These are the input data for the following calculation:
- An office area has length: 20 meter; width: 10 meter; height: 3 meter.
- The ceiling to desk height is 2 meters.
- The area is to be illuminated to a general level of 250 lux using twin lamp 32 watt CFL luminaires with a SHR of 1.25.
- Each lamp has an initial output (Efficiency) of 85 lumen per watt.
- The lamps Maintenance factor (MF) is 0.63 ,Utilization Factor is 0.69 and space height ratio (SHR) is 1.25.
Calculation in 8 steps
1. Total wattage of fixtures:
|Total wattage of fixtures = Number of lamps x each lamp’s watt.|
|Total wattage of fixtures = 2 × 32 = 64 Watt.|
2. Lumen per fixtures
|Lumen per fixtures = Lumen efficiency (Lumen per Watt) x each fixture’s watt|
|Lumen per fixtures = 85 x 64 = 5440 Lumen|
3. Number of fixtures
|Required number of fixtures = Required Lux x Room area / MF x UF x Lumen per fixture|
|Required number of fixtures = (250 x 20 x 10) / (0.63 × 0.69 × 5440)|
|We will need 21 fixtures|
4. Minimum spacing between each fixture
|The ceiling to desk height is 2 meters and space height ratio is 1.25, so:|
|Maximum spacing between fixtures = 2 × 1.25 = 2.25 meter.|
5. Number of required rows of fixtures along with width of room
|Number of rows required = Width of room / Max. spacing = 10 / 2.25|
|Number of rows required is therefore = 4.|
6. Number of fixtures required in each row
|Number of fixtures required in each row = Total Fixtures / Number of rows = 21 / 4|
|Therefore, we have 5 fixtures in each row.|
7. Axial spacing between each fixture:
|Axial spacing between fixtures = Length of room / Number of fixtures in each row|
|… and that would be: 20 / 5 = 4 Meter|
8. Transverse spacing between each fixture:
|Transverse spacing between fixtures = Width of room / Number of fixtures in row|
|… and that would be: 10 / 4 = 2.5 Meter.|
5 things to conclude
So far, we have been calculated the following:
- Number of rows with lighting fixtures = 4
- Number of lighting fixtures in each row = 5
- Axial spacing between fixtures = 4.0 meter
- Transverse spacing between fixtures = 2.5 meter
- Required total number of fixtures = 21