The Foundation: Why a Fixed Spacing Number Fails
Properly lighting a commercial building's exterior is a balance of security, aesthetics, and energy efficiency. Get it right, and your property looks inviting and feels safe. Get it wrong, and you end up with glaring hotspots, unsettling dark zones, and wasted electricity. The most common error I see facility managers and contractors make is searching for a single, magic number for wall pack spacing. In reality, there is no such thing.
Effective spacing is not a fixed distance; it is a calculation based on three interconnected factors:
- Mounting Height: How high on the wall the fixture is installed.
- Beam Angle: The angle of the light distribution from the fixture, measured in degrees.
- Target Illuminance: The amount of light required on the ground or wall surface, measured in foot-candles (FC) or lux.
Ignoring this relationship is the primary cause of poor lighting layouts. A fixture mounted at 12 feet will require different spacing than the same fixture mounted at 20 feet to achieve the same level of brightness on the ground.
Debunking the "One-Size-Fits-All" Spacing Myth
A pervasive myth is that wall packs should always be placed "15 feet apart." This oversimplification leads to uneven coverage, often creating a "scalloping" effect of bright crescents and dark voids along a pathway. The correct approach is to use a Spacing-to-Mounting-Height (S/MH) ratio. This ratio, provided by lighting manufacturers, helps determine the maximum distance fixtures can be placed apart relative to their height.
As a practical rule of thumb drawn from field experience:
- Wide Distribution (approx. 110°-120°): Start with a spacing of 0.6 to 1.2 times the mounting height.
- Narrow Distribution (approx. 60°-90°): Use a tighter spacing of 0.4 to 0.6 times the mounting height.
This calculation provides a starting point, which must then be refined based on the specific needs of the site.
Step 1: Define Your Illuminance and Uniformity Goals
Before you can calculate spacing, you must define what you are trying to achieve. Different areas of a commercial property have different lighting needs. The goal is not just brightness, but uniformity—the smooth transition of light from one area to the next. According to the Illuminating Engineering Society (IES) Recommended Practice for Lighting Industrial Facilities (RP-7), different tasks require different light levels.
Target Illuminance Levels
For commercial exteriors, the goal is to provide enough light for safe navigation and security without creating glare or light pollution. A good practice is to design for 30-50% beam overlap between fixtures. This intentional overlap is what eliminates dark spots and ensures a uniform appearance, which is especially critical for retail storefronts where curb appeal directly impacts business.
Here are some typical target illuminance levels for commercial building exteriors:
| Application Area | Average Maintained Illuminance (Lux) | Average Maintained Illuminance (Foot-Candles) | Notes |
|---|---|---|---|
| Pedestrian Walkways & Entrances | 10–20 lux | 1–2 FC | Focus on safety and wayfinding. |
| Storefronts & Facades | 20–50 lux | 2–5 FC | Emphasize architectural details and attract customers. |
| Loading Docks & Service Areas | 30–75 lux | 3–7 FC | Higher levels needed for task performance and security. |
| General Parking Areas | 5–10 lux | 0.5–1 FC | Basic security and vehicle navigation. |
Consider Surface Reflectance
One of the most overlooked factors in lighting design is the color and material of the building facade. A dark, textured brick wall absorbs significantly more light than smooth, white stucco. My field experience shows that a dark or non-reflective surface can require 20-40% more lumen output to achieve the same perceived brightness as a light-colored surface. Always factor this in when selecting your fixtures; failing to do so can leave your beautifully textured brick wall looking dim and unwelcoming.
Step 2: A Practical Guide to Wall Pack Layout
With your illuminance targets set, you can now proceed with the physical layout. This process ensures your design is both effective and compliant.
1. Select the Right Fixture
Choosing the correct wall pack is critical. Look for fixtures with clear photometric data, including IES files. An IES file is a digital model of the light's distribution pattern that can be used in lighting design software. For B2B projects, having this data is non-negotiable for proving compliance and qualifying for rebates.
Key features to consider:
- Lumen Output: Must be sufficient to meet your target illuminance levels after accounting for wall reflectance.
- Beam Angle: Determines the fixture's distribution pattern and influences spacing.
- Full-Cutoff Design: These fixtures direct all light downward, preventing upward light spill (light pollution) and glare. This is often a requirement in local building codes. An adjustable model like the Wall Pack Light - Wall E series provides precise aiming capabilities to keep light on your property and away from neighbors.
- IP Rating: An Ingress Protection rating indicates resistance to dust and water. For any outdoor application, a rating of IP65 is the baseline, ensuring the fixture is "dust tight" and protected against water jets, as defined by IEC Standard 60529.
2. Calculate Spacing Based on Mounting Height
Once you have a fixture and a mounting height, you can perform the initial spacing calculation.
- Example Scenario: You are lighting a walkway along a building with a wide-distribution (120°) wall pack mounted at 15 feet.
- Calculation: Using the 1.2x rule of thumb (1.2 * 15 ft), your initial spacing would be approximately 18 feet between fixtures.
This is your starting point. Now, you must adjust for uniformity and real-world conditions.
3. Mock-Up and Verify On-Site
Photometric software is an excellent tool, but it can never fully replicate the unique conditions of a specific site. This is why I always recommend a physical mock-up. Before committing to the entire installation, mount two or three fixtures at the proposed height and spacing.
Wait until it is fully dark, then use a handheld lux meter to take measurements on the ground. Check the light levels directly beneath the fixtures, and more importantly, check them at the halfway point between fixtures. Is the light level in the middle acceptable? Is the transition smooth, or is there a noticeable drop-off? Adjust the spacing inward or outward until you achieve the desired uniformity. This simple test prevents costly mistakes and ensures the final installation meets expectations.
Common Installation Mistakes and How to Avoid Them
Even with a solid plan, installation errors can compromise performance. Here are some common "gotchas" I've seen on job sites:
- Improper Photocell Placement: A dusk-to-dawn photocell is a great energy-saving feature, but only if placed correctly. I often find them installed under a deep soffit or right next to a brighter, upward-facing light. This tricks the sensor into thinking it's always daytime, preventing the light from turning on. Ensure the photocell has a clear, unobstructed view of the open sky.
- Using Shoebox Fixtures for Facade Work: Large, powerful shoebox lights designed for parking lots are the wrong tool for lighting a wall. When mounted on a building, their powerful, forward-throw optics create intense, glaring hotspots on the upper wall and leave the ground in shadow. Stick to fixtures designed for wall mounting.
- Incorrect Aiming: Wall packs should be aimed downward to illuminate the ground and the lower portion of the facade. A common mistake is aiming them too far out, which creates glare for passersby and contributes to light pollution. For fixtures on low parapets, an aiming angle of 0-10° downward is sufficient. On taller walls, an angle between 10-25° helps cast light further out while still controlling glare.
Wrapping Up: Key Takeaways for a Professional Layout
Achieving a professional-grade lighting layout for a commercial building is not about guesswork. It is a systematic process that balances technical calculations with real-world verification. By moving past the idea of a single fixed spacing number, you can create a lighting system that is effective, efficient, and aesthetically pleasing.
Remember these core principles:
- Spacing is a Function of Height and Optics: Use the Spacing-to-Mounting-Height ratio as your starting point, not a generic distance.
- Define Your Goals: Determine the required illuminance levels for each zone before you begin.
- Plan for Uniformity: Design for 30-50% beam overlap to eliminate dark spots and create a smooth, professional look.
- Verify in the Field: Always mock up a small section and measure the results with a lux meter. The real world always has surprises that software can't predict.
By following these steps, you can ensure your building's exterior is not only well-lit for security but also presents a polished and welcoming image to clients and customers.
Frequently Asked Questions (FAQ)
What is the best spacing for LED wall packs? There is no single "best" spacing. It depends on the mounting height, the wall pack's beam angle, and the target light level. A common starting point is to space fixtures at a distance of 0.6 to 1.2 times the mounting height for wide-beam fixtures.
How many lumens do I need for an exterior wall light? This depends on the application. A pedestrian entrance may only need 10-20 lux (1-2 FC), while a loading dock could require 30-75 lux (3-7 FC). You must also consider the color of the wall; a dark brick wall may need up to 40% more lumens than a white wall to appear equally bright.
What is a full-cutoff wall pack? A full-cutoff fixture has optics that direct all light downward, below the horizontal plane of the light. This prevents light from shining up into the night sky (light pollution) or causing glare for people on or off the property. Many municipalities now require full-cutoff fixtures in their building codes.
Do I need a photocell for my wall packs? A photocell automatically turns lights on at dusk and off at dawn. It is a highly recommended feature for energy efficiency and automated operation, ensuring lights are only on when needed. This can lead to significant savings on electricity bills, often qualifying for additional utility rebates under programs managed by organizations like the DesignLights Consortium (DLC).