When upgrading an industrial facility from legacy fluorescent lighting to modern solid-state lighting (SSL), the most common challenge is accurately sizing the replacement. For warehouses and workshops currently utilizing 4-lamp T5HO (High Output) fixtures, the transition to linear LED high bays offers a significant opportunity for energy reduction and improved visual acuity. However, a simple "one-to-one" lumen replacement often results in over-lighting the space, leading to unnecessary energy expenditure and increased glare.
The primary technical conclusion for facility managers is this: A 150W to 180W linear LED high bay, delivering between 22,000 and 27,000 lumens, is the optimal replacement for a standard 4-lamp T5HO fixture. This sizing accounts for the superior optical efficiency of LEDs and the typical light loss factors (LLF) associated with aging fluorescent systems.
Understanding the Legacy 4-Lamp T5HO System
To size a replacement correctly, one must first understand the baseline performance of the existing system. A standard 4-lamp T5HO fixture typically utilizes 54W lamps. While the nominal lamp wattage is 216W, the total system draw, including ballast factor and losses, is approximately 250W.
According to the U.S. Department of Energy (DOE) Federal Energy Management Program (FEMP), legacy T5 fluorescent high bays have an initial lamp efficacy of roughly 104 lumens per watt (lm/W). However, this is "lamp lumens," not "delivered lumens." Fluorescent fixtures are omnidirectional, meaning light is emitted in 360 degrees. Much of this light is trapped within the fixture housing or lost through multiple reflections before exiting the reflector.
In contrast, LED linear high bays are inherently directional. The IES LM-79-19 standard defines the approved method for measuring the electrical and photometric properties of SSL products, focusing on delivered lumens—the actual light that leaves the fixture and reaches the work plane. Because of this directional efficiency, you can typically target 70-80% of the total fluorescent system lumens to achieve equivalent perceived brightness.
| Metric | 4-Lamp T5HO Fluorescent | Linear LED High Bay (Equivalent) |
|---|---|---|
| System Wattage | ~250W | 130W - 180W |
| Initial Lumens | ~20,000 (Lamp) | 18,200 - 27,000 (Delivered) |
| Luminous Efficacy | 80 - 90 lm/W (System) | 140 - 160 lm/W |
| Lumen Maintenance | Rapid Depreciation | 90% at 50,000 Hours (L90) |
| Lifespan | 20,000 - 30,000 Hours | 50,000 - 100,000+ Hours |
The Economics of Retrofitting: A 50,000 Sq Ft Case Study
The decision to retrofit is rarely based on light quality alone; ROI (Return on Investment) and payback periods are the primary drivers for commercial stakeholders. Based on a simulated retrofit of a 50,000 sq ft warehouse with 100 fixtures, the financial implications are profound.
In this scenario, replacing 250W T5HO fixtures with 180W premium linear LED units results in a 70W reduction per fixture. At an electricity rate of $0.18/kWh and 6,000 annual operating hours (typical for 24/7 operations), the annual energy savings reach $7,560. When factoring in the avoidance of T5HO lamp replacements—which occur roughly every 1.67 years in high-use environments—maintenance savings add an additional $9,600 annually.
The "Glass Box" Logic of HVAC Interaction
A non-obvious factor in sizing and ROI is the HVAC interactive effect. Because LEDs convert a higher percentage of energy into light rather than heat, they reduce the cooling load in a facility. In hot climates, this can result in an additional 10-15% in energy savings (calculated using an interactive factor of 0.33). Conversely, in extremely cold climates, the reduction in "waste heat" from lighting may slightly increase heating costs. For a 100-fixture warehouse, the net HVAC impact often results in a modest but measurable credit, further shortening the payback period.
With available utility rebates for DesignLights Consortium (DLC) Premium listed products, which can range from $45 to $80 per fixture, the total payback period for such a project can be as low as 8.3 months.
Sizing by Footprint and Mounting Height
Linear high bays are designed to match the physical footprint of legacy fluorescent fixtures, typically available in 2ft and 4ft lengths. This allows contractors to utilize existing mounting points and wiring entries, significantly reducing labor costs.
1. Matching the Length
- 2ft Linear LED High Bays: Best suited for replacing 2-lamp or 4-lamp T5HO fixtures in smaller workshops or areas with lower ceiling heights (15-20ft).
- 4ft Linear LED High Bays: The standard replacement for 4-lamp and 6-lamp T5HO fixtures in large warehouses with ceiling heights of 20-40ft.
2. Lumen Package Selection
Selecting the correct lumen package is critical to maintaining the required foot-candles (FC) on the work surface. According to the ANSI/IES RP-7-21 Recommended Practice for Lighting Industrial Facilities, warehouse aisles typically require 10-30 FC, while manufacturing areas may require 50-100 FC.
- Low Ceiling (15-20ft): Target 15,000 - 18,000 lumens. This prevents excessive glare while providing uniform coverage.
- Medium Ceiling (20-30ft): Target 20,000 - 24,000 lumens. This is the "sweet spot" for 4-lamp T5HO replacements.
- High Ceiling (30ft+): Target 28,000+ lumens. Higher wattage units (240W-300W) may be necessary to overcome the inverse square law of light over long distances.
The Linear High Bay LED Lights -HPLH01 Series provides a pragmatic solution for these varying needs, offering adjustable wattage and color temperature (CCT) settings. This flexibility allows facility managers to fine-tune the light levels post-installation, addressing the "over-lighting" risk common in fixed-wattage retrofits.
Linear High Bay LED Lights -HPLH01 Series, 18200lumens, Adjustable Wattage & CCT, 120-277V
Compliance, Controls, and Future-Proofing
Modern energy codes, such as ASHRAE Standard 90.1-2022 and the 2024 International Energy Conservation Code (IECC), have significantly lowered the allowable Lighting Power Density (LPD) for industrial spaces. Meeting these codes requires more than just efficient fixtures; it requires intelligent controls.
Mandatory Control Strategies
- Occupancy/Vacancy Sensing: Required by most state codes (including California Title 24) for spaces over 5,000 sq ft. Implementing sensors can add an additional 15-30% in energy savings by dimming or turning off lights in unoccupied aisles.
- Daylight Harvesting: In warehouses with skylights or clerestory windows, fixtures must dim automatically in response to available natural light to maintain a constant illumination level.
- 0-10V Dimming: This is the industry standard for commercial LED control. It allows for smooth, flicker-free dimming from 100% down to 10%.
When selecting fixtures, ensure they carry the DLC 5.1 Premium certification. This version of the standard introduces requirements for "integral controls," meaning the fixture is tested and verified to work with occupancy and daylight sensors. This is often a prerequisite for the highest tier of utility rebates. For a deeper look at industry trends, refer to the 2026 Commercial & Industrial LED Lighting Outlook: The Guide to Project-Ready High Bays & Shop Lights.
Installation Pitfalls and Best Practices
Transitioning from fluorescent to LED involves more than just a physical swap. Experienced contractors look for specific technical indicators to ensure long-term reliability.
Thermal Management
LED lifespan is inversely proportional to operating temperature. While T5HO lamps struggle in cold environments, LEDs thrive. However, in high-ambient-temperature facilities (foundries, non-conditioned warehouses in the South), heat is the enemy. Always verify the fixture's maximum ambient operating temperature. Sustained temperatures above 104°F (40°C) can lead to premature driver failure. Look for fixtures with rugged cold-rolled steel or aluminum housings designed for maximum heat dissipation.
Wiring and Interference
Lower-quality LED drivers can generate electromagnetic interference (EMI), which may disrupt sensitive warehouse equipment or radio communications. Ensure all fixtures comply with FCC Part 15 Regulations. Additionally, when wiring 0-10V dimming circuits, installers must differentiate between Class 1 and Class 2 wiring to comply with the National Electrical Code (NEC). Mixing high-voltage and low-voltage control wires in the same conduit without proper insulation is a common code violation.
Photometric Validation
Before committing to a large-scale purchase, request a photometric layout. Using IES files (the industry-standard format for photometric data), designers can simulate the light distribution in your specific space. This validates that the chosen sizing and spacing will achieve the desired foot-candle levels and uniformity across all aisles and work zones.
Summary Checklist for Sizing replacements
- Identify Legacy Load: Confirm if existing fixtures are 4-lamp or 6-lamp and note the ballast factor.
- Verify Ceiling Height: Use 15,000-18,000 lumens for <20ft; 22,000-27,000 lumens for 20-30ft.
- Check DLC QPL: Ensure the specific model is listed on the DLC Qualified Products List to secure rebates.
- Plan for Controls: Incorporate occupancy sensors to meet IECC 2024 or Title 24 requirements.
- Review Thermal Limits: Confirm the fixture can handle the facility's peak summer temperatures.
Disclaimer: This article is for informational purposes only and does not constitute professional electrical engineering or financial advice. All electrical installations must be performed by a licensed contractor in accordance with local and national building codes.
Sources
- DesignLights Consortium (DLC) Qualified Products List
- IES LM-79-19: Optical and Electrical Measurements of Solid-State Lighting
- DOE FEMP – Purchasing Energy-Efficient Commercial and Industrial LED Luminaires
- ASHRAE Standard 90.1-2022 Energy Standard for Buildings
- ANSI/IES RP-7-21: Recommended Practice for Lighting Industrial Facilities