Skip to main content

Drawing hall 40 × 25 × 6 m, 2×36 W fluorescent

Short answer

With mounting height 4 m above working plane, lumen method yields 31.3 but spacing limit 5.6 m requires 8×5=40 luminaires of 2×36 W, spaced 5 m both ways, giving 115.2 lux average illuminance.

A drawing hall 40m×25m×6m height is to be illuminated with 2×36W fluorescent lamp to an average illumination of 90 Lm/m² on a working plane 1 m above the floor. Estimate suitable numbers, sizes of lamps. Mounting height 1m below the ceiling. Sketch the spacing layout. Assume: coefficient of utilization 0.8 depreciation factor 1.2, space height ratios 1.4 and the lamp efficiency is 60lumen/watt.

Given: hall 40 m × 25 m × 6 m, E = 90 lm/m² on a plane 1 m above floor, 2 × 36 W fluorescent (72 W per luminaire) at 60 lm/W, CU = 0.8, depreciation factor 1.2, SHR = 1.4, luminaires 1 m below ceiling.

Mounting height above working plane: Hm = (6 − 1) − 1 = 4 m.

Number of luminaires

Lumen method: N = (E × A) / (φ × UF × MF), where φ is the output of one luminaire.

Area A        = 40 × 25 = 1000 m²
φ per luminaire = 72 W × 60 lm/W = 4320 lm
MF            = 1/DF = 1/1.2 = 0.833
Total lumens  = E × A × DF / UF
              = 90 × 1000 × 1.2 / 0.8
              = 135000 lm
N             = 135000 / 4320 = 31.25

Rounding up and fitting a regular grid: 8 × 5 = 40 luminaires (8 along the 40 m side, 5 along the 25 m side).

Arrangement and spacing

Spacing along length = 40/8 = 5 m
Spacing along width  = 25/5 = 5 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.4 × 4 = 5.6 m
5 m and 5 m ≤ 5.6 m  → OK
Actual E = 40 × 4320 × 0.8 × 0.833 / 1000
         = 115.2 lux (≥ 90 lux required)

The lumen calculation needs only 32 luminaires, but the 5.6 m spacing limit governs: 40/5.6 = 7.1 → 8 luminaires along the length and 25/5.6 = 4.5 → 5 across the width. A 7 × 5 grid gives 5.71 m spacing (> 5.6 m) and an 8 × 4 grid gives 6.25 m, so both are too wide. So 8 × 5 = 40 luminaires of 2 × 36 W are used at 5 m × 5 m spacing (2.5 m from the walls). Total load = 40 × 72 = 2880 W.

Layout diagram

Plan 40 m × 25 m (o = luminaire)
+-----------------+
| o o o o o o o o |
| o o o o o o o o |
| o o o o o o o o |
| o o o o o o o o |
| o o o o o o o o |
+-----------------+
Spacing between luminaires = 5 m both ways
End spacing to each wall   = 2.5 m
Mounting height: 5 m above floor (Hm = 4 m)

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Industrial Power Distribution and Illumination chapter 7 questions →