Input information
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
Related electrical guides & articles
Good day.
Very good material and well presented.
I am working on the design of mini (500-600w) solar power plants.
This material helped a lot.
Thank You.
It is very interesting and brief except the mistakes as satendra had highlited but everything is fine. I like it
It’s good, but under calculation many mistakes are there like step 4 – 2*1.25 = 2.5 but there is 2.25 & step 8 – instead of 10/5 there is 10/4. Correct it. Except calculation mistake everything is really too good. Thank you so much for explanations.
It is very good.
The 8th step answer was wrong I guess. Because “width of room/no.of fixtures in row is the formula right? Then 10/5 is the numbers right?
I appreciate this page because it gives me the full package of what I’m looking for. Thank your very much.
Dear sir can you explain industrial lighting same as step by step procedure ….
Really, it’s so nice
I need a villa with full lighting calculation for it’s room and elactrical installations on AUTO CAD too, can you send me on my email now, please
Thanks for this calculation. This is really helpful for me.
the 21st has no place we equip with none odd fixtures and It’s good.
It’s good
In this example, we calculated the number of lighting fixtures as 21 but, why we only placed 20 in the figure (as 5×4)? Can anyone please explain?