Section 8.4.1/Interior Lighting Power Allowance
ASHRAE 90.1 Section 8.4 establishes maximum interior lighting power density (LPD) limits using either the building area method or the space-by-space method.
The interior lighting power allowance limits the total installed lighting wattage based on building type or individual space type. Section 8.4.1 offers two methods. The Building Area Method (Table 8.4.1) assigns a single lighting power density (LPD) in watts per square foot to the entire building based on the building type (e.g., office 0.64 W/sf, retail 0.84 W/sf, school 0.72 W/sf, warehouse 0.41 W/sf). The Space-by-Space Method (Table 8.4.2) assigns LPD values to individual space types (e.g., open office 0.61 W/sf, conference room 0.77 W/sf, corridor 0.41 W/sf, restroom 0.63 W/sf), allowing trading between spaces within the building. Additional power allowances are permitted for decorative lighting, display lighting in retail, and lighting for specific visual tasks per Section 8.4.3. The total installed interior lighting power must not exceed the allowance calculated by either method. The IECC Section C405 references ASHRAE 90.1 for the LPD values in many jurisdictions.
Why this section exists
Lighting is one of the largest energy consumers in commercial buildings, historically accounting for 25 to 40 percent of total electricity use. The LPD limits drive the adoption of high- efficiency light sources (LED), optimized luminaire design, and appropriate light levels. The values in the tables are updated each code cycle to reflect improving LED technology, and have decreased significantly over the past 20 years. The space-by-space method allows designers to allocate more power to spaces with higher visual demands (retail display, labs) while using less in spaces with lower requirements (storage, corridors), providing flexibility without increasing total building energy use.
What plan reviewers look for
Plan reviewers check the lighting power calculation on the electrical drawings or energy compliance documentation. They verify the method used (building area or space-by-space) and confirm the correct LPD value from the applicable table. For the building area method, they check the total installed wattage against the building type LPD times the gross floor area. For the space-by-space method, they verify each space type is correctly classified and check the total allowance against the total installed wattage. They verify that additional allowances (decorative, display) are claimed only where permitted. They check the lighting fixture schedule for total wattage per fixture type and confirm the calculation sums match.