Best Practices for Site Preparation and Installation of In-Grade Fixtures  

Some projects demand that designers find a way to distribute light upward from the ground in highly trafficked outdoor areas. Recessed in-grade luminaires are the solution for uplighting features of an architectural structure or landscape, without creating a tripping hazard or maintenance obstacle in the area of intrigue. The in-grade luminaires are recessed into the ground and installed flush with the walking surface, enabling passersby to admire the sculpture, column, or canopy of trees and move safely through the space. Achieving the desired illumination goals with these luminaires requires many of the typical considerations that are made when selecting any luminaire, interior or exterior, such as reviewing the light distribution pattern and evaluating the energy needs and lamp life of the fixture. These recessed in-grade fixtures also require attention to details that are less typical in the sphere of lighting fixture specification, namely, the type of soil available on a project and the drainage it can provide.

Many designers may be surprised to know what a difference good drainage makes on the overall longevity and performance of a recessed in-grade installation. In fact, poor drainage is the predominant cause of water ingress that leads to the failure of an in-grade fixture. This article will provide best practices for matching the type of recessed in-grade fixture with the available soil type, creating adequate drainage, and options for bringing power to the fixtures.

The Dirt on Drainage

Recessed in-grade fixtures need drainage, because, despite impressive technological developments, water and electrical components still do not mix. As it relates to recessed in-grade lighting fixtures, relentless exposure to water can rust out the interior hardware or short out a ballast or socket, ultimately causing the fixture to fail. Although electrical products destined to be installed in the ground are often designed to better manage exposure to water and precipitation, best practices suggest that measures be taken to reduce the total soak time of an in-grade fixture.

The natural ability of the soil to drain water away from the surface, and away from the luminaire, is a powerful ally in keeping the recessed in-grade luminaire a little drier, but different types of soil drain water away with varying degrees of proficiency. It is the texture of the soil, its composition of gravel, sand, silt, and clay, that determines how quickly water will drain through the soil, also referred to as drainage. Sandy and granular soils typically drain water quickly from the surface. Soils with larger compositions of silt and clay tend to drain more poorly and, as a result, remain soggier or constantly saturated, posing more of a threat to the longevity of the recessed in-grade luminaire.

Identifying Soil Type

Before selecting a specific recessed in-grade luminaire, best practices recommend first identifying the type of soil present at the project site. Soils have been classified into two distinct groups, Type I and Type II, as it relates to a soil's ability to drain.

Type I describes a soil that is an easy or free draining soil. In general, this type of soil has very little or no clays and may have a sandy feel or appearance. Type II describes a soil that has more difficulty draining. In many cases, even small amounts of water will pool and can take hours or days to drain away from the surface. Typically, this type of soil consists of varied amount of clay or hardpan, which inhibits subsurface drainage.

The Soil Type Test

Determining whether the soil on a project is Type I or Type II is a fairly straightforward process. There is a simple test that will reveal the soil type in two hours. First, dig a hole at least 18 inches in diameter and 18 to 24 inches deep. Then fill half of the hole with water and track how long it takes for the water to drain into the soil. If complete drainage occurs in less than two hours, the soil is Type I. If complete drainage takes more than two hours, the soil is considered Type II.

Matching Installation Practices and Soil Type

The type of soil available on a project heavily influences how the recessed in-grade luminaire should be installed in order to reduce the fixture's exposure to standing water. While it is generally understood that the contractor will need to provide some type of drainage during installation, there are different ways to create drainage and these different approaches can deliver very different results. A technique that provides adequate drainage on one project may provide terribly insufficient drainage in another, ultimately creating a problematic installation. It is critical to match the site-specific drainage needs with the installation technique that will provide the necessary degree of drainage.

Installing In-Grade Fixtures in Type I Soil

Type I soil is considered a preferred site condition with ideal drainage properties. No additional installation practices are required if the project site offers Type I soil. However, review the Other Installation Considerations section on the next page, because different installation practices may be recommended in certain site-specific situations including areas covered in concrete, planter areas and walkways, and areas subject to heavy rainfall.

Installing In-Grade Fixtures in Type II Soil

There are three different installation practices recommended for installing recessed in-grade fixtures in Type II soil. They are a Standard Type II installation, an Advanced Type II installation, and a Premium Type II installation. Once an installer determines that the project site hosts Type II soil, the degree of drainage support necessary can be determined by assessing the prevalence of water in the location of the installation. Irrigation systems designed to water the area, a downspout draining into the vicinity of the installation, or a topography that slopes down toward the in-grade fixture location would all indicate that greater degrees of drainage should be provided to protect the functionality of the system.

Standard Type II installation. Over-excavating is the critical piece of a Standard Type II installation. This over-excavation creates an opportunity to backfill the excessively large hole with a material that drains better than the Type II soil readily available at the jobsite. Sand is the most highly recommended material. Backfilling with gravel, rock, or crushed rock should be avoided, because geotechnical research indicates that a migration of fine soil particles to this coarser backfill will occur over time, which could cause the ground around the fixture installation to sink, settle irregularly, or shift. Compacted sand does not allow for the migration of adjacent soil particles, preventing problematic soil settlement without compromising the installation of the fixture.

Specifically, the Standard Type II installation recommends that the installer create a hole for the housing that is 12 to 18 inches deeper than required by the fixture. The additional depth is then backfilled with moistened sand that is compacted into the space. The sand is easily adjusted to maintain the proper grade for the installation of the recessed in-grade fixture housing.

Placing an effective filter between the fixture housing and the sand backfill is the second critical component of a Standard Type II installation. Prior to the final setting of the housing, it is recommended that a filter material, such as a porous landscape fabric, be placed between the sand and the bottom of the housing. This filter will help keep sand out of the bottom of the open or vented fixture housing, without inhibiting the self-draining properties of the outer housing.

Advanced Type II installation. The Advanced Type II installation expands the excavation for the in-grade fixture system by pairing the additional depth of the Standard Type II installation with a trench between the fixture housings. The trench helps to facilitate the drainage of water away from the surface by removing the poorly draining soil from the immediate surrounding area of the fixture.

The connecting trenches should also be over-excavated and backfilled with moistened sand. A filter material should also be placed between the sand and the bottom of the fixture housing, allowing water to migrate down through the sand and away from the fixture, without allowing the sand to work itself up into the luminaire, potentially clogging the fixture housing.

Premium Type II installation. The Premium Type II installation, commonly referred to as a French Drain, also requires over-excavation, a sand backfill, trenching and further increases drainage on the project with the addition of a subsurface perforated pipe. The perforated pipe can be PVC or another type of gravity sewer pipe that is used for landscape drainage.

Drainage pipes can be pre-perforated or perforated in the field, but they should contain perforations at the 8:30, 10:30, 1:30, and 3:30 positions, when the pipe is placed directly under the housing. The perforations allow the water to migrate down through the sand and leach into the pipe for swift evacuation. The pipe should be at least 3 inches from the bottom of the sand backfill or 3 inches from the bottom of the fixture housing. A continuous pipe can be constructed and positioned to aid the drainage of multiple housings. Beyond the perforations, the pipe should also be wrapped with filter material so the sand backfill does not clog the drainage system.

The drainage pipe will need to maintain a sufficient grade difference to ensure that the collected water flows to the outlet and does not remain sitting in the pipe. A minimum slope of 0.15 percent is generally considered sufficient to promote adequate drainage. Optimum drainage will be achieved if the pipe is installed at ½ percent slope (0.5 foot of drop per 100 feet) or greater.

Other Installation Considerations

The presence of concrete, walkways, buildings, and other landscape and architectural features can materially affect how water travels through an environment, regardless of soil type. Some elements, like concrete, for example, may offer the installation area additional protection against super-saturation, while the presence of other items, such as a building's downpipe, irrigation systems, or the rain-heavy climate of the location may warrant additional drainage as a prudent precaution.

Concrete. If the lighting fixtures are being installed in concrete and there is a minimum of 5 feet between the lighting fixtures and nearby soil, Type I installation practices are recommended, because concrete protects the nearby soil from saturation and the presence of standing water may not be an environmental factor.

Planter areas and walkway. If the lighting fixtures will be installed between a walkway and another structure, such as a building or sidewalk, Type II installation practices are recommended to better manage the water that will run off of the pavement and onto the dirt where the fixtures are installed.

Areas subject to heavy rainfall. In areas that receive heavy rainfall, such as Seattle, Advanced or Premium Type II installation practices are recommended unless the existing soil demonstrates good Type I characteristics.

A word about clean gaskets. While poor drainage is the predominant cause of water ingress that leads to the failure of an in-grade fixture, poorly sealed gaskets are the next most common cause of water-related damage. To avoid leaky gaskets, it is critically important that the gaskets and the surface areas they are attempting to seal are clean. The presence of any grit, dust, or grime can interfere with the formation of a watertight seal, compromising the integrity of the gasket. Contractors should clean the gaskets and adhering surfaces before installation to ensure a tight and effective fit.

Selecting the Right In-Grade Fixture for a Project

Well lights and direct burial lights are two of the most popular types of recessed in-grade luminaires. Here is a closer look at the similarities and differences between these two fixture types, a few of the features that are unique to in-grade fixtures, and some general recommendations on the types of applications that may be best suited for each.

Well Lights

A well light is designed to allow water to pass through the fixture and drain into the soil below. The optical compartment inside the fixture housing is sealed, keeping water from the electrical components that would be damaged by water exposure, but the outer housing is vented and often referred to as a flow-through housing, because it allows water to drain from the surface and flow through the fixture.

Well lights are often installed in areas that require surface water drainage, such as tree grates. In a tree grate application, the luminaire can be integrated into the drain system and uplight the tree too.

Well lights should never be installed in applications where there is no drainage or poor drainage, because if the water is unable to drain out of the fixture, it will sit in the fixture and eventually cause the fixture to fail. If a well light is to be installed on a project where the soil is not permeable, a draining system will need to be installed.

Direct Burial Lights

Direct burial lights do not offer a method for draining surface water. These fixture housings are completely sealed and self-contained, designed for installation in concrete or soil. Despite being completely sealed, providing adequate drainage at each fixture is still a recommended practice, because extended exposure to saturated soil or submersion increases opportunities for the fixtures to leak and increases the risk of fixture failure.

Direct burial lights are often used to illuminate building perimeters surrounded by a sidewalk or to uplight structures such as flagpoles, sculptures, and monuments, because the direct burial lights can be installed into the sidewalk or the concrete base that is often poured to support these elements.

Recessed In-Grade Luminaire Features

Recessed in-grade fixtures are installed in the ground, flush with the grade. This elevation poses unique environmental challenges not faced by a luminaire installed above the ground. Fortunately, fixtures are being developed with features designed to better accommodate the elements. Some well lights and direct burial lights can be designed to withstand complete submersion in water, for short periods of time, to keep the temperature of the lens low enough to touch, and to hold up when driven over by an SUV.

Environmental protection. When fixtures are installed outdoors, in the ground, they will be regularly exposed to water, dirt, and dust. Unfortunately these elements can wreak havoc on electrical components, so recessed in-grade luminaires are designed to provide a certain degree of environmental protection to the more environmentally sensitive electrical items. The degree of protection is quantified by an Ingress Protection (IP) rating. The IP rating of a device typically has two numbers, although some have three. The first number describes the level of protection the enclosure provides against solid objects and dust. The second number denotes the level of protection provided against water.

The first IP number ranges from zero to six. Zero specifies that the luminaire provides no special protection from dirt, dust, and other solids and a six indicates total protection. The second IP number ranges from zero to eight. With zero indicating that the enclosure provides no protection from water and eight designating that the luminaire is suitable for intermittent submersion in water.

As it relates to recessed in-grade fixtures, an IP rating of 68 is the highest rating possible and indicates that the fixture can withstand exposure to dust, dirt, and particles and complete submersion in water over 1 meter in depth for one day. Few recessed in-grade fixtures are available that have achieved an IP rating of 68. In-grade fixtures with an IP rating of 67 are also completely protected against the ingress of dust, dirt, and particles, but are only protected against submersion in water that does not exceed 1 meter in depth. Recessed in-grade fixtures with an IP rating of 66 and lower are not rated for immersion and should not be submerged.

Thermally controlled lens system. Some recessed in-grade fixtures house light sources that generate a considerable amount of heat, such as halogen or metal halide. This can be particularly problematic in an in-grade fixture being used in a pedestrian area, because the lens is accessible to the public. Children can easily touch it. Animals can rub up against it. People can walk over it. If the lens becomes dangerously hot, it has the potential to cause an injury.

Some of these fixtures have been equipped to regulate the temperature of the lens, preventing it from becoming dangerously hot. This temperature-regulating system is referred to as a thermally controlled lens system. In-grade fixtures with a thermally controlled lens system employ a thermal mechanism that prevents the thermal characteristics of the lamp from transferring to the lens.

Drive-over rated. Some applications require that recessed in-grade fixtures withstand the weight of an automobile. Parking decks, circular driveways for restaurants and hotels, and even an entryway for a manufacturing facility or warehouse can benefit from the presence of recessed in-grade fixtures, but the fixtures are positioned in spaces that are designed to be driven over. Recessed in-grade fixtures can be designed to be driven over too.

Some manufacturers design recessed in-grade fixtures with a lens that is drive-over rated. The lens can be rated to withstand the weight of large trucks and SUVs.

Bringing Power to an In-Grade Fixture

Deciding how a recessed in-grade fixture system will be powered is another important piece of completing a successful installation. Designers can select line-voltage or low-voltage fixtures and this decision makes a significant difference in how the system will be installed.

Basics in Line Voltage and Low Voltage

At its most basic, a discussion about bringing power to any piece of equipment must begin with a quick explanation of line voltage and low voltage. Line voltage is supplied by the local power company in 120V and 277V. It is carried from the power company through power lines and into a residential or commercial building. Low-voltage fixtures and devices operate at a dramatically lower voltage, 12V or 24V, which is much more energy efficient, but requires a transformer to step down the line voltage to the lower voltage.

Transformer Options: Integral or Remote

Many of the recessed in-grade fixtures available today are low voltage, because of the incredible energy savings that the low-voltage fixtures are able to provide. These low-voltage recessed in-grade fixtures require a transformer to step down the line voltage. The transformer can be integral to the fixture, which means included as part of the fixture, or the transformer can be entirely separate from the fixture, also referred to as remote.

Integral vs. Remote Transformers

Whether the transformer is integral or remote affects how the power needs to be run to the recessed in-grade system. If a recessed in-grade fixture has an integral transformer, then the installer can bring line voltage directly to the fixture, which then steps down the voltage internally. If the low-voltage fixture does not have an integral transformer, it requires that the line voltage be run to the remote transformer and then low-voltage wires must be run out to each low-voltage fixture to power them.

There are a variety of reasons why a specifier may choose a recessed in-grade fixture with an integral transformer over a remote solution and vice versa. Depth constraints at the project site, pre-existing power configurations, project budget, and even the total size of the system design can impact which type of transformer best fits a particular project scenario.

Depth constraints. Some project sites have depth limitations for the recessed in-grade fixtures and require a shallower fixture housing to accommodate. Parking decks, commercial patio areas, and projects constructed on bedrock are all examples of applications where available depth is often limited. In areas where fixture depth is a concern, remote transformers may be the preferred solution, as fixture selection may be limited when trying to find a product that can pack a transformer into a shallower housing design. In these instances it may make sense to locate the ballast or transformer in a remote location.

Cost of installation. There is a dramatic difference in the number of man-hours required to run line voltage to a destination and the number of man-hours necessary to run a low-voltage wire to the same spot. Code mandates that line voltage encased in conduit must be run in a trench 18 inches deep. An unsheathed wire must be buried 24 inches below the surface. Either way, a contractor must dig a trench at least 18 inches deep all the way from the power source, often the building, to each location of the recessed in-grade luminaires to install a line-voltage system. Alternatively, low-voltage wire can be run on, or very near, the surface of the lawn. An installer will need to create the 18-inch trench from the power source to the transformer and then run the low-voltage wire from the transformer to each fixture drop. While local codes may vary, many installers use a spade and drop the low-voltage wire 6 inches below grade, instead of leaving the wire exposed on the ground, but it is tremendously less expensive to install low-voltage recessed in-grade fixtures when compared to the installation costs of a line-voltage system.

Pre-existing power configuration. In some instances, a line voltage supply already exists. In this scenario, it makes sense to use what is readily available. Designers will need to select a recessed in-grade fixture with an integral transformer, rather than creating another trench to a remote transformer and running low-voltage wire to each luminaire location.

Serviceability. In some cases, simplifying servicing and maintenance of the recessed in-grade system may be a primary concern for the design team. Applications prone to power issues may benefit substantially from a low-voltage system, because trouble-shooting a system with a single, remote transformer is much easier than trouble-shooting a system where each fixture has its own power source.

Design distance. Another important design factor to consider when selecting the right type of recessed in-grade fixture for a project is the length of the installation. When running low-voltage wire from one location to another, a voltage drop occurs over distance that can make a significant difference in the light output of a fixture at one end of the design and the other. Some manufacturers have defined a maximum allowable voltage drop of 5 percent. Designs which allow more than a 5 percent drop across the fixtures may reduce light output to a level that is not acceptable.

Designers can avoid unacceptable voltage drops by selecting a wire size that will accommodate the required wattage over the pre-determined distance to the fixture locations. Low-voltage wire sizes include 12-, 10-, 8- and 6-gauge wire. As the gauge of the wire decreases, the size of the copper wire becomes thicker, so 6-gauge wire is thicker than 12-gauge wire. The diameter of the copper is important, because thicker copper can transmit greater amounts of power.

Line voltage circuits do not experience a voltage drop in relationship to distance. Recessed in-grade fixtures with integral transformers can be positioned 100, 500, or 1,000 feet apart without affecting the light output of the fixture in the least.

A note about LED fixtures. The most common LED fixtures are low-voltage fixtures, but LEDs require additional power management to preserve the longevity of the diodes than just stepping down the power from 120V to 12V. The utility company delivers line-voltage power in the form of alternating current (AC), which is then stepped down to a lower voltage by a transformer. LED devices must convert the low-voltage AC into direct current (DC). LEDs use a component called a driver to make this conversion. LED drivers are entirely different from the transformers and ballasts that are sometimes also referred to as drivers in the industry.

Beyond the potential confusion in vernacular, transformer selection for LED fixtures requires special consideration not required when selecting a transformer for another type of low-voltage fixture. Many electronic transformers have a minimum starting load requirement. If the minimum is not met, the transformer will cycle off and cause the LEDs to flicker. The minimum is important to be aware of, because LED fixtures are known to have very small loads and it is entirely possible to have a system of recessed in-grade LED fixtures that would not exceed the minimum load requirement. Most electronic transformers also have a maximum wire length to limit voltage drop from one LED fixture to another. A magnetic transformer is capable of igniting at 1W, a much lower minimum than some electronic transformers, and has greater distances capabilities. Although a magnetic transformer is often not as energy efficient as an electronic transformer, it can be a more reliable and durable solution for an LED recessed in-grade lighting installation.

In any application that calls for a recessed in-grade fixture, regardless of whether it is a well light or a direct burial light, using best practices to provide the appropriate drainage and to select how the system will be powered will help to protect the performance of the fixtures and support trouble-free uplighting for passersby to enjoy year after year.

 

Since 1984, B-K Lighting has manufactured the highest quality and most innovative standard and custom fixtures for interior and exterior lighting including path, sign, area, flood, recessed, and in-grade applications. B-K Lighting products are installed in residential, commercial, and landmark projects worldwide. www.bklighting.com