Illumination Design Principles

A lighting scheme is the way luminaires are arranged to provide light for the work in an area. Industries use the following schemes:

1. General lighting

Luminaires are spread in a regular pattern over the whole area to give uniform illumination (e.g. 200–300 lux) on the working plane, independent of where machines are placed. Spacing–height ratio is kept within limits for uniformity. Used in assembly halls, warehouses and general workshops.

2. Localised (local) lighting

Extra lamps are fixed close to particular work points (on a lathe, inspection bench, drawing board) to give high illumination (500–1000+ lux) only where fine work is done. Saves energy but gives strong contrast if used alone.

3. General plus local (supplementary) lighting

Moderate general lighting for movement and safety, plus local lamps at critical tasks. This is the usual scheme in machine shops: it is economical and avoids dark surroundings and eye strain.

4. Localised general lighting

General-type luminaires are concentrated in the zones where work is done (above machine rows), with lower lighting over gangways.

5. Emergency lighting

Battery-backed or generator-fed lights that come on when the normal supply fails, lighting escape routes, exits, stairs and critical machines (minimum about 1 lux on escape routes).

Classification by light distribution

In factories, direct general lighting with high-bay LED or metal-halide luminaires plus local lighting at machines is the normal choice, with phases distributed to avoid stroboscopic effect.

Types of lighting schemes

By light distribution: direct, semi-direct, general diffusing, semi-indirect and indirect lighting (decided by the fraction of light thrown downward, from 90–100 % for direct to 0–10 % for indirect).

By coverage of the area: general lighting, local (localised) lighting, general plus local lighting, localised general lighting and emergency lighting.

General lighting

General lighting provides a uniform level of illumination over the whole floor area, irrespective of the position of work. Luminaires of the same type are installed in a regular grid at a spacing within the allowed spacing-to-height ratio, usually with direct or semi-direct distribution.

• Gives good uniformity (minimum/average ≥ 0.7) and allows machines to be moved freely.
• Typical levels: 150–300 lux in workshops, stores and assembly areas.
• Disadvantage: lighting the whole area to the level needed for fine work wastes energy.

Local lighting

Local lighting gives high illumination on a small working area, using lamps fixed on or near the machine (adjustable arm lamps, bench lights, machine lamps).

• Gives 500–2000 lux where fine work, inspection or measurement is done.
• Can be aimed to show surface texture and reduce shadows from the worker's body.
• Must be used with some general lighting to avoid sharp contrast and eye fatigue; local lamps should be shielded to prevent glare and be low-voltage (24/48 V) in hazardous places.

Emergency lighting

Emergency lighting operates automatically when the normal supply fails, so that people can leave safely and dangerous processes can be shut down.

• Escape lighting: lights exits, corridors, stairs and exit signs (about 1 lux minimum on escape routes).
• Standby lighting: keeps essential work going (control rooms, hospitals, critical processes).
• Sources: self-contained battery luminaires, central battery systems, UPS or DG sets with automatic changeover; batteries are usually rated for 1–3 hours.
• Must be tested regularly.

Normal supply ──┬── General & local lights
                │
            [Charger]──[Battery]──┐
                │                 │
           (supply fails) → [Changeover] → Emergency lights

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

Given: hall 120 m × 30 m, E = 300 lux, UF = 0.8, MF = 0.8, twin-tube LED fitting 2 × 20 W = 40 W at 100 lm/W. Assumptions: mounting height 3 m above work plane, SHR ≤ 1.5, pf 0.9 for LED, 3-phase 400/230 V supply, all motors and machines 3-phase.

Number of luminaires

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

Area A        = 120 × 30 = 3600 m²
φ per luminaire = 40 W × 100 lm/W = 4000 lm
Total lumens  = E × A / (UF × MF)
              = 300 × 3600 / (0.8 × 0.8)
              = 1687500 lm
N             = 1687500 / 4000 = 421.88

Rounding up and fitting a regular grid: 47 × 9 = 423 luminaires (47 along the 120 m side, 9 along the 30 m side).

Arrangement and spacing

Spacing along length = 120/47 = 2.55 m
Spacing along width  = 30/9 = 3.33 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.5 × 3 = 4.5 m
2.55 m and 3.33 m ≤ 4.5 m  → OK
Actual E = 423 × 4000 × 0.8 × 0.8 / 3600
         = 300.8 lux (≥ 300 lux required)

Layout diagram

Plan 120 m × 30 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 |
| 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 |
| 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 |
| 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 |
| o o o o o o o o o o ... o o o o o o o o o o |
+---------------------------------------------+
[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 40 W, so up to 10 luminaires per sub-circuit. Number of circuits is made a multiple of 3 for phase balance.

Largest light circuit: 400 W → I = 400/(230 × 0.9) = 1.93 A → 6 A MCB, 1.5 mm² Cu.

Power load calculation

Pump rating: 7.5 hp × 746 = 5595 W output; input = 5595/0.8 = 6993.75 W; kVA = 6993.75/0.8 = 8.742 kVA.

Assumed utilisation factor ku: 0.75 for machine tools, 0.8 for compressor and pumps, 1.0 for oven and ventilation. CB rating ≥ 1.25 × full-load current, D-curve for motors (high starting current), C-curve for the oven.

PDB demand = ks × Σ(ku × kVA) = 0.8 × 60.24 = 48.19 kVA
Lighting load    = 16920 W / 0.9 pf      = 18.8 kVA
Total max demand = 48.19 + 18.8 = 66.99 kVA
With 25 % future growth: 1.25 × 66.99 = 83.74 kVA

Transformer: standard 100 kVA, 11/0.4 kV, Dyn11 (full-load LV current = 100000/(√3 × 400) = 144.3 A).

Breakers and DBs

PDB current = 48.19 kVA/(√3×400 V) = 69.56 A
            → 1.25I = 86.95 A
            → 100 A TP MCCB for PDB feeder
LDB current = 27.25 A per phase  → 1.25I = 34.06 A
            → 40 A TPN MCB + 40 A 4P RCCB (100 mA)
Main LT breaker (transformer secondary 144.3 A)
            → 160 A TPN MCCB

• Power DB (PDB): 12-way TP DB with 100 A TP MCCB incomer and outgoing TP MCBs as in the table (3 × lathe, 2 × drill, 2 × ventilation, compressor, oven, 2 × pump = 11 ways + 1 spare).
• Lighting DB (LDB): TPN DB, 16 ways per phase, 45 light circuits of 9–10 luminaires (6 A SP MCB each), 15 per phase, with 40 A TPN MCB + RCCB incomer.

Single line diagram

 11 kV supply
     |
  [HT fuse/VCB]
     |
  (( 100 kVA, 11/0.4 kV ))
     |
  [160 A TPN MCCB]  Main LT panel
     |
 ====+==========+====  LT busbar
     |          |
 [100 A MCCB] [40 A TPN MCB+RCCB]
     |          |
    PDB        LDB
  |  |  |      | | | ...
 L1 L2 L3 ...  L1..L45 light
 (lathes,      circuits
 drills, fans, (6 A each)
 comp, oven,
 pumps)

Layout of power and light sub-circuits

+-------------------- 120 m --------------------+
| o o o o o o o o ... 47 luminaires per row     |
| (9 rows; 45 circuits of 9–10 lamps, R-Y-B)    |
|                                               |
| [Lathe][Lathe][Lathe]  [Drill][Drill]  [Oven] |  30 m
|                                               |
| [Fan]        [Compressor]   [Pump][Pump] [Fan]|
+--[LDB]-[PDB]--- entrance ---------------------+
 Power circuits: radial TP cables PDB → each machine

Given: hall 55.75 m × 40.25 m, E = 250 lux (lm/m²), 30 W LED panel at 100 lm/W, UF = 0.8, MF = 0.6, supply 400 V, 3-phase, 4-wire. Assumptions: mounting height 3 m above the working plane, maximum spacing-to-height ratio 1.5, LED driver pf 0.9.

Number of luminaires

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

Area A        = 55.75 × 40.25 = 2243.938 m²
φ per luminaire = 30 W × 100 lm/W = 3000 lm
Total lumens  = E × A / (UF × MF)
              = 250 × 2243.938 / (0.8 × 0.6)
              = 1168717 lm
N             = 1168717 / 3000 = 389.57

Rounding up and fitting a regular grid: 26 × 15 = 390 luminaires (26 along the 55.75 m side, 15 along the 40.25 m side).

Arrangement and spacing

Spacing along length = 55.75/26 = 2.14 m
Spacing along width  = 40.25/15 = 2.68 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.5 × 3 = 4.5 m
2.14 m and 2.68 m ≤ 4.5 m  → OK
Actual E = 390 × 3000 × 0.8 × 0.6 / 2243.938
         = 250.3 lux (≥ 250 lux required)

Layout diagram

Plan 55.75 m × 40.25 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 |
| 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 |
| 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 |
|                      :                      |
| 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 |
| 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 |
| 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 |
+---------------------------------------------+
[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 30 W, so up to 10 luminaires per sub-circuit. Number of circuits is made a multiple of 3 for phase balance.

Largest light circuit: 300 W → I = 300/(230 × 0.9) = 1.45 A → 6 A MCB, 1.5 mm² Cu.

Distribution board design

Total connected load = 11700 W (lighting)
Most loaded phase    = 3900 W
Iph = 3900 / (230 × 0.9) = 18.84 A
Main MCB ≥ 1.25 × 18.84 = 23.55 A → 25 A TPN MCB

DB specification: 16-way-per-phase TPN DB (48 ways in all, spares included) with 25 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 2.5 mm² Cu PVC (rating 27 A ≥ 25 A), plus earth continuity conductor.

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

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.

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

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

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.

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.

Distribution board design

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.

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.

Given: shop 30 m × 25 m, E = 150 lm/m², 40 W LED lamp at 100 lm/W, supply 400 V, 3-phase, 50 Hz. Assumptions: walls and ceiling brightly painted → UF = 0.7, clean industrial area → MF = 0.8; mounting height 3 m above working plane, SHR ≤ 1.5; pf 0.9; 12 power sockets of 16 A (1 kW each).

Number of luminaires

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

Area A        = 30 × 25 = 750 m²
φ per luminaire = 40 W × 100 lm/W = 4000 lm
Total lumens  = E × A / (UF × MF)
              = 150 × 750 / (0.7 × 0.8)
              = 200893 lm
N             = 200893 / 4000 = 50.22

Rounding up and fitting a regular grid: 9 × 6 = 54 luminaires (9 along the 30 m side, 6 along the 25 m side).

Arrangement and spacing

Spacing along length = 30/9 = 3.33 m
Spacing along width  = 25/6 = 4.17 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.5 × 3 = 4.5 m
3.33 m and 4.17 m ≤ 4.5 m  → OK
Actual E = 54 × 4000 × 0.7 × 0.8 / 750
         = 161.3 lux (≥ 150 lux required)

Layout diagram

Plan 30 m × 25 m (letter = phase of lamp circuit)
+-------------------+
| R R R R R R R R R |
| Y Y Y Y Y Y Y Y Y |
| B B B B B B B B B |
| R R R R R R R R R |
| Y Y Y Y Y Y Y Y Y |
| B B 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.

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.

Distribution board design

Connected load = 2160 W light + 12000 W power = 14160 W
Most loaded phase    = 4720 W
Iph = 4720 / (230 × 0.9) = 22.8 A
Main MCB ≥ 1.25 × 22.8 = 28.5 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.

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  ...

Given: hall 16 m × 12 m, E = 300 lm/m², 55 W LED at 90 lm/W, UF = 0.8, MF = 0.8, supply 400 V, 3-phase, 4-wire. Assumptions: mounting height 3 m above working plane, SHR ≤ 1.5, pf 0.9.

Number of luminaires

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

Area A        = 16 × 12 = 192 m²
φ per luminaire = 55 W × 90 lm/W = 4950 lm
Total lumens  = E × A / (UF × MF)
              = 300 × 192 / (0.8 × 0.8)
              = 90000 lm
N             = 90000 / 4950 = 18.18

Rounding up and fitting a regular grid: 5 × 4 = 20 luminaires (5 along the 16 m side, 4 along the 12 m side).

Arrangement and spacing

Spacing along length = 16/5 = 3.2 m
Spacing along width  = 12/4 = 3 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.5 × 3 = 4.5 m
3.2 m and 3 m ≤ 4.5 m  → OK
Actual E = 20 × 4950 × 0.8 × 0.8 / 192
         = 330 lux (≥ 300 lux required)

Layout diagram

Plan 16 m × 12 m (letter = phase of lamp circuit)
+-----------+
| R R R R R |
| Y Y Y R R |
| Y Y Y Y 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.

Sub-circuits

Rule (IS 732 / NEA practice): a light sub-circuit carries at most 10 points and 800 W. Here one luminaire is 55 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 = 1100 W (lighting)
Most loaded phase    = 385 W
Iph = 385 / (230 × 0.9) = 1.86 A
Main MCB ≥ 1.25 × 1.86 = 2.32 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
  |    |    |    |    |
  L1   L2   L3  ...

Given: store 36 m × 15 m, 45 W LED panel 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 (so spacing ≤ 4.5 m); 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.

Area A        = 36 × 15 = 540 m²
φ per luminaire = 4800 lm
Total lumens  = E × A / (UF × MF)
              = 200 × 540 / (0.8 × 0.8)
              = 168750 lm
N             = 168750 / 4800 = 35.16

Rounding up and fitting a regular grid: 9 × 4 = 36 luminaires (9 along the 36 m side, 4 along the 15 m side).

Arrangement and spacing

Spacing along length = 36/9 = 4 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
4 m and 3.75 m ≤ 4.5 m  → OK
Actual E = 36 × 4800 × 0.8 × 0.8 / 540
         = 204.8 lux (≥ 200 lux required)

Layout diagram

Plan 36 m × 15 m (letter = phase of lamp circuit)
+-------------------+
| R R R R R R Y Y Y |
| B B B B B B Y Y Y |
| R R R R R R Y Y Y |
| B B B B B B Y Y Y |
+-------------------+
[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 45 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): 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.

Distribution board design

Connected load = 1620 W light + 12000 W power = 13620 W
Most loaded phase    = 4540 W
Iph = 4540 / (230 × 0.9) = 21.93 A
Main MCB ≥ 1.25 × 21.93 = 27.42 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.

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  ...

Given: store 30 m × 15 m, twin 55 W LED luminaire (110 W per luminaire) giving 4950 lm, Hm = 3 m, E = 300 lux, CU = 0.8, MF = 0.8, supply 400 V, 3-phase, 50 Hz. Assumptions: 4950 lm is the output of one twin luminaire; SHR ≤ 1.5 (spacing ≤ 4.5 m); 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 = 4950 lm
Total lumens  = E × A / (UF × MF)
              = 300 × 450 / (0.8 × 0.8)
              = 210938 lm
N             = 210938 / 4950 = 42.61

Rounding up and fitting a regular grid: 9 × 5 = 45 luminaires (9 along the 30 m side, 5 along the 15 m side).

Arrangement and spacing

Spacing along length = 30/9 = 3.33 m
Spacing along width  = 15/5 = 3 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.5 × 3 = 4.5 m
3.33 m and 3 m ≤ 4.5 m  → OK
Actual E = 45 × 4950 × 0.8 × 0.8 / 450
         = 316.8 lux (≥ 300 lux required)

Layout diagram

Plan 30 m × 15 m (letter = phase of lamp circuit)
+-------------------+
| R R R R R Y Y Y Y |
| R R R B B B B B Y |
| R R Y Y Y Y Y B B |
| Y R R R R R B B B |
| Y Y Y Y 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.

Sub-circuits

Rule (IS 732 / NEA practice): a light sub-circuit carries at most 10 points and 800 W. Here one luminaire is 110 W, so up to 7 luminaires per sub-circuit. Number of circuits is made a multiple of 3 for phase balance.

Distribution board design

Total connected load = 4950 W (lighting)
Most loaded phase    = 1650 W
Iph = 1650 / (230 × 0.9) = 7.97 A
Main MCB ≥ 1.25 × 7.97 = 9.96 A → 10 A TPN MCB

DB specification: 4-way-per-phase TPN DB (12 ways in all, spares included) with 10 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 ≥ 10 A), plus earth continuity conductor.

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

Given: room 30 m × 20 m, 42 W LED at 110 lm/W, Hm = 3 m, CU = 0.75, depreciation factor 1.2, supply 400 V, 3-phase, 50 Hz. Assumptions: the required illumination is not given, so E = 300 lux is taken (medium industrial work, IS 3646); SHR ≤ 1.5; pf 0.9; 12 power sockets of 16 A (1 kW each).

Number of luminaires

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

Area A        = 30 × 20 = 600 m²
φ per luminaire = 42 W × 110 lm/W = 4620 lm
MF            = 1/DF = 1/1.2 = 0.833
Total lumens  = E × A × DF / UF
              = 300 × 600 × 1.2 / 0.75
              = 288000 lm
N             = 288000 / 4620 = 62.34

Rounding up and fitting a regular grid: 9 × 7 = 63 luminaires (9 along the 30 m side, 7 along the 20 m side).

Arrangement and spacing

Spacing along length = 30/9 = 3.33 m
Spacing along width  = 20/7 = 2.86 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.5 × 3 = 4.5 m
3.33 m and 2.86 m ≤ 4.5 m  → OK
Actual E = 63 × 4620 × 0.75 × 0.833 / 600
         = 303.2 lux (≥ 300 lux required)

Layout diagram

Plan 30 m × 20 m (letter = phase of lamp circuit)
+-------------------+
| R R R R R R R Y Y |
| B B B B Y Y Y Y Y |
| B B B R R R R R R |
| B Y Y Y Y Y Y Y R |
| 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 42 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): 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.

Distribution board design

Connected load = 2646 W light + 12000 W power = 14646 W
Most loaded phase    = 4882 W
Iph = 4882 / (230 × 0.9) = 23.58 A
Main MCB ≥ 1.25 × 23.58 = 29.48 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.

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  ...

Given: luminous intensity I = 250 cd (c.p.) in all directions below the horizontal; illuminance directly below the lamp E₀ = 100 lux (lm/m²).

Height of the lamp

For a point directly below the lamp, θ = 0 and distance = h:

E₀ = I / h²
h  = √(I / E₀) = √(250 / 100) = √2.5 = 1.581 m

Illuminance at 1.2 m from the vertical axis

By the cosine law, E = (I / d²) × cos θ, where d is the distance from lamp to point and cos θ = h/d.

d     = √(h² + x²) = √(2.5 + 1.2²) = √3.94 = 1.985 m
cos θ = h/d = 1.581/1.985 = 0.7966   (θ = 37.2°)
E     = I × h / d³
      = 250 × 1.581 / 1.985³
      = 395.3 / 7.821
      = 50.54 lux

Answer: lamp height = 1.58 m above the working table; illuminance at 1.2 m from the axis = 50.54 lux (lm/m²).

Given: hall 40 m × 25 m × 6 m, E = 250 lux on a plane 1 m above floor, CU = 0.8, depreciation factor 1.2, 45 W LED at 90 lm/W. The supply "400 V, 150 Hz" is read as 400 V, 50 Hz (standard frequency in Nepal). Assumptions: lamps suspended 1 m below ceiling → 5 m above floor, so Hm = 4 m; SHR ≤ 1.5; pf 0.9; 12 power sockets of 16 A (1 kW each).

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 = 45 W × 90 lm/W = 4050 lm
MF            = 1/DF = 1/1.2 = 0.833
Total lumens  = E × A × DF / UF
              = 250 × 1000 × 1.2 / 0.8
              = 375000 lm
N             = 375000 / 4050 = 92.59

Rounding up and fitting a regular grid: 12 × 8 = 96 luminaires (12 along the 40 m side, 8 along the 25 m side).

Arrangement and spacing

Spacing along length = 40/12 = 3.33 m
Spacing along width  = 25/8 = 3.13 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.5 × 4 = 6 m
3.33 m and 3.13 m ≤ 6 m  → OK
Actual E = 96 × 4050 × 0.8 × 0.833 / 1000
         = 259.2 lux (≥ 250 lux required)

Layout diagram

Plan 40 m × 25 m (letter = phase of lamp circuit)
+-------------------------+
| R R R R R R R R Y Y Y Y |
| B B B B B B B B Y Y Y Y |
| R R R R R R R R Y Y Y Y |
| B B B B B B B B Y Y Y Y |
| R R R R R R R R Y Y Y Y |
| B B B B B B B B Y Y Y Y |
| R R R R R R R R Y Y Y Y |
| B B B B B B B B Y Y Y Y |
+-------------------------+
[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 45 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): 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.

Distribution board design

Connected load = 4320 W light + 12000 W power = 16320 W
Most loaded phase    = 5440 W
Iph = 5440 / (230 × 0.9) = 26.28 A
Main MCB ≥ 1.25 × 26.28 = 32.85 A → 40 A TPN MCB

DB specification: 8-way-per-phase TPN DB (24 ways in all, spares included) with 40 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 ≥ 40 A), plus earth continuity conductor.

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

Result: 96 LED lamps of 45 W, at 5 m above floor (Hm = 4 m), 12 × 8 grid.

Given: hall 40 m × 20 m × 5.5 m, 2 × 36 W CFL set (72 W per set) at 60 lm/W, E = 108 lm/m², 3-phase 4-wire supply. Assumptions: UF and MF not given, so UF = 0.6 (medium-light room surfaces) and MF = 0.8 (clean room); working plane 0.8 m and sets suspended 0.5 m below ceiling → Hm = 5.5 − 0.5 − 0.8 = 4.2 m; SHR ≤ 1.5; pf 0.9; 12 power sockets of 16 A (1 kW each).

Number of luminaires

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

Area A        = 40 × 20 = 800 m²
φ per luminaire = 72 W × 60 lm/W = 4320 lm
Total lumens  = E × A / (UF × MF)
              = 108 × 800 / (0.6 × 0.8)
              = 180000 lm
N             = 180000 / 4320 = 41.67

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

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 × 4.2 = 6.3 m
4.44 m and 4 m ≤ 6.3 m  → OK
Actual E = 45 × 4320 × 0.6 × 0.8 / 800
         = 116.6 lux (≥ 108 lux required)

Layout diagram

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 72 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): 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.

Distribution board design

Connected load = 3240 W light + 12000 W power = 15240 W
Most loaded phase    = 5080 W
Iph = 5080 / (230 × 0.9) = 24.54 A
Main MCB ≥ 1.25 × 24.54 = 30.68 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.

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  ...

Generalised (general) lighting is a scheme in which luminaires are arranged in a regular pattern over the whole area to give a uniform level of illumination on the working plane, regardless of where the work is done. It lights tasks, gangways and surroundings alike.

Example: a classroom or a textile weaving shed lit by rows of LED panels/tube fittings at equal spacing to give about 300 lux everywhere.

Why localised lighting cannot be used alone

Localised lighting puts lamps close to particular work points (machine lamp on a lathe, bench lamp). If it is the only lighting:

1. High contrast and eye fatigue: the eye moves between a very bright task and dark surroundings and keeps re-adapting, which causes strain and headache.
2. Safety hazard: gangways, stairs, moving cranes and obstacles stay dark, so workers may trip or collide; finding exits in an emergency is difficult.
3. Harsh shadows and glare: a single nearby source throws deep shadows and may reflect off shiny work.
4. Poor supervision and cleaning: the general area cannot be inspected or maintained easily.
5. Gloomy, unpleasant environment that lowers morale and productivity.

Example: in a machine shop, a lamp on each lathe gives 1000 lux on the tool, but a worker walking between machines with no general light cannot see oil spills or moving trolleys. So local lighting is always used with general lighting (general-plus-local scheme), with the general level at least about one-third of the local task level.

Given: hall 40 m × 25 m × 6 m, E = 90 lm/m² on a plane 1 m above floor, CU = 0.8, depreciation factor 1.2, SHR = 1.2, 45 W LED at 90 lm/W, 3-phase 4-wire supply. Assumptions: luminaires mounted on the ceiling, so Hm = 6 − 1 = 5 m; pf 0.9; 12 power sockets of 16 A (1 kW each).

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 = 45 W × 90 lm/W = 4050 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 / 4050 = 33.33

Rounding up and fitting a regular grid: 7 × 5 = 35 luminaires (7 along the 40 m side, 5 along the 25 m side).

Arrangement and spacing

Spacing along length = 40/7 = 5.71 m
Spacing along width  = 25/5 = 5 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.2 × 5 = 6 m
5.71 m and 5 m ≤ 6 m  → OK
Actual E = 35 × 4050 × 0.8 × 0.833 / 1000
         = 94.5 lux (≥ 90 lux required)

Here the spacing limit (SHR 1.2) decides the grid: 34 lamps are needed for lux, and 7 × 5 = 35 just satisfies spacing ≤ 6 m. If the lamps were suspended 1 m (Hm = 4 m), spacing ≤ 4.8 m would need a 9 × 6 = 54 grid, so ceiling mounting is chosen.

Layout diagram

Plan 40 m × 25 m (letter = phase of lamp circuit)
+---------------+
| R R R R R R Y |
| B B Y Y Y Y Y |
| B B B B R R R |
| Y Y Y Y R R R |
| Y Y 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 45 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): 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.

Distribution board design

Connected load = 1575 W light + 12000 W power = 13575 W
Most loaded phase    = 4540 W
Iph = 4540 / (230 × 0.9) = 21.93 A
Main MCB ≥ 1.25 × 21.93 = 27.42 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.

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  ...

Given: hall 15 m × 12 m, 70 W metal halide lamp at 90 lm/W, E = 200 lm/m², UF = 0.7, MF = 0.6, supply 400 V, 3-phase, 4-wire. Assumptions: high-bay mounting with Hm = 4 m, SHR ≤ 1.5; pf 0.85 with compensated ballast; ballast loss neglected.

Number of luminaires

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

Area A        = 15 × 12 = 180 m²
φ per luminaire = 70 W × 90 lm/W = 6300 lm
Total lumens  = E × A / (UF × MF)
              = 200 × 180 / (0.7 × 0.6)
              = 85714 lm
N             = 85714 / 6300 = 13.61

Rounding up and fitting a regular grid: 5 × 3 = 15 luminaires (5 along the 15 m side, 3 along the 12 m side).

Arrangement and spacing

Spacing along length = 15/5 = 3 m
Spacing along width  = 12/3 = 4 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.5 × 4 = 6 m
3 m and 4 m ≤ 6 m  → OK
Actual E = 15 × 6300 × 0.7 × 0.6 / 180
         = 220.5 lux (≥ 200 lux required)

Layout diagram

Plan 15 m × 12 m (letter = phase of lamp circuit)
+-----------+
| R R R R R |
| Y Y Y Y Y |
| 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.

Sub-circuits

Rule (IS 732 / NEA practice): a light sub-circuit carries at most 10 points and 800 W. Here one luminaire is 70 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 = 1050 W (lighting)
Most loaded phase    = 350 W
Iph = 350 / (230 × 0.85) = 1.79 A
Main MCB ≥ 1.25 × 1.79 = 2.24 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
  |    |    |    |    |
  L1   L2   L3  ...

Metal halide lamps take 3–5 minutes to reach full output and 10–15 minutes to restrike, so a few LED lamps on a separate circuit are advisable for emergency/escape lighting.

The coefficient of utilization (CU) or utilisation factor is the ratio of the lumens that actually reach the working plane to the total lumens emitted by the lamps:

CU = lumens received on the working plane / total lumens emitted by the lamps

It is always less than 1 (typically 0.4–0.8), because some light is absorbed by the luminaire, walls and ceiling or falls outside the working plane. It is used in the lumen method: N = E × A / (φ × CU × MF).

Factors considered while selecting CU

1. Type of luminaire and light distribution: direct luminaires send most light downward and give a high CU; indirect lighting depends on ceiling reflection and gives a low CU. Luminaire efficiency (light output ratio) also matters.
2. Room index (room dimensions and mounting height): RI = L × W / [Hm (L + W)]. Large, low rooms (high RI) give high CU; narrow, tall rooms lose more light on walls (low CU).
3. Reflection factors of ceiling, walls and floor: light-coloured, brightly painted surfaces reflect more light back and raise CU; dark surfaces lower it.
4. Mounting height: higher mounting spreads more light onto walls, lowering CU.
5. Colour and condition of surroundings and obstructions: machinery and partitions block light.

CU is read from manufacturers' tables for the chosen luminaire, entering with room index and reflectances.

Given: room 30 m × 15 m × 5 m, E = 200 lux, CU = 0.8, MF = 0.7, twin 40 W fluorescent (80 W per luminaire) at 60 lm/W, Hm = 3 m. Assumptions: SHR ≤ 1.5 (spacing ≤ 4.5 m); electronic ballast, pf 0.9, ballast loss neglected; 3-phase 4-wire supply for the DB.

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 = 80 W × 60 lm/W = 4800 lm
Total lumens  = E × A / (UF × MF)
              = 200 × 450 / (0.8 × 0.7)
              = 160714 lm
N             = 160714 / 4800 = 33.48

Rounding up and fitting a regular grid: 9 × 4 = 36 luminaires (9 along the 30 m side, 4 along the 15 m side).

Arrangement and spacing

Spacing along length = 30/9 = 3.33 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
3.33 m and 3.75 m ≤ 4.5 m  → OK
Actual E = 36 × 4800 × 0.8 × 0.7 / 450
         = 215 lux (≥ 200 lux required)

Layout diagram

Plan 30 m × 15 m (letter = phase of lamp circuit)
+-------------------+
| R R R R R R Y Y Y |
| B B B B B B Y Y Y |
| R R R R R R Y Y Y |
| B B B B B B Y Y Y |
+-------------------+
[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 = 2880 W (lighting)
Most loaded phase    = 960 W
Iph = 960 / (230 × 0.9) = 4.64 A
Main MCB ≥ 1.25 × 4.64 = 5.8 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
  |    |    |    |    |
  L1   L2   L3   L4   L5  ...

Given: room 40 m × 15 m, E = 75 lux, Hm = 4 m, UF = 0.8, MF = 0.7, 1 × 36 W FTL with electronic ballast at 60 lm/W, 3-phase 4-wire supply. Assumptions: suitable space–height ratio 1.5 → maximum spacing 1.5 × 4 = 6 m; pf 0.9 with electronic ballast, ballast loss neglected.

Number of luminaires

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

Area A        = 40 × 15 = 600 m²
φ per luminaire = 36 W × 60 lm/W = 2160 lm
Total lumens  = E × A / (UF × MF)
              = 75 × 600 / (0.8 × 0.7)
              = 80357 lm
N             = 80357 / 2160 = 37.2

Rounding up and fitting a regular grid: 10 × 4 = 40 luminaires (10 along the 40 m side, 4 along the 15 m side).

Arrangement and spacing

Spacing along length = 40/10 = 4 m
Spacing along width  = 15/4 = 3.75 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.5 × 4 = 6 m
4 m and 3.75 m ≤ 6 m  → OK
Actual E = 40 × 2160 × 0.8 × 0.7 / 600
         = 80.6 lux (≥ 75 lux required)

Layout diagram

Plan 40 m × 15 m (letter = phase of lamp circuit)
+---------------------+
| R R R R R R R Y Y Y |
| B B B B B B Y Y Y Y |
| B R R R R R R R Y Y |
| B B B B B B Y Y Y Y |
+---------------------+
[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 36 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 = 1440 W (lighting)
Most loaded phase    = 504 W
Iph = 504 / (230 × 0.9) = 2.43 A
Main MCB ≥ 1.25 × 2.43 = 3.04 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
  |    |    |    |    |
  L1   L2   L3   L4   L5  ...

Factory lighting must give enough light, of the right quality, safely and economically, so that workers can work accurately, comfortably and safely.

General requirements

1. Adequate illumination: lux level suited to the task (e.g. 150 lux stores, 300 lux general machining, 500–1000 lux fine assembly and inspection, as per IS 3646).
2. Uniformity: minimum/average illuminance ≥ about 0.7 over the working area; achieved by keeping spacing within the spacing-to-height ratio.
3. Freedom from glare: luminaires shielded or mounted high; no bright reflections from shiny surfaces.
4. Shadow control: light coming from several directions so that machines and workers do not cast deep shadows.
5. No stroboscopic effect: adjacent lamps on different phases, electronic ballasts or flicker-free LED drivers near rotating machines.
6. Proper colour rendering: suitable CRI where colours must be judged.
7. Reliability and safety: emergency lighting on escape routes; luminaires suited to the environment (dust-proof, vapour-proof, flameproof in hazardous areas).
8. Economy and energy efficiency: high-efficacy lamps (LED, metal halide), good maintenance factor, controls (zoning, daylight sensors).
9. Easy maintenance: luminaires accessible for cleaning and lamp replacement.

Types of installation

The choice depends on ceiling height, nature of work, environment and cost; most factories use high- or low-bay general lighting with local lighting at critical machines.

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 spacing limit 5.6 m governs: 40/5.6 = 7.1 → 8 rows and 25/5.6 = 4.5 → 5 rows. A 7 × 5 grid gives 5.71 m (> 5.6 m) and an 8 × 4 grid gives 6.25 m, both too wide. So 8 × 5 = 40 luminaires of 2 × 36 W are used at 5 m × 5 m spacing (2.5 m from 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 |
+-----------------+

Given: room 45 m × 27 m, twin 36 W fluorescent (72 W per luminaire) at 70 lm/W, E = 150 lm/m², 3-phase 4-wire supply. Assumptions: brightly painted walls and ceiling → UF = 0.7; MF = 0.8; Hm = 3 m, SHR ≤ 1.5; 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        = 45 × 27 = 1215 m²
φ per luminaire = 72 W × 70 lm/W = 5040 lm
Total lumens  = E × A / (UF × MF)
              = 150 × 1215 / (0.7 × 0.8)
              = 325446 lm
N             = 325446 / 5040 = 64.57

Rounding up and fitting a regular grid: 11 × 6 = 66 luminaires (11 along the 45 m side, 6 along the 27 m side).

Arrangement and spacing

Spacing along length = 45/11 = 4.09 m
Spacing along width  = 27/6 = 4.5 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.5 × 3 = 4.5 m
4.09 m and 4.5 m ≤ 4.5 m  → OK
Actual E = 66 × 5040 × 0.7 × 0.8 / 1215
         = 153.3 lux (≥ 150 lux required)

Layout diagram

Plan 45 m × 27 m (letter = phase of lamp circuit)
+-----------------------+
| R R R R R R R R Y Y Y |
| B B B B B B Y Y Y Y Y |
| B B R R R R R R R Y Y |
| B B B B B B Y Y Y Y Y |
| B R R R R R R R Y Y Y |
| B B B B B B B Y Y Y Y |
+-----------------------+
[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 72 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 = 4752 W (lighting)
Most loaded phase    = 1584 W
Iph = 1584 / (230 × 0.9) = 7.65 A
Main MCB ≥ 1.25 × 7.65 = 9.57 A → 10 A TPN MCB

DB specification: 4-way-per-phase TPN DB (12 ways in all, spares included) with 10 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 ≥ 10 A), plus earth continuity conductor.

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

The point-to-point method (inverse-square / direct-ratio method) calculates the illuminance at a particular point due to one or more lamps from their luminous intensity, using the inverse square law and the Lambert cosine law. It is used where the illumination at specific points matters: street lighting, flood lighting, local lighting and checking uniformity.

Principle

For a lamp of intensity I (cd) in the direction of point P, at distance d from P, with θ the angle between the light ray and the normal to the surface:

E = (I / d²) × cos θ        lux

On a horizontal plane, lamp at height h,
point at horizontal distance x:
  d = √(h² + x²),  cos θ = h/d
  E_H = I h / d³ = (I / h²) cos³θ
On a vertical plane:
  E_V = I x / d³ = (I / h²) cos²θ sin θ

   Lamp L
     |\
   h |  \ d
     |θ   \
     +-----P   (horizontal plane)
        x

Steps

1. Find the intensity I of each lamp towards P from the polar curve of the luminaire.
2. Find d and θ from the geometry.
3. Calculate E from each lamp; add the contributions of all lamps at that point.
4. Repeat for several points to check minimum, maximum and uniformity.

Example: I = 400 cd, h = 2 m: directly below, E = 400/2² = 100 lux; at x = 2 m, d = 2.83 m, E = 400 × 2/2.83³ = 35.4 lux.

Limitation: it ignores light reflected from walls and ceiling, so for general indoor lighting the lumen method is used instead.

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
  |    |    |    |    |
  L1   L2   L3   L4   L5  ...

Given: room 40 m × 20 m, twin 40 W fluorescent (80 W) at 60 lm/W, Hm = 3 m, CU = 0.75, depreciation factor 1.2, supply 400 V, 3-phase, 50 Hz. Assumption: the required illumination is not given, so E = 200 lux (general process/industrial work) is taken; SHR ≤ 1.5; 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        = 40 × 20 = 800 m²
φ per luminaire = 80 W × 60 lm/W = 4800 lm
MF            = 1/DF = 1/1.2 = 0.833
Total lumens  = E × A × DF / UF
              = 200 × 800 × 1.2 / 0.75
              = 256000 lm
N             = 256000 / 4800 = 53.33

Rounding up and fitting a regular grid: 9 × 6 = 54 luminaires (9 along the 40 m side, 6 along the 20 m side).

Arrangement and spacing

Spacing along length = 40/9 = 4.44 m
Spacing along width  = 20/6 = 3.33 m
End spacing (to wall) = half of these
Max spacing = SHR × Hm = 1.5 × 3 = 4.5 m
4.44 m and 3.33 m ≤ 4.5 m  → OK
Actual E = 54 × 4800 × 0.75 × 0.833 / 800
         = 202.5 lux (≥ 200 lux required)

Layout diagram

Plan 40 m × 20 m (letter = phase of lamp circuit)
+-------------------+
| R R R R R R R R R |
| Y Y Y Y Y Y Y Y Y |
| B B B B B B B B B |
| R R R R R R R R R |
| Y Y Y Y Y Y Y Y Y |
| B B 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.

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 = 4320 W (lighting)
Most loaded phase    = 1440 W
Iph = 1440 / (230 × 0.9) = 6.96 A
Main MCB ≥ 1.25 × 6.96 = 8.7 A → 10 A TPN MCB

DB specification: 4-way-per-phase TPN DB (12 ways in all, spares included) with 10 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 ≥ 10 A), plus earth continuity conductor.

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

The amount of illumination needed at a place depends mainly on what visual task is done there, by whom, and for how long. A good lighting scheme must give this illumination with good quality (no glare, no harsh shadows, correct colour) at the lowest running cost.

Factors governing the amount of illumination

1. Nature of work (visual task): fine work (drawing, watch repair, inspection) needs 500–1500 lux; ordinary work (assembly, office) 300–500 lux; corridors and stores only 50–150 lux.
2. Size of detail and contrast: small objects, or objects with poor contrast against the background, need more light.
3. Speed and accuracy required: fast moving parts or precision work need higher levels.
4. Duration of work: long continuous work needs more light to avoid eye fatigue.
5. Age and eyesight of workers: older people need roughly 2–3 times more light for the same task.
6. Reflection factors of surroundings: light-coloured ceiling, walls and floor reflect light and raise the useful illumination; dark surfaces absorb it.
7. Size and shape of room (room index): in tall narrow rooms more light is lost on the walls, so the utilisation factor is lower.
8. Availability of daylight and the hours when artificial light is used.
9. Safety and economy: hazardous areas need a minimum safe level; the level chosen must also be affordable to install and run.

Recommended levels are taken from lighting codes (e.g. IS 3646 / CIBSE / IES tables).

Points to keep in mind while executing a lighting scheme

1. Adequate illumination: give the recommended lux level on the working plane (usually 0.75–0.85 m above floor).
2. Uniformity: the ratio of minimum to average illuminance should be about 0.7 or more; keep spacing-to-mounting-height ratio within the luminaire's limit.
3. Freedom from glare: shield lamps, mount them above the line of sight, use diffusers or louvres; avoid shiny surfaces that cause reflected glare.
4. Avoid harsh shadows: use several sources and diffused light, but keep some shadow for depth.
5. Correct colour rendering: choose lamps with suitable colour temperature and CRI for the task (e.g. high CRI for colour matching).
6. Avoid stroboscopic effect: near rotating machines, connect adjacent luminaires to different phases or use high-frequency electronic ballasts.
7. Suitable luminaire and mounting: dust-tight, vapour-proof or flame-proof fittings where required; easy access for maintenance.
8. Maintenance factor: allow for dirt and lamp lumen depreciation; plan regular cleaning and group replacement.
9. Energy efficiency and cost: use high-efficacy sources (LED, T5, HPSV), separate switching, daylight and occupancy controls.
10. Aesthetics and safety: the scheme should look neat and the wiring must follow electrical regulations.

 Task needs ──> Lux level ──> Lamp & luminaire
     │                              │
     └─> Room data (RI, reflect.) ──┴─> Number,
                                       spacing, layout