A high bay spec'd on wattage alone is a guess. The number that actually determines whether a warehouse aisle is properly lit is lumens delivered to the work plane, and that depends on mounting height, beam angle, and spacing together, not wattage by itself. Two fixtures rated at the same wattage can put out very different lumen totals depending on efficacy, and the same fixture can look underlit or overlit depending on how high it hangs and how far apart it's spaced. This guide walks through how to spec a high bay or low bay LED fixture the way a distributor actually does it: by mounting height first, then lumens, then spacing and beam angle, with a separate section on when the standard line isn't the right answer.
Start with mounting height, not wattage
Mounting height sets almost everything else in the spec. A fixture hung at 15 ft needs a different beam angle and lumen package than the same fixture hung at 40 ft to deliver comparable light on the floor. As a starting point:
- 12 to 20 ft ceilings: this is the low bay range. A narrower beam angle and a lower lumen package usually cover the area without wasting light on the walls.
- 20 to 30 ft ceilings: the standard high bay range for most warehouses and distribution centers. Round (UFO-style) or linear high bays both work here.
- 30 to 45 ft ceilings: higher-output high bays with a narrower beam angle to concentrate light on the floor rather than spreading it wide and losing intensity.
NemaLED's UFO-C high bay covers this range in one family: 100 W to 300 W with field-adjustable power, so the same fixture can be tuned down for a lower ceiling or up for a higher one rather than stocking a separate SKU per mounting height. The Q1/Q2 low bay and high bay series runs a wider 20 W to 200 W band across the same ceiling range, with four driver-cover options that set the mounting method: ceiling, pendant, surface, wall, or 25° stanchion.
Lumens per fixture, not just watts
Once mounting height is set, the next question is how many lumens each fixture needs to put out. Efficacy (lumens per watt) is what makes wattage alone misleading: a 150 W fixture at 170 lm/W puts out roughly 25,500 lumens, while a 150 W fixture at 100 lm/W puts out only 15,000. Two "150 W high bays" can light very different amounts of floor.
As a general target for warehouse and industrial floors, the IES Lighting Handbook and most LED manufacturer guides point to roughly 200 to 300 lux (about 20 to 30 footcandles) for general storage and circulation aisles, and 300 to 500 lux where picking, packing, or fine work happens. Those are starting targets, not a substitute for a proper layout calculation for your specific aisle width and rack height. NemaLED's free photometric layout service runs that calculation from your actual dimensions and returns fixture count, spacing, and a sample point-by-point light-level plan rather than a rule of thumb.
On efficacy: NemaLED's UFO-C delivers up to 170 lm/W, and the Q1/Q2 family runs up to 160 lm/W, both published on the fixture's own spec sheet rather than a blanket line-wide claim. Always confirm the number for the specific wattage and configuration you're ordering, since efficacy typically varies across a fixture's wattage range rather than staying flat.
Spacing: how far apart fixtures actually go
Spacing and mounting height move together. A commonly used starting ratio in warehouse lighting design is a spacing-to-mounting-height ratio of roughly 1:1 to 1.5:1, meaning fixtures spaced 1 to 1.5 times the mounting height apart, before uniformity starts to suffer and you get bright pools directly under each fixture with darker gaps between them. A fixture mounted at 25 ft, for example, commonly lands in the 25 to 35 ft spacing range depending on beam angle and the light level target. Aisle-focused racking layouts often warrant a tighter ratio than open floor areas, since the aisle itself is the target, not the top of the rack.
This is a starting point for budgeting fixture count, not a final layout. Racking height, aisle width, reflectance of the floor and rack surfaces, and the specific beam angle of the fixture you're speccing all shift the real number. If you want the actual math run against your building instead of a rule of thumb, send us your dimensions through the photometric layout request and we'll return a computed plan, not an estimate.
Beam angle: round vs. linear, wide vs. narrow
Beam angle decides how a fixture's lumens spread across the floor. A wide beam angle covers more floor area per fixture but delivers less intensity directly below it; a narrow beam angle concentrates light for higher ceilings or where a specific aisle or work surface needs to be well lit without spilling into adjacent zones.
- Round high bays (like the UFO-C) throw a symmetric circular pattern, which suits open floor areas: general warehouse storage, sports halls, workshops, and logistics centers where light needs to spread evenly in every direction.
- Linear or rectangular high bays concentrate light along a line, which suits narrow aisles between racking, where a round fixture would waste lumens on the rack tops instead of the aisle floor.
Match the beam pattern to the layout before the wattage. A high-output round fixture in a narrow aisle wastes lumens on the racks; a narrow linear fixture in an open floor area leaves gaps between rows.
When a standard high bay isn't the right spec
The high bay and low bay category covers the general industrial case, but two conditions call for a different line entirely, and neither is optional once the condition applies.
Ambient heat. A standard high bay is rated for typical warehouse ambient, not the 70-80°C air found near a furnace, kiln, or foundry floor. Running a standard-rated fixture in that heat shortens driver life and LED lumen maintenance well before the rated lifespan, often without an obvious failure on day one. If the mounting location runs hot, the high-temperature LED lighting line, or its high-temperature high bay category, carries fixtures with a verified ambient rating printed on the spec sheet, not just a "heat resistant" label. See the high-temperature LED high bay buyer guide for how those ratings are actually tested.
Hazardous classification. If the space is classified, a paint booth, fuel storage area, chemical handling zone, or anywhere flammable gas or vapor can be present under abnormal conditions, a standard high bay is not code-legal regardless of how well it's built. That's a Class I, Division 2 call, and Class I, Division 2 is the most common hazardous rating tied to high bay fixtures because it covers the "intermittent, abnormal-condition" case most industrial spaces actually have (continuous flammable atmospheres are rarer and fall under Class I, Division 1 instead). Confirm the classification on your area drawing before ordering; the fixture rating has to match the zone, not a guess at what "probably" applies.
Both of these are certification questions, not upgrades. A hazardous-rated or high-temperature-rated fixture costs more because it's built and tested to a different standard, not because it's a premium version of the same product.
A short spec checklist
Before requesting a quote, have these five numbers ready:
- Mounting height at the fixture location, not the building's overall ceiling height if they differ.
- Aisle width or floor area the fixture needs to cover.
- Target light level, or the industry/task if you're not sure (general storage vs. picking/packing vs. fine work).
- Ambient temperature at the mounting location, especially near any heat source.
- Hazardous classification, if any, per your area-classification drawing.
Send those five and we'll match a fixture and spec, or run the full layout through the photometric layout request if you want spacing and fixture count calculated rather than estimated.
Common questions
How many lumens do I need per high bay fixture? It depends on mounting height, spacing, and target light level together, not a single fixed number. As a rough starting point, a fixture mounted around 25 to 30 ft commonly needs somewhere in the 20,000 to 30,000 lumen range to hit typical warehouse light levels at normal spacing, but this varies with beam angle and how far apart fixtures are spaced. Run the actual numbers for your building rather than assuming a round figure.
What's the difference between a high bay and a low bay? The terms describe mounting height, not a different product category. Low bay generally means roughly 12 to 20 ft ceilings, with a wider beam angle and lower output; high bay covers 20 ft and up, typically with higher output and a narrower beam angle to project light further down. Some fixture families, like NemaLED's UFO-C and Q1/Q2 lines, cover both ranges through adjustable power rather than needing separate low bay and high bay SKUs.
Do I need a different fixture for a narrow aisle versus open floor? Not necessarily a different fixture, but often a different beam angle or format. A round high bay throws light evenly in a circle, which suits open floor space. A narrow rack aisle is better served by a linear or narrower-beam fixture that concentrates light along the aisle instead of spilling it onto the tops of the racks on either side.
Can I use a standard high bay in a warehouse that also stores flammable materials? Only in the unclassified parts of the building. If any area is classified as hazardous under your area-classification drawing, paint booths, fuel storage, chemical handling, that zone needs a certified Class I Division 2 (or stricter) fixture, not a standard industrial high bay, regardless of how sealed or well-built the standard fixture is.
How do I know if my ceiling runs hotter than a standard high bay is rated for? Standard high bays are typically rated for normal warehouse ambient, well under the 70-80°C air found near a furnace, kiln, or foundry floor. If your mounting location is near a heat source like that, measure the actual ambient temperature at the fixture location rather than assuming the room's general temperature applies, and check the fixture's printed ambient rating against it. Our high-temperature buyer guide covers how to verify a real rating before you buy.
Is a round or linear high bay more efficient? Neither format is inherently more efficient; efficacy (lumens per watt) is a property of the specific fixture, not the shape. The shape decides how the light spreads, round for even coverage across open floor, linear for concentrated light along a line, so the right choice depends on the layout, not a general efficiency difference between the two formats.
Sources: IES Lighting Handbook (illuminance guidance for industrial and warehouse spaces), manufacturer spec sheets for the fixtures referenced above.
Need a fixture spec'd to your environment? Send the room dimensions, classification, and ambient, we'll come back same day.
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