## 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**

mohammed nizamuddin

Number of rows with lighting fixtures = 4

Number of lighting fixtures in each row = 5

Required total number of fixtures = 21

TOTAL NUMBER OF FIXTURES SHOULD BE 20 OR 21?? IF 21 PLEASE EXPLAIN ? ACCORDING TO DATA AT THE BOTTOM SIDE (ALSO MENTIONED IN 1ST 2 LINES OF COMMENT) I THINK SO TOTAL NUMBER OF FIXTURES SHOULD BE 20.

REGARDS

mohammed nizamuddin

how to download this one?

kavi

tiny mistake in figures..

step 4: spacing should be 2.5 instead of 2.25…1.25×2=2.5..

step 5: the calculation sticks to 10/2.5=4

Alexander Nowak

Nice demonstation of a classical calculation without the help of a fat light planning software.

But let me please make some comments: A lighting level of 250 Lux for an office area is not compliant to standards. For offices you should plan with 500 Lux instead.

Also I would consider to use not CFL but linear fluorescent lamps (T5), which have much higher efficiency of approx. 100 Lumen per Watt. Than you get also a better MF of 0,8 and this together reduces the number of fixtures.

ugbana joseph

Thanks a lot its useful mostly to me because am still a fresher in the field of engineering.