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

Tips for designing a trouble-free installation in outdoor settings
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Sponsored by B-K Lighting
Jeanette Fitzgerald Pitts
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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.

Flight 93 Memorial, Union City, California. The length of a project must be considered when selecting how the installation will be powered, because low-voltage wires experience a voltage drop over long distances, which affects light output.

Photo courtesy of B-K Lighting

Flight 93 Memorial, Union City, California. The length of a project must be considered when selecting how the installation will be powered, because low-voltage wires experience a voltage drop over long distances, which affects light output.

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.


B-K Lighting 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.



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Originally published in Architectural Record
Originally published in December 2014