Lighting Design by the Lumen Method — Fixtures and Circuits
The lumen method gives a defensible fixture count in a few lines of arithmetic — and that count drives the lighting load, the circuits, and the BOM.
Pick the target illuminance
| Space | Target (lux) |
|---|---|
| Corridor / storage | 100–150 |
| General office | 300–500 |
| Drawing / detailed work | 500–750 |
| Retail sales floor | 500–1000 |
The lumen method formula
Where N = number of luminaires, E = target lux, A = floor area (m²), F = rated lumens per luminaire, UF = utilisation factor, MF = maintenance factor.
Utilisation and maintenance factors
UF comes from the manufacturer’s photometric table, indexed by the room index and surface reflectances:
where Hm is the mounting height above the working plane. UF is typically 0.4–0.7. MF (light loss over the cleaning cycle) is typically 0.7–0.8 for a normal environment.
Worked example
Office 12 m × 8 m, target 400 lux, LED panels at 4000 lm each, UF = 0.55, MF = 0.8:
From fixtures to circuits
At ~40 W per panel, 24 fixtures = 960 W. A 20 A, 230 V lighting circuit loaded to 80% carries ~3680 W, so this room fits one circuit with room to spare. Keep lighting circuits under 12–16 outlets and separate from receptacle circuits.
EngEst Pro turns your lighting layout into load: enter fixture wattage and quantity per area and it computes the lighting load, assigns circuits, and lists luminaires, switches, and wiring in the electrical BOM.
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