ASHRAE 90.1-2019 Lighting Power Density: LPD Limits and Controls

ASHRAE 90.1-2019 §9 limits connected interior lighting power by building type or by space type, and §9.4 adds mandatory lighting controls that are checked separately. The 2019 edition lowered allowances by about 5% on average from 90.1-2016. This guide gives the values the U.S. Department of Energy’s Building Energy Codes Program publishes for both editions, and states the edition for each number.

What are the LPD limits under ASHRAE 90.1-2019?

Lighting power density (LPD) is connected lighting power divided by floor area, in W/ft². Compliance compares the total installed lighting power of a building (or each space) with the allowance from the table. Standard 90.1-2019 offers two interior methods: the Building Area Method (§9.5, Table 9.5.1) and the Space-by-Space Method (§9.6, Table 9.6.1). A Simplified Building Method (§9.3) is available for buildings or tenant improvements under 25,000 ft² where at least 80% of the floor area is office, retail or school.

Space-by-Space Method, Table 9.6.1 (selected values, W/ft²):

Building Area Method, Table 9.5.1 (selected values, W/ft²):

Source: DOE Building Energy Codes Program, 90.1-2019 lighting training (slides on Addenda BB and CG). Table 9.5.1 has 32 building types and Table 9.6.1 many more space types; read the tables in the standard for the others. Building Area values are derived from the Space-by-Space values, so they move together between editions.

Luminaire wattage in the calculation is the manufacturer’s labeled maximum for line-voltage equipment, and the total input wattage of all components where a ballast, driver or transformer is used (§9.1.4). Exit signs and lighting integral to equipment installed by its manufacturer are among the exemptions the DOE training lists; the standard lists the rest.

What are the Building Area and Space-by-Space methods?

Building Area Method. Multiply the gross lighted area of each building type by its Table 9.5.1 allowance and sum. The DOE training describes it as fewer calculations, limited choice of building types and generally more restrictive than the Space-by-Space Method. It can be used for the entire building or for separate building-type areas.

Space-by-Space Method. Multiply the area of each space type by its Table 9.6.1 allowance and sum. It provides additional allowances for difficult room configurations, decorative and retail lighting, and advanced controls. A room geometry adjustment (§9.6.4) allows a 20% increase where the room cavity ratio exceeds the table’s threshold, and for corridors under 8 ft wide. A space with several functions is split into subspaces, except subspaces under 20% of the space and under 1,000 ft².

Both methods need a fixture schedule showing input watts per fixture and total watts per area.

What mandatory lighting controls does §9.4 require?

Controls are a separate compliance track from LPD. Section 9.4.1.1 lists the control functions: local control, restricted to manual ON, restricted to partial automatic ON, bilevel lighting control, automatic daylight responsive controls for sidelighting and toplighting, automatic partial OFF, automatic full OFF, scheduled shutoff, and scheduled OFF during nonbusiness hours. Which apply depends on the space type. Selected 90.1-2019 details from the DOE training:

• Local control: at least one control for all lighting in the space; in spaces up to 10,000 ft², each control serves 2,500 ft² at most.
• Bilevel control: at least one control step between 30% and 70% of full power, in addition to off.
• Sidelighting daylight control: photocontrols are required where the combined general lighting input power in primary sidelighted areas is 150 W or more, or in primary and secondary sidelighted areas combined is 300 W or more. Exceptions include glazing under 20 ft² and retail spaces.
• Toplighting daylight control: photocontrols are required where general lighting under skylights and roof monitors totals 150 W or more.
• Dimming: 90.1-2019 requires continuous dimming for daylight responsive controls (step dimming was removed), down to 20% or less, or off.

A project can pass the LPD calculation and still fail review if the electrical drawings omit the controls. Show control zones, sensor locations and daylight zone boundaries consistently on the reflected ceiling plans, electrical plans and specifications.

What mistakes cause LPD compliance failures?

1. Carrying over an older fixture schedule. Table values dropped between editions; open-plan office allowance fell from 0.81 to 0.61 W/ft² from 2016 to 2019. Check the connected wattage against the edition the AHJ has adopted.

2. Using lamp output instead of input watts. Use total input watts, including ballast, driver or transformer, per §9.1.4.

3. Misclassifying the space type. Pick the Table 9.6.1 space type by the room’s actual function, not the label on the architectural drawings.

4. Omitting controls documentation. Show the §9.4 controls that apply to each space on the electrical drawings.

Frequently asked questions

What is the ASHRAE 90.1-2019 LPD limit for an open-plan office?

0.61 W/ft² under the Space-by-Space Method (Table 9.6.1), down from 0.81 in 90.1-2016. The Building Area Method value for an office building is 0.64 W/ft² in 2019 (0.79 in 2016). The AHJ sets the applicable edition.

What is the difference between the Space-by-Space and Building Area methods?

The Building Area Method assigns one allowance per building type (Table 9.5.1); the Space-by-Space Method assigns an allowance per space type (Table 9.6.1) and adds allowances for room geometry, decorative and retail lighting, and advanced controls. DOE describes the Building Area Method as generally more restrictive.

What lighting is exempt from the LPD calculation?

The DOE training lists, among others, exit signs, lighting integral to equipment installed by its manufacturer, retail display windows enclosed by ceiling-height partitions, and casino gaming areas. See the standard for the complete list and the controls that still apply to some exempt lighting.

Do lighting controls need to appear on the construction drawings?

Yes, for permit review the controls that §9.4.1.1 requires for each space should be documented. LPD compliance alone does not satisfy §9.

Sources

• U.S. DOE Building Energy Codes Program, Standard 90.1-2019 Lighting training (energycodes.gov slides as hosted by Oregon Building Codes Division): Tables 9.5.1 and 9.6.1 values, §9.1.4, §9.3, §9.4.1.1, §9.6.4.
• ASHRAE 90.1 itself is paywalled; read §9 of the adopted edition before specifying.