Office lighting design with CFL luminaires

Given: room 20 m × 10 m × 3 m, mounting height above desk Hm = 2 m, E = 250 lux, SHR(max) = 1.25, MF = 0.63, UF = 0.69, lamp efficacy 120 lm/W. Assumptions: ceiling-mounted CFL/LED-type luminaires, pf 0.9; 6 power sockets (16 A, 1 kW each) for office equipment; 3-phase, 400/230 V, 4-wire supply.

Number and size of lamps

The number is fixed by the maximum spacing (for uniformity), then the lamp size is fixed by the lumens needed.

Max spacing  S = SHR × Hm = 1.25 × 2 = 2.5 m
Along length: 20/2.5 = 8 rows;  along width: 10/2.5 = 4
Number of luminaires N = 8 × 4 = 32

Total lumens = E × A / (UF × MF)
             = 250 × 200 / (0.69 × 0.63) = 115022 lm
Lumens per luminaire = 115022 / 32 = 3594.4 lm
Lamp wattage = 3594.4 / 120 = 29.95 W  → choose 30 W lamp
Lumens per 30 W lamp = 30 × 120 = 3600 lm

Answer: 32 luminaires of 30 W each (3600 lm), in 8 × 4 grid, total lighting load 960 W.

Arrangement and spacing

Spacing along length = 20/8 = 2.5 m
Spacing along width  = 10/4 = 2.5 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.25 × 2 = 2.5 m
2.5 m and 2.5 m ≤ 2.5 m  → OK
Actual E = 32 × 3600 × 0.69 × 0.63 / 200
         = 250.4 lux (≥ 250 lux required)

Layout diagram

Plan 20 m × 10 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. 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 30 W, so up to 10 luminaires per sub-circuit. Number of circuits is made a multiple of 3 for phase balance.

Power sub-circuits (assumed): 6 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.

Distribution board design

Connected load = 960 W light + 6000 W power = 6960 W
Most loaded phase    = 2330 W
Iph = 2330 / (230 × 0.9) = 11.26 A
Main MCB ≥ 1.25 × 11.26 = 14.07 A → 16 A TPN MCB

DB specification: 4-way-per-phase TPN DB (12 ways in all, spares included) with 16 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 and 20 A SP MCBs for power circuits. Incoming cable: 4-core 1.5 mm² Cu PVC (rating 21 A ≥ 16 A), plus earth continuity conductor.

3-phase, 400/230 V, 4-wire supply
        |
   [Energy meter]
        |
   [16 A TPN MCB]
        |
   [25 A 4P RCCB]
        |
==R====Y====B====N== busbars
  |    |    |    |    |
  L1   L2   L3   P1   P2  ...

Wiring layout

       20 m
+-----------------------------------+
| x   x   x   x   x   x   x   x  [P]|
| x   x   x   x   x   x   x   x     | 10 m
| x   x   x   x   x   x   x   x  [P]|
| x   x   x   x   x   x   x   x     |
+[P]-----[P]----[SB][DB]-------[P]--+
                 door
x = 30 W luminaire   [P] = 16 A socket
Conduit runs along rows; each row-group
is looped back to its switch on SB.

Wiring is in concealed PVC conduit: 1.5 mm² Cu (phase, neutral, earth) for light circuits and 2.5 mm² Cu for socket circuits, all drawn from the DB near the door.

Distribution system layout

NEA LT supply 400 V, 3φ 4-wire
     |
 [Energy meter] → [Main switch / TPN MCB]
     |
   Office DB (TPN) ── L circuits → luminaires
                   └─ P circuits → sockets
     |
   Earth electrode (pipe/plate) ── ECC to all points