IS 3646 : 2009Code of Practice for Interior Illumination

IS 3646 is the code of practice for interior illumination — Part 1 covers principles of good lighting and the design method; Part 2 the schedule of recommended illuminance and glare-limit values by interior and task. It is what an electrical/services engineer designs building lighting against, and what a commissioning check verifies on handover.

It is read with the regulatory and product stack:

• NBC 2016 Part 8 Section 1 — lighting and natural ventilation (the statutory trigger)
• ECBC / Energy Conservation Building Code — caps lighting power density (LPD), so design must satisfy *both* lux and watts
• IS 16107 (parts) — LED luminaires performance
• IS 732 — the wiring installation feeding the lighting

IS 3646 sets maintained illuminance (lux on the task plane, after light loss) and limits glare. Representative task levels:

• General office / classroom: 300–500 lux
• Drawing office, detailed/precision work: 600–750 lux and above
• Corridors, stairs, lobbies: 100–150 lux
• Industrial work: from ~200 lux (rough) to 1000+ lux (fine assembly/inspection)

Also specified: a limiting glare index for the interior, uniformity of illuminance over the task area (typically ≥ 0.7–0.8), and appropriate colour rendering (Ra) and colour temperature for the task. The design tool is the lumen method, which converts these targets into a luminaire count using the utilisation factor (room/luminaire geometry) and the maintenance factor (lamp lumen depreciation + dirt).

Brief: general office 10 m × 8 m (area 80 m²), target 400 lux maintained, LED luminaire of 4000 lm.

Lumen method: N = (E × A) / (Φ × UF × MF)

• E = 400 lux, A = 80 m², Φ = 4000 lm per luminaire
• Utilisation factor UF ≈ 0.50 (typical for the room index and mounting)
• Maintenance factor MF ≈ 0.80 (clean office, LED)

N = (400 × 80) / (4000 × 0.50 × 0.80) = 32000 / 1600 = 20 luminaires

Lay out as a uniform 5 × 4 grid; then check ECBC LPD: 20 × ~36 W = 720 W over 80 m² ≈ 9 W/m², which must be ≤ the ECBC limit for the space type. If it exceeds, choose higher-efficacy luminaires — you cannot trade away the lux.

1. **Designing to lamp lumens, not *maintained* lux.** Omitting the maintenance factor over-states delivered light by 20–40% — the installation is under-lit within a year.

2. Ignoring uniformity and glare. Hitting average lux with hot-spots and dark corners fails the IS 3646 uniformity/glare criteria even if the average 'passes'.

3. Designing lux without checking ECBC LPD. Modern compliance is dual — adequate lux *and* within the lighting-power-density cap; one without the other fails approval.

4. Mixing colour temperatures/CRI across a space, or using low-CRI sources where colour judgement matters.

5. No daylight integration or controls. IS 3646/ECBC expect daylight-linked dimming and occupancy control in many spaces — designing a static all-on scheme wastes energy and misses compliance credits.

• NBC 2016 Part 8 Section 1 — lighting and natural ventilation
• ECBC — lighting power density limits and controls
• IS 16107 (parts) — LED luminaires / modules performance
• IS 3646 Part 1 & Part 2 — principles and the schedule of illuminance values
• IS 732 — electrical wiring installations
• IS 6665 — code of practice for industrial lighting · IS 4347 — emergency lighting

The IS 3646 *task lux* targets are stable and still authoritative, but the technology and compliance context have moved decisively to LED + lighting controls + ECBC. Practical lighting design in 2026 means: meet the IS 3646 task lux and uniformity, stay under the ECBC lighting-power-density cap, integrate daylight-linked and occupancy controls, and select luminaires by efficacy (lm/W), CRI and glare rating (UGR) — not just wattage.

The most common field outcome — spaces that measure well below the design lux at 12–18 months — is almost always a missing or optimistic maintenance factor and no cleaning/relamping regime. Specify the maintained lux, state the MF assumption, and hand over a maintenance schedule; that is what makes an IS 3646 design hold up in service.