Department store 30 × 15 m twin 40 W fluorescent

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.

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

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

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.

Distribution board design

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.

3-phase, 400/230 V, 4-wire supply
        |
   [Energy meter]
        |
   [6 A TPN MCB]
        |
   [25 A 4P RCCB]
        |
==R====Y====B====N== busbars
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  L1   L2   L3   L4   L5  ...