Sheet CALC-17 · Design Guide & Calculator
Lighting Design: The Lumen Method, Worked Properly
How many luminaires does a room actually need, will they distribute evenly, and what will they cost to run? Work it room by room below, then read the guide underneath to understand what the numbers mean and where they stop being trustworthy.
The Guide
What the Lumen Method Does
The lumen method answers one question: how many luminaires does it take to hold an average maintained illuminance across a working plane. It works backwards from the target. Take the lux you need, multiply by the area to get the lumens that must land on the plane, then divide by the lumens each luminaire actually delivers there — which is its output reduced by the utilisation factor and the maintenance factor.
Those two factors carry the engineering. Utilisation factor is the proportion of a luminaire’s output that reaches the working plane after bouncing around the room; it rises with a squatter, brighter room and falls in a tall, narrow, dark one. Maintenance factor accounts for the light you lose over time to dirt on the fitting, dirt on the surfaces, and lamp depreciation — design to the dirty condition, not the day it was installed.
The Method in Five Steps
Set the target
Pick maintained illuminance from the task, not the room name. A store where labels are read is not a store where boxes are stacked.
Find the room index
RI = (L x W) / (Hm x (L + W)), where Hm is mounting height above the working plane. It captures the room shape in one number.
Look up utilisation
UF comes from RI plus ceiling and wall reflectances. Dark walls in a small room can cost you a third of your light.
Apply maintenance
MF of 0.8 suits a clean office on a normal cleaning cycle. Dusty or industrial environments justify 0.7 or lower.
Round to a layout
N = (E x A) / (F x UF x MF), then round up to a grid that divides the room sensibly. Eleven luminaires rarely lays out well; twelve does.
Check spacing
Spacing must stay within the spacing-to-height ratio, typically about 1.25 for a direct luminaire. Meeting the average with too few fittings gives you bright pools and dark gaps.
Typical Maintained Illuminance
Starting points for common spaces — always check the project brief and any applicable standard
| Space | Maintained lux | Notes |
|---|---|---|
| Corridors, circulation | 100 | Transition zones: avoid a large step from adjacent spaces |
| Stores, plant rooms | 150 | Higher locally where labels or gauges must be read |
| Reception, lobby | 200 | Appearance matters as much as the number |
| Residential living | 200 | Layered: ambient, task and accent rather than one flat level |
| Classrooms | 300 | Board face needs its own vertical illuminance |
| General office | 500 | Task plane; screen-based work needs glare control more than lux |
| Laboratories, workshops | 500 | Local task lighting for fine work |
| Drawing, detailed inspection | 750 | Consider colour rendering as well as level |
These are working defaults consistent with common practice in the codes and standards used in Kenya. They are a starting point for a design conversation, not a substitute for reading the standard that applies to your project, and any specific client or statutory brief takes precedence.
Beyond the Average
Where the Lumen Method Stops
An average is a poor description of a room. The lumen method tells you nothing about the four things clients actually complain about:
- Uniformity. The same average can be a comfortable even wash or a row of bright pools with gloom between them. Spacing discipline is what separates the two.
- Glare. Discomfort glare is a function of luminaire luminance, position and background — expressed as UGR, and not derivable from lux. Screen-based work usually needs UGR of about 19 or better; the fitting’s photometry decides this, not the quantity.
- Vertical illuminance. Faces in a meeting room, a classroom board, goods on a retail shelf: none of these live on the horizontal plane you just calculated.
- Colour. Colour rendering and colour temperature govern how the space feels and whether inspection work is possible. A scheme can be numerically perfect and still look wrong.
So treat the output above as the quantity and layout check, then confirm with a photometric calculation in DIALux or Relux using the actual manufacturer photometry before anything is issued for construction.
In Kenyan Practice
What Actually Bites on Site
Overstated lumen figures. Fittings arrive on Kenyan sites with catalogue outputs that no independent test supports. Ask for LM-79 test data, and where the supply chain cannot produce it, derate your assumption rather than your expectations.
Controls that get disabled. Occupancy sensing saves real energy right up until the first complaint about lights dropping out during a still meeting, after which someone overrides the lot. Commission the time delays properly and explain them to the facilities team, or the saving you calculated evaporates.
Voltage and driver quality. Where supply voltage swings, cheap drivers fail early and take your maintenance factor with them. The luminaire schedule is a specification document: name the driver standard, not just the lumen output.
Lighting is a diversity story too. The connected load you calculate here feeds the load schedule, and the load schedule feeds the cable, the board and the transformer — which is why lighting design is the second calculation in a building services set, not an afterthought.
Related Tools
Where This Goes Next
The connected lighting load from this calculation belongs in the load schedule, which drives cable sizing and protective device selection. If the scheme is part of an energy retrofit, test the wider case in the electricity bill calculator. All sixteen tools sit in the calculator suite.
Beyond the Calculator
Need the Lighting Design Done, or Want to Learn It?
We prepare lighting layouts, luminaire schedules and photometric calculations as part of a full electrical design package — or teach you to produce them yourself on real project drawings.