How Many LED High Bays Do You Need for a Warehouse?

There is a fast way to get close and a slow way to get it right. The fast way takes about two minutes with a calculator and gets you within a fixture or two, which is usually enough to price a job. The slow way is a photometric layout, and you want one before you commit four hundred fixtures to a building.

Here is the fast way.

Step 1. Decide how much light the space actually needs

Everything starts here, and it is where most quotes go wrong. A warehouse that stores pallets and a warehouse where people read part numbers off a shelf are not the same lighting job, even if the building is identical.

Foot candles are lumens per square foot landing on a surface. When a facility manager says the building is dark, they are almost always describing a foot candle number, not a wattage. Targets for other space types are in foot candle requirements by space type.

Step 2. Run the lumen method

The lumen method answers one question: how many total lumens does this room need. Then you divide by the output of the fixture you want to use.

That 0.7 is doing real work. It bundles two losses together: the coefficient of utilization, which is the share of a fixture's output that actually reaches the work plane rather than being absorbed by walls, racking and dark ceiling deck, and the light loss factor, which covers dirt on the lens and lumen depreciation over the fixture's life. In a clean open warehouse with light walls you can push it toward 0.8. In a dusty building with black racking and a dark deck, drop it to 0.6.

A worked example

Twenty thousand square feet, twenty five foot ceiling, bulk storage with some picking, so call it 20 foot candles.

Twenty seven fixtures on a twenty thousand square foot floor works out to roughly one fixture per 740 square feet, which on a grid is about 27 feet apart. Hold that number, because the next step checks it.

Sanity check the wattage

Twenty seven fixtures at 150 watts is 4,050 watts across 20,000 square feet, or about 0.20 watts per square foot. Energy codes generally allow warehouse lighting power densities in the range of roughly 0.6 to 0.95 watts per square foot depending on which code the jurisdiction has adopted, so a modern LED layout lands comfortably under the cap. If your math comes out above 0.8, either the target foot candle level is high for the space or the fixture efficacy is low. Both are worth a second look.

Step 3. Check the spacing against the mounting height

The lumen method tells you how many fixtures. It does not tell you whether the light will be even. That is what the spacing to mounting height ratio is for.

In the example above, if fixtures hang at 24 feet and the work plane is the floor, spacing of 24 to 36 feet keeps the ratio in range. Our 27 foot grid fits. If the count had come out at 15 fixtures spaced 45 feet apart, the average would still be 20 foot candles but you would see bright pools under each fixture and gloom between them.

Go tighter than 1.0 and you are spending money on overlap. Go wider than about 1.5 and uniformity falls apart, which is what people are reacting to when they say a space feels dim despite a meter reading that looks fine.

Step 4. Pick the beam angle, and let the racking decide

This is the step that separates a layout that works from one that technically hits the number.

Racked aisles want linear, not round

A round UFO high bay throws a circle. An aisle is a long rectangle. Put UFOs over racking and a large share of the output lands on the top of the rack instead of down in the aisle where the picker is.

In a racked building, run linear high bays down the aisles, oriented with the aisle rather than across it. You will use fewer fixtures to hit the same aisle foot candle level than you would with round fixtures, because far less of the light is wasted on the rack tops.

What the fast method will not catch

• Sprinkler clearance. Fixtures have to sit clear of sprinkler heads and their spray pattern. This regularly forces a mounting height change, which changes the count.
• Existing structure. Bar joists, ductwork and crane rails decide where a fixture can actually hang. A perfect grid on paper is rarely a perfect grid on the deck.
• Rack height versus mounting height. If racks are 20 feet and fixtures hang at 22, you are lighting rack tops. Fixtures generally want to be well above the rack or dropped into the aisle.
• Emergency and egress. Separate requirement, separate fixtures. Check it before the inspector does. See exit and emergency.
• Controls. Occupancy sensors in low traffic aisles often save more energy than any fixture choice, and most high bays we carry take an integral sensor.

Two things that make the order easier

Selectable wattage. A high bay that switches between 100, 150 and 200 watts on a DIP switch lets you lock the layout now and set the output after you have walked the space with a meter. One part number covers the building. How selectable fixtures work.

Check the DLC listing. High bay retrofits are the most rebated commercial lighting project there is, because the runtime is long and the fixtures they replace are so inefficient. If a rebate is in play on your job, the fixture has to be on the DesignLights Consortium list on the day it is installed. That listing is shown on every high bay product page here, and our short rebate guide covers how to check what your utility pays.