Lighting power allowance
Allowance (W) = LPD limit (W/ft2) x Area (ft2)
The code allowance for a space type multiplied by the floor area served by that lighting.
Guide
Commercial Lighting Compliance: Lux Targets, Power Density, and Control Credits
What a commercial lighting design has to prove: enough light on the task, not too much connected power, and controls that actually deliver the credited savings.
Get the illuminance target from the activity
Office desk work sits around 300 to 500 lux, classrooms and retail floors similar, corridors 100 to 150, and storage or dock areas 150 to 300. The number belongs to the task, not to the fixture or the room name on the drawing. Where a project has a client standard, use it, and note in the design file which standard governs so the reviewer is not comparing against a different table.
Power density is a watts per square foot budget
The code gives each space type an allowance in watts per square foot. Multiply by the area served, and that is the lighting budget for the space. Fill it with luminaires, but count input watts including driver losses, because the allowance is measured at the panel, not at the LED module. Spaces with different activities are usually counted separately, and using the space-by-space method is what keeps a retail floor from borrowing the office allowance.
Control credits are earned in writing
Occupancy sensing, daylight responsive dimming, and scheduled shutoff all earn a credit in most commercial codes, and the credit is only valid if the sequence of operation describes what the controls do and the drawings show the devices. Keep the credit conservative: an allowance with a thirty percent credit that nobody can commission is worth less than a modest installation that meets the budget without one.
What the reviewer and the commissioning agent both check
Fixture schedule against the drawings, input wattage per position, control devices on the plan, and the space-by-space area takeoff. Commissioning then tests the same list in the field. The screens in this section exist so both checks happen before the fixtures are ordered, when the wattage can still be changed.
Formulas used
Installed power density
LPD = Fixtures x Input watts / Area
Compare against the allowance. Input watts include the driver or ballast loss, and any credited control reduction is applied to the installed figure.
Average illuminance check
E = N x Phi x UF x MF / A
The same lumen method as residential work, with the utilization factor taken from the actual room surfaces and cavity.
Worked examples
A 5,000 square foot office floor
Inputs: 60 fixtures at 40 input watts, an allowance of 0.82 W/ft2, office target 400 lux.
The allowance is 0.82 x 5,000 = 4,100 watts. The installation draws 60 x 40 = 2,400 watts, or 0.48 W/ft2, which is fifty-eight percent of the allowance. The headroom of 1,700 watts is where extra fixtures, a higher output fixture, or a feature area can go.
The same floor with a two thousand square foot tenant area
Inputs: 2,000 ft2, the same 60 fixtures, a twenty percent control credit.
The area is smaller, so the same load is denser: 2,400 watts over 2,000 ft2 is 1.20 W/ft2 before controls. With the credit applied the figure drops to 0.96 W/ft2, still above the 0.82 allowance, which is a layout problem rather than a controls problem.
Questions readers ask
Which code edition should I use for the allowance?
The one the authority having jurisdiction has adopted for that project. Editions published between 2010 and 2019 differ by space type, so two projects in the same city can legitimately use different numbers. Confirm the adopted edition in writing before the numbers are fixed in a submittal.
Does the power density allowance cover exterior lighting?
Exterior lighting is normally counted in its own allowances, by zone or by area, and not in the interior budget. Keep exterior luminaires, canopy lighting, and signage on separate lines so an interior space is not charged for the parking lot.
Can a control credit push a design under the allowance on its own?
Sometimes, but not by much. Credits in the ten to thirty percent range close a modest overage. If the uncredited load is above the allowance by more than a third, the fixture count or output is the problem and controls will not hide it.
Check the power density of your design
Sources
- Energy Explained: Electricity U.S. Energy Information Administration · 2026-10-15
- Standards and guidelines ASHRAE · 2026-10-15
- Standards and recommended practice Illuminating Engineering Society · 2026-10-15