 450/(0.8 x 0.8) = 140625 lm, divided by 4800 gives 29.3. (Key point: Use of given lumen output)

2. Why is 8 x 4 = 32 chosen instead of 6 x 5 = 30?
   A. 30 gives less than 200 lux
   B. 6 x 5 spacing (5 m x 3 m) exceeds the 4.5 m limit
   C. 30 cannot be divided among 3 phases
   D. The grid must have even numbers
   Answer: B. Max spacing is 1.5 x 3 = 4.5 m; the 8 x 4 grid gives 3.75 m both ways, which is acceptable. (Key point: Grid and spacing check)

3. How is the current of the R phase (880 W) found?
   A. 880/400
   B. 880/(230 x 0.9) = 4.25 A
   C. 880 x 0.9/230
   D. 880/(400 x 0.9) = 2.44 A
   Answer: B. Each phase supplies lamps at 230 V phase voltage and pf 0.9, giving 4.25 A. (Key point: Phase current)

4. Which table of phase loads matches the design (circuits of 480, 480, 400, 400, 400, 400 W)?
   A. R 880, Y 880, B 800 W
   B. R 1440, Y 560, B 560 W
   C. R 480, Y 480, B 400 W
   D. R 880, Y 800, B 880 W
   Answer: A. Pairing L1+L5, L2+L6 and L3+L4 gives 880, 880 and 800 W, nearly balanced; total 2560 W. (Key point: Phase balance)

## Close variants set in other exams

- [Department store 30 × 15 m twin 55 W LED](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Adepartment-store-30x15-twin-55w) — 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×

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