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# An example of calculating the number of indoor lighting fixtures

Home / Technical Articles / An example of calculating the number of indoor lighting fixtures An example of calculating the number of indoor lighting fixtures (on photo: Columbus, OH Ohio History Center reading room; by army.arch via Flickr)

## Input information

These are the input data for the following calculation:

1. An office area has length: 20 meter; width: 10 meter; height: 3 meter.
2. The ceiling to desk height is 2 meters.
3. 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.
4. Each lamp has an initial output (Efficiency) of 85 lumen per watt.
5. 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

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer. ### Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.

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