Lighting, Electrical, and Mechanical Systems
Lighting, Electrical, and Mechanical Systems
This domain tests coordination of building systems with interior design. Expect lighting metrics and calculations, lighting power density, controls, electrical load basics, HVAC distribution, and ceiling/space coordination.
Core rules
- Lighting metrics: lumens measure output, footcandles (lux) measure illuminance, CRI measures color rendering, CCT describes color appearance. Do not treat CCT as brightness.
- Lumen method: fixtures = (target fc × area) ÷ (lumens per fixture × CU × LLF). Use coefficients of utilization and light-loss factors appropriate to the fixture and maintenance.
- LPD limits installed lighting power per square foot; office values are commonly around 0.8–0.9 W/sf depending on code edition and space type. Controls-occupancy sensors, daylight dimming, time clocks-are part of compliance.
- Electrical: 20 A, 120 V branch circuits are typical; keep loads within 80% continuous capacity; coordinate receptacle and data locations with furniture.
- Mechanical: supply/return diffusers need throw and clearance; ducts, VAV boxes, and sprinklers compete for ceiling space. Check ceiling plenum depth and access panels.
Worked example
A 600 sf office needs 50 fc. Use 4,000-lumen fixtures, CU 0.70, LLF 0.80. Required lumens = 50 × 600 = 30,000. Delivered lumens per fixture = 4,000 × 0.70 × 0.80 = 2,240. Fixtures = 30,000 ÷ 2,240 ≈ 13.4, so specify 14. Check LPD: 14 × 40 W = 560 W; 560 ÷ 600 = 0.93 W/sf-verify against the applicable limit and adjust wattage or count.
Traps
- Confusing footcandles with lumens or using CCT as a brightness measure.
- Ignoring LPD and controls while focusing only on footcandles.
- Forgetting maintenance/LLF or using a generic CU.
- Coordinating lighting without ducts, sprinklers, structure, or ceiling height.