---
title: "Drawing hall 40 × 20 × 6 m lighting design"
subject: "Industrial Power Distribution and Illumination"
chapter: 7
key: "semester_6/industrial_power_distribution_and_illumination:drawing-hall-40x20-led"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/drawing-hall-40x20-led
site: BCT Engineering
---

# Drawing hall 40 × 20 × 6 m lighting design

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

- Asked in: 2077 Chaitra, 12 marks
- Video: [ILLUMINATION ENGINEERING - PART - 11 - LUMEN OR FLUX METHOD OF  CALCULATION AND THREE PROBLEMS](https://www.youtube.com/watch?v=mB573i9S39A) — 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/drawing-hall-40x20-led
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-7

## Question

A drawing hall 40m×20m×6m is to be illuminated with metal filament gas filled lamps to an average illumination of 150 lm/m² on a working plane 1m above the floor. Estimate suitable number, size and mounting height of lamps. Assume coefficient of utilization of 0.8, depreciation factor of 1.2. Size of lamps is 40 Watt LED panel light and luminous efficiency of lamp 100 lm/watt. Draw layout diagram showing arrangement of luminaries, switches, power socket and distribution board. Assume the supply system to be 3-phase, 400V, 50Hz.

## Short answer

Mounting height above working plane: 4 m. Number of 40 W LED luminaires: 45, arranged in a 9 ×5 grid. Distribution board: 32 A TPN incomer, 40 A RCCB, 6 A SP MCBs for six lighting circuits, 20 A SP MCBs for power sockets.

## Answer

**Given:** hall 40 m × 20 m × 6 m, E = 150 lm/m² on a working plane 1 m above floor, CU = 0.8, depreciation factor 1.2, 40 W LED panel at 100 lm/W (the question's "metal filament" lamp is replaced by this LED panel as stated).
**Assumptions:** luminaires suspended 1 m below the 6 m ceiling, i.e. at 5 m above floor, so **mounting height above working plane Hm = 5 − 1 = 4 m**; SHR ≤ 1.5; 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        = 40 × 20 = 800 m²
φ per luminaire = 40 W × 100 lm/W = 4000 lm
MF            = 1/DF = 1/1.2 = 0.833
Total lumens  = E × A × DF / UF
              = 150 × 800 × 1.2 / 0.8
              = 180000 lm
N             = 180000 / 4000 = 45
```

Rounding up and fitting a regular grid: **9 × 5 = 45 luminaires** (9 along the 40 m side, 5 along the 20 m side).

#### Arrangement and spacing

```text
Spacing along length = 40/9 = 4.44 m
Spacing along width  = 20/5 = 4 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.5 × 4 = 6 m
4.44 m and 4 m ≤ 6 m  → OK
Actual E = 45 × 4000 × 0.8 × 0.833 / 800
         = 150 lux (≥ 150 lux required)
```

#### Layout diagram

```text
Plan 40 m × 20 m (letter = phase of lamp circuit)
+-------------------+
| R R R R R R R R Y |
| B B Y Y Y Y Y Y Y |
| B B B B B B R R R |
| Y Y Y Y Y R R R R |
| Y Y B B B B B B B |
+-------------------+
[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 40 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 | 8 | 320 | 1.55 | R | 6 A |
| L2 | 8 | 320 | 1.55 | Y | 6 A |
| L3 | 8 | 320 | 1.55 | B | 6 A |
| L4 | 7 | 280 | 1.35 | R | 6 A |
| L5 | 7 | 280 | 1.35 | Y | 6 A |
| L6 | 7 | 280 | 1.35 | 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 | 4600 | 22.22 |
| Y | L2, L5, P2, P5 | 4600 | 22.22 |
| B | L3, L6, P3, P6 | 4600 | 22.22 |

```text
Connected load = 1800 W light + 12000 W power = 13800 W
Most loaded phase    = 4600 W
Iph = 4600 / (230 × 0.9) = 22.22 A
Main MCB ≥ 1.25 × 22.22 = 27.78 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  ...
```

**Result:** 45 LED panels of **40 W** each, mounted at **5 m above floor (Hm = 4 m)** in a 9 × 5 grid.

## Check yourself

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

1. Luminaires hang at 5 m above the floor and the working plane is 1 m above the floor. What is Hm?
   A. 6 m
   B. 5 m
   C. 4 m
   D. 1 m
   Answer: C. Hm is measured from the working plane: 5 - 1 = 4 m, so the maximum spacing is 1.5 x 4 = 6 m. (Key point: Mounting height)

2. How is the depreciation factor used in the lumen calculation?
   A. MF = DF, so multiply
   B. MF = 1/DF = 0.833, so total lumens = E x A x DF / UF
   C. It is ignored
   D. It is added to the UF
   Answer: B. Maintenance factor is the reciprocal of the depreciation factor, so lumens = 150 x 800 x 1.2 / 0.8 = 180000 lm. (Key point: Depreciation factor)

3. With 180000 lm needed and 4000 lm per luminaire, how many are fitted?
   A. 45 in a 9 x 5 grid
   B. 36
   C. 180
   D. 450
   Answer: A. 180000 / 4000 = 45, which fits exactly a 9 x 5 grid. (Key point: Number of luminaires)

4. How are the 12 power sockets of 16 A grouped?
   A. All on one circuit
   B. At most 2 sockets per power sub-circuit, giving 6 circuits spread over the phases
   C. One socket per phase only
   D. On the lighting circuits
   Answer: B. Taking 1 kW per socket and at most 2 sockets per circuit gives P1-P6, spread over R, Y and B. (Key point: Power sockets)

## Close variants set in other exams

- [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×

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